macros: split diagnostic derives into modules

Split `SessionDiagnostic` and `SessionSubdiagnostic` derives and the
various helper functions into multiple modules.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_macros/src/diagnostics/diagnostic.rs

@@ -0,0 +1,703 @@
+#![deny(unused_must_use)]
+
+use crate::diagnostics::error::{span_err, throw_span_err, SessionDiagnosticDeriveError};
+use crate::diagnostics::utils::{
+    option_inner_ty, report_error_if_not_applied_to_span, type_matches_path, FieldInfo, HasFieldMap,
+};
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use std::collections::HashMap;
+use syn::{spanned::Spanned, Attribute, Meta, MetaList, MetaNameValue, Type};
+use synstructure::Structure;
+
+/// The central struct for constructing the `as_error` method from an annotated struct.
+pub(crate) struct SessionDiagnosticDerive<'a> {
+    structure: Structure<'a>,
+    builder: SessionDiagnosticDeriveBuilder,
+}
+
+impl<'a> SessionDiagnosticDerive<'a> {
+    pub(crate) fn new(diag: syn::Ident, sess: syn::Ident, structure: Structure<'a>) -> Self {
+        // Build the mapping of field names to fields. This allows attributes to peek values from
+        // other fields.
+        let mut fields_map = HashMap::new();
+
+        // Convenience bindings.
+        let ast = structure.ast();
+
+        if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &ast.data {
+            for field in fields.iter() {
+                if let Some(ident) = &field.ident {
+                    fields_map.insert(ident.to_string(), quote! { &self.#ident });
+                }
+            }
+        }
+
+        Self {
+            builder: SessionDiagnosticDeriveBuilder {
+                diag,
+                sess,
+                fields: fields_map,
+                kind: None,
+                code: None,
+                slug: None,
+            },
+            structure,
+        }
+    }
+
+    pub(crate) fn into_tokens(self) -> TokenStream {
+        let SessionDiagnosticDerive { mut structure, mut builder } = self;
+
+        let ast = structure.ast();
+        let attrs = &ast.attrs;
+
+        let (implementation, param_ty) = {
+            if let syn::Data::Struct(..) = ast.data {
+                let preamble = {
+                    let preamble = attrs.iter().map(|attr| {
+                        builder
+                            .generate_structure_code(attr)
+                            .unwrap_or_else(|v| v.to_compile_error())
+                    });
+
+                    quote! {
+                        #(#preamble)*;
+                    }
+                };
+
+                // Generates calls to `span_label` and similar functions based on the attributes
+                // on fields. Code for suggestions uses formatting machinery and the value of
+                // other fields - because any given field can be referenced multiple times, it
+                // should be accessed through a borrow. When passing fields to `set_arg` (which
+                // happens below) for Fluent, we want to move the data, so that has to happen
+                // in a separate pass over the fields.
+                let attrs = structure.each(|field_binding| {
+                    let field = field_binding.ast();
+                    let result = field.attrs.iter().map(|attr| {
+                        builder
+                            .generate_field_attr_code(
+                                attr,
+                                FieldInfo {
+                                    vis: &field.vis,
+                                    binding: field_binding,
+                                    ty: &field.ty,
+                                    span: &field.span(),
+                                },
+                            )
+                            .unwrap_or_else(|v| v.to_compile_error())
+                    });
+
+                    quote! { #(#result);* }
+                });
+
+                // When generating `set_arg` calls, move data rather than borrow it to avoid
+                // requiring clones - this must therefore be the last use of each field (for
+                // example, any formatting machinery that might refer to a field should be
+                // generated already).
+                structure.bind_with(|_| synstructure::BindStyle::Move);
+                let args = structure.each(|field_binding| {
+                    let field = field_binding.ast();
+                    // When a field has attributes like `#[label]` or `#[note]` then it doesn't
+                    // need to be passed as an argument to the diagnostic. But when a field has no
+                    // attributes then it must be passed as an argument to the diagnostic so that
+                    // it can be referred to by Fluent messages.
+                    if field.attrs.is_empty() {
+                        let diag = &builder.diag;
+                        let ident = field_binding.ast().ident.as_ref().unwrap();
+                        quote! {
+                            #diag.set_arg(
+                                stringify!(#ident),
+                                #field_binding.into_diagnostic_arg()
+                            );
+                        }
+                    } else {
+                        quote! {}
+                    }
+                });
+
+                let span = ast.span().unwrap();
+                let (diag, sess) = (&builder.diag, &builder.sess);
+                let init = match (builder.kind, builder.slug) {
+                    (None, _) => {
+                        span_err(span, "diagnostic kind not specified")
+                            .help("use the `#[error(...)]` attribute to create an error")
+                            .emit();
+                        return SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some((kind, _)), None) => {
+                        span_err(span, "`slug` not specified")
+                            .help(&format!("use the `#[{}(slug = \"...\")]` attribute to set this diagnostic's slug", kind.descr()))
+                            .emit();
+                        return SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some((SessionDiagnosticKind::Error, _)), Some((slug, _))) => {
+                        quote! {
+                            let mut #diag = #sess.struct_err(
+                                rustc_errors::DiagnosticMessage::fluent(#slug),
+                            );
+                        }
+                    }
+                    (Some((SessionDiagnosticKind::Warn, _)), Some((slug, _))) => {
+                        quote! {
+                            let mut #diag = #sess.struct_warn(
+                                rustc_errors::DiagnosticMessage::fluent(#slug),
+                            );
+                        }
+                    }
+                };
+
+                let implementation = quote! {
+                    #init
+                    #preamble
+                    match self {
+                        #attrs
+                    }
+                    match self {
+                        #args
+                    }
+                    #diag
+                };
+                let param_ty = match builder.kind {
+                    Some((SessionDiagnosticKind::Error, _)) => {
+                        quote! { rustc_errors::ErrorGuaranteed }
+                    }
+                    Some((SessionDiagnosticKind::Warn, _)) => quote! { () },
+                    _ => unreachable!(),
+                };
+
+                (implementation, param_ty)
+            } else {
+                span_err(
+                    ast.span().unwrap(),
+                    "`#[derive(SessionDiagnostic)]` can only be used on structs",
+                )
+                .emit();
+
+                let implementation = SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                let param_ty = quote! { rustc_errors::ErrorGuaranteed };
+                (implementation, param_ty)
+            }
+        };
+
+        let sess = &builder.sess;
+        structure.gen_impl(quote! {
+            gen impl<'__session_diagnostic_sess> rustc_session::SessionDiagnostic<'__session_diagnostic_sess, #param_ty>
+                    for @Self
+            {
+                fn into_diagnostic(
+                    self,
+                    #sess: &'__session_diagnostic_sess rustc_session::parse::ParseSess
+                ) -> rustc_errors::DiagnosticBuilder<'__session_diagnostic_sess, #param_ty> {
+                    use rustc_errors::IntoDiagnosticArg;
+                    #implementation
+                }
+            }
+        })
+    }
+}
+
+/// What kind of session diagnostic is being derived - an error or a warning?
+#[derive(Copy, Clone)]
+enum SessionDiagnosticKind {
+    /// `#[error(..)]`
+    Error,
+    /// `#[warn(..)]`
+    Warn,
+}
+
+impl SessionDiagnosticKind {
+    /// Returns human-readable string corresponding to the kind.
+    fn descr(&self) -> &'static str {
+        match self {
+            SessionDiagnosticKind::Error => "error",
+            SessionDiagnosticKind::Warn => "warning",
+        }
+    }
+}
+
+/// Tracks persistent information required for building up the individual calls to diagnostic
+/// methods for the final generated method. This is a separate struct to `SessionDiagnosticDerive`
+/// only to be able to destructure and split `self.builder` and the `self.structure` up to avoid a
+/// double mut borrow later on.
+struct SessionDiagnosticDeriveBuilder {
+    /// Name of the session parameter that's passed in to the `as_error` method.
+    sess: syn::Ident,
+    /// The identifier to use for the generated `DiagnosticBuilder` instance.
+    diag: syn::Ident,
+
+    /// Store a map of field name to its corresponding field. This is built on construction of the
+    /// derive builder.
+    fields: HashMap<String, TokenStream>,
+
+    /// Kind of diagnostic requested via the struct attribute.
+    kind: Option<(SessionDiagnosticKind, proc_macro::Span)>,
+    /// Slug is a mandatory part of the struct attribute as corresponds to the Fluent message that
+    /// has the actual diagnostic message.
+    slug: Option<(String, proc_macro::Span)>,
+    /// Error codes are a optional part of the struct attribute - this is only set to detect
+    /// multiple specifications.
+    code: Option<proc_macro::Span>,
+}
+
+impl HasFieldMap for SessionDiagnosticDeriveBuilder {
+    fn get_field_binding(&self, field: &String) -> Option<&TokenStream> {
+        self.fields.get(field)
+    }
+}
+
+impl SessionDiagnosticDeriveBuilder {
+    /// Establishes state in the `SessionDiagnosticDeriveBuilder` resulting from the struct
+    /// attributes like `#[error(..)#`, such as the diagnostic kind and slug. Generates
+    /// diagnostic builder calls for setting error code and creating note/help messages.
+    fn generate_structure_code(
+        &mut self,
+        attr: &Attribute,
+    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+        let span = attr.span().unwrap();
+
+        let name = attr.path.segments.last().unwrap().ident.to_string();
+        let name = name.as_str();
+        let meta = attr.parse_meta()?;
+
+        if matches!(name, "help" | "note") && matches!(meta, Meta::Path(_) | Meta::NameValue(_)) {
+            let diag = &self.diag;
+            let slug = match &self.slug {
+                Some((slug, _)) => slug.as_str(),
+                None => throw_span_err!(
+                    span,
+                    &format!(
+                        "`#[{}{}]` must come after `#[error(..)]` or `#[warn(..)]`",
+                        name,
+                        match meta {
+                            Meta::Path(_) => "",
+                            Meta::NameValue(_) => " = ...",
+                            _ => unreachable!(),
+                        }
+                    )
+                ),
+            };
+            let id = match meta {
+                Meta::Path(..) => quote! { #name },
+                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                    quote! { #s }
+                }
+                _ => unreachable!(),
+            };
+            let fn_name = proc_macro2::Ident::new(name, attr.span());
+
+            return Ok(quote! {
+                #diag.#fn_name(rustc_errors::DiagnosticMessage::fluent_attr(#slug, #id));
+            });
+        }
+
+        let nested = match meta {
+            Meta::List(MetaList { nested, .. }) => nested,
+            Meta::Path(..) => throw_span_err!(
+                span,
+                &format!("`#[{}]` is not a valid `SessionDiagnostic` struct attribute", name)
+            ),
+            Meta::NameValue(..) => throw_span_err!(
+                span,
+                &format!("`#[{} = ...]` is not a valid `SessionDiagnostic` struct attribute", name)
+            ),
+        };
+
+        let kind = match name {
+            "error" => SessionDiagnosticKind::Error,
+            "warning" => SessionDiagnosticKind::Warn,
+            other => throw_span_err!(
+                span,
+                &format!("`#[{}(...)]` is not a valid `SessionDiagnostic` struct attribute", other)
+            ),
+        };
+        self.set_kind_once(kind, span)?;
+
+        let mut tokens = Vec::new();
+        for attr in nested {
+            let span = attr.span().unwrap();
+            let meta = match attr {
+                syn::NestedMeta::Meta(meta) => meta,
+                syn::NestedMeta::Lit(_) => throw_span_err!(
+                    span,
+                    &format!(
+                        "`#[{}(\"...\")]` is not a valid `SessionDiagnostic` struct attribute",
+                        name
+                    )
+                ),
+            };
+
+            let path = meta.path();
+            let nested_name = path.segments.last().unwrap().ident.to_string();
+            match &meta {
+                // Struct attributes are only allowed to be applied once, and the diagnostic
+                // changes will be set in the initialisation code.
+                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                    match nested_name.as_str() {
+                        "slug" => {
+                            self.set_slug_once(s.value(), s.span().unwrap());
+                        }
+                        "code" => {
+                            tokens.push(self.set_code_once(s.value(), s.span().unwrap()));
+                        }
+                        other => {
+                            let diag = span_err(
+                                span,
+                                &format!(
+                                    "`#[{}({} = ...)]` is not a valid `SessionDiagnostic` struct attribute",
+                                    name, other
+                                ),
+                            );
+                            diag.emit();
+                        }
+                    }
+                }
+                Meta::NameValue(..) => {
+                    span_err(
+                        span,
+                        &format!(
+                            "`#[{}({} = ...)]` is not a valid `SessionDiagnostic` struct attribute",
+                            name, nested_name
+                        ),
+                    )
+                    .help("value must be a string")
+                    .emit();
+                }
+                Meta::Path(..) => {
+                    span_err(
+                        span,
+                        &format!(
+                            "`#[{}({})]` is not a valid `SessionDiagnostic` struct attribute",
+                            name, nested_name
+                        ),
+                    )
+                    .emit();
+                }
+                Meta::List(..) => {
+                    span_err(
+                        span,
+                        &format!(
+                            "`#[{}({}(...))]` is not a valid `SessionDiagnostic` struct attribute",
+                            name, nested_name
+                        ),
+                    )
+                    .emit();
+                }
+            }
+        }
+
+        Ok(tokens.drain(..).collect())
+    }
+
+    #[must_use]
+    fn set_kind_once(
+        &mut self,
+        kind: SessionDiagnosticKind,
+        span: proc_macro::Span,
+    ) -> Result<(), SessionDiagnosticDeriveError> {
+        match self.kind {
+            None => {
+                self.kind = Some((kind, span));
+                Ok(())
+            }
+            Some((prev_kind, prev_span)) => {
+                let existing = prev_kind.descr();
+                let current = kind.descr();
+
+                let msg = if current == existing {
+                    format!("`{}` specified multiple times", existing)
+                } else {
+                    format!("`{}` specified when `{}` was already specified", current, existing)
+                };
+                throw_span_err!(span, &msg, |diag| diag
+                    .span_note(prev_span, "previously specified here"));
+            }
+        }
+    }
+
+    fn set_code_once(&mut self, code: String, span: proc_macro::Span) -> TokenStream {
+        match self.code {
+            None => {
+                self.code = Some(span);
+            }
+            Some(prev_span) => {
+                span_err(span, "`code` specified multiple times")
+                    .span_note(prev_span, "previously specified here")
+                    .emit();
+            }
+        }
+
+        let diag = &self.diag;
+        quote! { #diag.code(rustc_errors::DiagnosticId::Error(#code.to_string())); }
+    }
+
+    fn set_slug_once(&mut self, slug: String, span: proc_macro::Span) {
+        match self.slug {
+            None => {
+                self.slug = Some((slug, span));
+            }
+            Some((_, prev_span)) => {
+                span_err(span, "`slug` specified multiple times")
+                    .span_note(prev_span, "previously specified here")
+                    .emit();
+            }
+        }
+    }
+
+    fn generate_field_attr_code(
+        &mut self,
+        attr: &syn::Attribute,
+        info: FieldInfo<'_>,
+    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+        let field_binding = &info.binding.binding;
+        let option_ty = option_inner_ty(&info.ty);
+        let generated_code = self.generate_non_option_field_code(
+            attr,
+            FieldInfo {
+                vis: info.vis,
+                binding: info.binding,
+                ty: option_ty.unwrap_or(&info.ty),
+                span: info.span,
+            },
+        )?;
+
+        if option_ty.is_none() {
+            Ok(quote! { #generated_code })
+        } else {
+            Ok(quote! {
+                if let Some(#field_binding) = #field_binding {
+                    #generated_code
+                }
+            })
+        }
+    }
+
+    fn generate_non_option_field_code(
+        &mut self,
+        attr: &Attribute,
+        info: FieldInfo<'_>,
+    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+        let diag = &self.diag;
+        let span = attr.span().unwrap();
+        let field_binding = &info.binding.binding;
+
+        let name = attr.path.segments.last().unwrap().ident.to_string();
+        let name = name.as_str();
+
+        let meta = attr.parse_meta()?;
+        match meta {
+            Meta::Path(_) => match name {
+                "skip_arg" => {
+                    // Don't need to do anything - by virtue of the attribute existing, the
+                    // `set_arg` call will not be generated.
+                    Ok(quote! {})
+                }
+                "primary_span" => {
+                    report_error_if_not_applied_to_span(attr, &info)?;
+                    Ok(quote! {
+                        #diag.set_span(*#field_binding);
+                    })
+                }
+                "label" | "note" | "help" => {
+                    report_error_if_not_applied_to_span(attr, &info)?;
+                    Ok(self.add_subdiagnostic(field_binding, name, name))
+                }
+                other => throw_span_err!(
+                    span,
+                    &format!("`#[{}]` is not a valid `SessionDiagnostic` field attribute", other)
+                ),
+            },
+            Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => match name {
+                "label" | "note" | "help" => {
+                    report_error_if_not_applied_to_span(attr, &info)?;
+                    Ok(self.add_subdiagnostic(field_binding, name, &s.value()))
+                }
+                other => throw_span_err!(
+                    span,
+                    &format!(
+                        "`#[{} = ...]` is not a valid `SessionDiagnostic` field attribute",
+                        other
+                    )
+                ),
+            },
+            Meta::NameValue(_) => throw_span_err!(
+                span,
+                &format!("`#[{} = ...]` is not a valid `SessionDiagnostic` field attribute", name),
+                |diag| diag.help("value must be a string")
+            ),
+            Meta::List(MetaList { path, nested, .. }) => {
+                let name = path.segments.last().unwrap().ident.to_string();
+                let name = name.as_ref();
+
+                match name {
+                    "suggestion" | "suggestion_short" | "suggestion_hidden"
+                    | "suggestion_verbose" => (),
+                    other => throw_span_err!(
+                        span,
+                        &format!(
+                            "`#[{}(...)]` is not a valid `SessionDiagnostic` field attribute",
+                            other
+                        )
+                    ),
+                };
+
+                let (span_, applicability) = self.span_and_applicability_of_ty(info)?;
+
+                let mut msg = None;
+                let mut code = None;
+
+                for attr in nested {
+                    let meta = match attr {
+                        syn::NestedMeta::Meta(meta) => meta,
+                        syn::NestedMeta::Lit(_) => throw_span_err!(
+                            span,
+                            &format!(
+                                "`#[{}(\"...\")]` is not a valid `SessionDiagnostic` field attribute",
+                                name
+                            )
+                        ),
+                    };
+
+                    let span = meta.span().unwrap();
+                    let nested_name = meta.path().segments.last().unwrap().ident.to_string();
+                    let nested_name = nested_name.as_str();
+
+                    match meta {
+                        Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                            match nested_name {
+                                "message" => {
+                                    msg = Some(s.value());
+                                }
+                                "code" => {
+                                    let formatted_str = self.build_format(&s.value(), s.span());
+                                    code = Some(formatted_str);
+                                }
+                                other => throw_span_err!(
+                                    span,
+                                    &format!(
+                                        "`#[{}({} = ...)]` is not a valid `SessionDiagnostic` field attribute",
+                                        name, other
+                                    )
+                                ),
+                            }
+                        }
+                        Meta::NameValue(..) => throw_span_err!(
+                            span,
+                            &format!(
+                                "`#[{}({} = ...)]` is not a valid `SessionDiagnostic` struct attribute",
+                                name, nested_name
+                            ),
+                            |diag| diag.help("value must be a string")
+                        ),
+                        Meta::Path(..) => throw_span_err!(
+                            span,
+                            &format!(
+                                "`#[{}({})]` is not a valid `SessionDiagnostic` struct attribute",
+                                name, nested_name
+                            )
+                        ),
+                        Meta::List(..) => throw_span_err!(
+                            span,
+                            &format!(
+                                "`#[{}({}(...))]` is not a valid `SessionDiagnostic` struct attribute",
+                                name, nested_name
+                            )
+                        ),
+                    }
+                }
+
+                let method = format_ident!("span_{}", name);
+
+                let slug = self
+                    .slug
+                    .as_ref()
+                    .map(|(slug, _)| slug.as_str())
+                    .unwrap_or_else(|| "missing-slug");
+                let msg = msg.as_deref().unwrap_or("suggestion");
+                let msg = quote! { rustc_errors::DiagnosticMessage::fluent_attr(#slug, #msg) };
+                let code = code.unwrap_or_else(|| quote! { String::new() });
+
+                Ok(quote! { #diag.#method(#span_, #msg, #code, #applicability); })
+            }
+        }
+    }
+
+    /// Adds a subdiagnostic by generating a `diag.span_$kind` call with the current slug and
+    /// `fluent_attr_identifier`.
+    fn add_subdiagnostic(
+        &self,
+        field_binding: &proc_macro2::Ident,
+        kind: &str,
+        fluent_attr_identifier: &str,
+    ) -> TokenStream {
+        let diag = &self.diag;
+
+        let slug =
+            self.slug.as_ref().map(|(slug, _)| slug.as_str()).unwrap_or_else(|| "missing-slug");
+        let fn_name = format_ident!("span_{}", kind);
+        quote! {
+            #diag.#fn_name(
+                *#field_binding,
+                rustc_errors::DiagnosticMessage::fluent_attr(#slug, #fluent_attr_identifier)
+            );
+        }
+    }
+
+    fn span_and_applicability_of_ty(
+        &self,
+        info: FieldInfo<'_>,
+    ) -> Result<(TokenStream, TokenStream), SessionDiagnosticDeriveError> {
+        match &info.ty {
+            // If `ty` is `Span` w/out applicability, then use `Applicability::Unspecified`.
+            ty @ Type::Path(..) if type_matches_path(ty, &["rustc_span", "Span"]) => {
+                let binding = &info.binding.binding;
+                Ok((quote!(*#binding), quote!(rustc_errors::Applicability::Unspecified)))
+            }
+            // If `ty` is `(Span, Applicability)` then return tokens accessing those.
+            Type::Tuple(tup) => {
+                let mut span_idx = None;
+                let mut applicability_idx = None;
+
+                for (idx, elem) in tup.elems.iter().enumerate() {
+                    if type_matches_path(elem, &["rustc_span", "Span"]) {
+                        if span_idx.is_none() {
+                            span_idx = Some(syn::Index::from(idx));
+                        } else {
+                            throw_span_err!(
+                                info.span.unwrap(),
+                                "type of field annotated with `#[suggestion(...)]` contains more than one `Span`"
+                            );
+                        }
+                    } else if type_matches_path(elem, &["rustc_errors", "Applicability"]) {
+                        if applicability_idx.is_none() {
+                            applicability_idx = Some(syn::Index::from(idx));
+                        } else {
+                            throw_span_err!(
+                                info.span.unwrap(),
+                                "type of field annotated with `#[suggestion(...)]` contains more than one Applicability"
+                            );
+                        }
+                    }
+                }
+
+                if let Some(span_idx) = span_idx {
+                    let binding = &info.binding.binding;
+                    let span = quote!(#binding.#span_idx);
+                    let applicability = applicability_idx
+                        .map(|applicability_idx| quote!(#binding.#applicability_idx))
+                        .unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
+
+                    return Ok((span, applicability));
+                }
+
+                throw_span_err!(info.span.unwrap(), "wrong types for suggestion", |diag| {
+                    diag.help("`#[suggestion(...)]` on a tuple field must be applied to fields of type `(Span, Applicability)`")
+                });
+            }
+            // If `ty` isn't a `Span` or `(Span, Applicability)` then emit an error.
+            _ => throw_span_err!(info.span.unwrap(), "wrong field type for suggestion", |diag| {
+                diag.help("`#[suggestion(...)]` should be applied to fields of type `Span` or `(Span, Applicability)`")
+            }),
+        }
+    }
+}

compiler/rustc_macros/src/diagnostics/error.rs

@@ -0,0 +1,60 @@
+use proc_macro::{Diagnostic, Level, MultiSpan};
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn;
+
+#[derive(Debug)]
+pub(crate) enum SessionDiagnosticDeriveError {
+    SynError(syn::Error),
+    ErrorHandled,
+}
+
+impl SessionDiagnosticDeriveError {
+    pub(crate) fn to_compile_error(self) -> TokenStream {
+        match self {
+            SessionDiagnosticDeriveError::SynError(e) => e.to_compile_error(),
+            SessionDiagnosticDeriveError::ErrorHandled => {
+                // Return ! to avoid having to create a blank DiagnosticBuilder to return when an
+                // error has already been emitted to the compiler.
+                quote! {
+                    { unreachable!(); }
+                }
+            }
+        }
+    }
+}
+
+pub(crate) fn span_err(span: impl MultiSpan, msg: &str) -> Diagnostic {
+    Diagnostic::spanned(span, Level::Error, msg)
+}
+
+/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+///
+/// Emit a diagnostic on span `$span` with msg `$msg` (optionally performing additional decoration
+/// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+macro_rules! throw_span_err {
+    ($span:expr, $msg:expr) => {{ throw_span_err!($span, $msg, |diag| diag) }};
+    ($span:expr, $msg:expr, $f:expr) => {{
+        return Err(crate::diagnostics::error::_throw_span_err($span, $msg, $f));
+    }};
+}
+
+pub(crate) use throw_span_err;
+
+/// When possible, prefer using `throw_span_err!` over using this function directly. This only
+/// exists as a function to constrain `f` to an `impl FnOnce`.
+pub(crate) fn _throw_span_err(
+    span: impl MultiSpan,
+    msg: &str,
+    f: impl FnOnce(Diagnostic) -> Diagnostic,
+) -> SessionDiagnosticDeriveError {
+    let diag = span_err(span, msg);
+    f(diag).emit();
+    SessionDiagnosticDeriveError::ErrorHandled
+}
+
+impl From<syn::Error> for SessionDiagnosticDeriveError {
+    fn from(e: syn::Error) -> Self {
+        SessionDiagnosticDeriveError::SynError(e)
+    }
+}

compiler/rustc_macros/src/diagnostics/mod.rs

@@ -0,0 +1,114 @@
+mod diagnostic;
+mod error;
+mod subdiagnostic;
+mod utils;
+
+use diagnostic::SessionDiagnosticDerive;
+use proc_macro2::TokenStream;
+use quote::format_ident;
+use subdiagnostic::SessionSubdiagnosticDerive;
+use synstructure::Structure;
+
+/// Implements `#[derive(SessionDiagnostic)]`, which allows for errors to be specified as a struct,
+/// independent from the actual diagnostics emitting code.
+///
+/// ```ignore (pseudo-rust)
+/// # extern crate rustc_errors;
+/// # use rustc_errors::Applicability;
+/// # extern crate rustc_span;
+/// # use rustc_span::{symbol::Ident, Span};
+/// # extern crate rust_middle;
+/// # use rustc_middle::ty::Ty;
+/// #[derive(SessionDiagnostic)]
+/// #[error(code = "E0505", slug = "borrowck-move-out-of-borrow")]
+/// pub struct MoveOutOfBorrowError<'tcx> {
+///     pub name: Ident,
+///     pub ty: Ty<'tcx>,
+///     #[primary_span]
+///     #[label]
+///     pub span: Span,
+///     #[label = "first-borrow-label"]
+///     pub first_borrow_span: Span,
+///     #[suggestion(code = "{name}.clone()")]
+///     pub clone_sugg: Option<(Span, Applicability)>
+/// }
+/// ```
+///
+/// ```fluent
+/// move-out-of-borrow = cannot move out of {$name} because it is borrowed
+///     .label = cannot move out of borrow
+///     .first-borrow-label = `{$ty}` first borrowed here
+///     .suggestion = consider cloning here
+/// ```
+///
+/// Then, later, to emit the error:
+///
+/// ```ignore (pseudo-rust)
+/// sess.emit_err(MoveOutOfBorrowError {
+///     expected,
+///     actual,
+///     span,
+///     first_borrow_span,
+///     clone_sugg: Some(suggestion, Applicability::MachineApplicable),
+/// });
+/// ```
+///
+/// See rustc dev guide for more examples on using the `#[derive(SessionDiagnostic)]`:
+/// <https://rustc-dev-guide.rust-lang.org/diagnostics/sessiondiagnostic.html>
+pub fn session_diagnostic_derive(s: Structure<'_>) -> TokenStream {
+    // Names for the diagnostic we build and the session we build it from.
+    let diag = format_ident!("diag");
+    let sess = format_ident!("sess");
+
+    SessionDiagnosticDerive::new(diag, sess, s).into_tokens()
+}
+
+/// Implements `#[derive(SessionSubdiagnostic)]`, which allows for labels, notes, helps and
+/// suggestions to be specified as a structs or enums, independent from the actual diagnostics
+/// emitting code or diagnostic derives.
+///
+/// ```ignore (pseudo-rust)
+/// #[derive(SessionSubdiagnostic)]
+/// pub enum ExpectedIdentifierLabel<'tcx> {
+///     #[label(slug = "parser-expected-identifier")]
+///     WithoutFound {
+///         #[primary_span]
+///         span: Span,
+///     }
+///     #[label(slug = "parser-expected-identifier-found")]
+///     WithFound {
+///         #[primary_span]
+///         span: Span,
+///         found: String,
+///     }
+/// }
+///
+/// #[derive(SessionSubdiagnostic)]
+/// #[suggestion_verbose(slug = "parser-raw-identifier")]
+/// pub struct RawIdentifierSuggestion<'tcx> {
+///     #[primary_span]
+///     span: Span,
+///     #[applicability]
+///     applicability: Applicability,
+///     ident: Ident,
+/// }
+/// ```
+///
+/// ```fluent
+/// parser-expected-identifier = expected identifier
+///
+/// parser-expected-identifier-found = expected identifier, found {$found}
+///
+/// parser-raw-identifier = escape `{$ident}` to use it as an identifier
+/// ```
+///
+/// Then, later, to add the subdiagnostic:
+///
+/// ```ignore (pseudo-rust)
+/// diag.subdiagnostic(ExpectedIdentifierLabel::WithoutFound { span });
+///
+/// diag.subdiagnostic(RawIdentifierSuggestion { span, applicability, ident });
+/// ```
+pub fn session_subdiagnostic_derive(s: Structure<'_>) -> TokenStream {
+    SessionSubdiagnosticDerive::new(s).into_tokens()
+}

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -0,0 +1,530 @@
+#![deny(unused_must_use)]
+
+use crate::diagnostics::error::{span_err, throw_span_err, SessionDiagnosticDeriveError};
+use crate::diagnostics::utils::{
+    option_inner_ty, report_error_if_not_applied_to_applicability,
+    report_error_if_not_applied_to_span, FieldInfo, HasFieldMap, SetOnce,
+};
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use std::collections::HashMap;
+use std::fmt;
+use std::str::FromStr;
+use syn::{spanned::Spanned, Meta, MetaList, MetaNameValue};
+use synstructure::{BindingInfo, Structure, VariantInfo};
+
+enum Applicability {
+    MachineApplicable,
+    MaybeIncorrect,
+    HasPlaceholders,
+    Unspecified,
+}
+
+impl FromStr for Applicability {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "machine-applicable" => Ok(Applicability::MachineApplicable),
+            "maybe-incorrect" => Ok(Applicability::MaybeIncorrect),
+            "has-placeholders" => Ok(Applicability::HasPlaceholders),
+            "unspecified" => Ok(Applicability::Unspecified),
+            _ => Err(()),
+        }
+    }
+}
+
+impl quote::ToTokens for Applicability {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.extend(match self {
+            Applicability::MachineApplicable => {
+                quote! { rustc_errors::Applicability::MachineApplicable }
+            }
+            Applicability::MaybeIncorrect => {
+                quote! { rustc_errors::Applicability::MaybeIncorrect }
+            }
+            Applicability::HasPlaceholders => {
+                quote! { rustc_errors::Applicability::HasPlaceholders }
+            }
+            Applicability::Unspecified => {
+                quote! { rustc_errors::Applicability::Unspecified }
+            }
+        });
+    }
+}
+
+#[derive(Clone, Copy)]
+enum SubdiagnosticSuggestionKind {
+    /// `#[suggestion]`
+    Normal,
+    /// `#[suggestion_short]`
+    Short,
+    /// `#[suggestion_hidden]`
+    Hidden,
+    /// `#[suggestion_verbose]`
+    Verbose,
+}
+
+#[derive(Clone, Copy)]
+enum SubdiagnosticKind {
+    /// `#[label(...)]`
+    Label,
+    /// `#[note(...)]`
+    Note,
+    /// `#[help(...)]`
+    Help,
+    /// `#[suggestion{,_short,_hidden,_verbose}]`
+    Suggestion(SubdiagnosticSuggestionKind),
+}
+
+impl FromStr for SubdiagnosticKind {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "label" => Ok(SubdiagnosticKind::Label),
+            "note" => Ok(SubdiagnosticKind::Note),
+            "help" => Ok(SubdiagnosticKind::Help),
+            "suggestion" => Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Normal)),
+            "suggestion_short" => {
+                Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Short))
+            }
+            "suggestion_hidden" => {
+                Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Hidden))
+            }
+            "suggestion_verbose" => {
+                Ok(SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Verbose))
+            }
+            _ => Err(()),
+        }
+    }
+}
+
+impl quote::IdentFragment for SubdiagnosticKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            SubdiagnosticKind::Label => write!(f, "label"),
+            SubdiagnosticKind::Note => write!(f, "note"),
+            SubdiagnosticKind::Help => write!(f, "help"),
+            SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Normal) => {
+                write!(f, "suggestion")
+            }
+            SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Short) => {
+                write!(f, "suggestion_short")
+            }
+            SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Hidden) => {
+                write!(f, "suggestion_hidden")
+            }
+            SubdiagnosticKind::Suggestion(SubdiagnosticSuggestionKind::Verbose) => {
+                write!(f, "suggestion_verbose")
+            }
+        }
+    }
+
+    fn span(&self) -> Option<proc_macro2::Span> {
+        None
+    }
+}
+
+pub(crate) struct SessionSubdiagnosticDerive<'a> {
+    structure: Structure<'a>,
+    diag: syn::Ident,
+}
+
+impl<'a> SessionSubdiagnosticDerive<'a> {
+    pub(crate) fn new(structure: Structure<'a>) -> Self {
+        let diag = format_ident!("diag");
+        Self { structure, diag }
+    }
+
+    pub(crate) fn into_tokens(self) -> TokenStream {
+        let SessionSubdiagnosticDerive { mut structure, diag } = self;
+        let implementation = {
+            let ast = structure.ast();
+            let span = ast.span().unwrap();
+            match ast.data {
+                syn::Data::Struct(..) | syn::Data::Enum(..) => (),
+                syn::Data::Union(..) => {
+                    span_err(
+                        span,
+                        "`#[derive(SessionSubdiagnostic)]` can only be used on structs and enums",
+                    );
+                }
+            }
+
+            if matches!(ast.data, syn::Data::Enum(..)) {
+                for attr in &ast.attrs {
+                    span_err(
+                        attr.span().unwrap(),
+                        "unsupported type attribute for subdiagnostic enum",
+                    )
+                    .emit();
+                }
+            }
+
+            structure.bind_with(|_| synstructure::BindStyle::Move);
+            let variants_ = structure.each_variant(|variant| {
+                // Build the mapping of field names to fields. This allows attributes to peek
+                // values from other fields.
+                let mut fields_map = HashMap::new();
+                for binding in variant.bindings() {
+                    let field = binding.ast();
+                    if let Some(ident) = &field.ident {
+                        fields_map.insert(ident.to_string(), quote! { #binding });
+                    }
+                }
+
+                let mut builder = SessionSubdiagnosticDeriveBuilder {
+                    diag: &diag,
+                    variant,
+                    span,
+                    fields: fields_map,
+                    kind: None,
+                    slug: None,
+                    code: None,
+                    span_field: None,
+                    applicability: None,
+                };
+                builder.into_tokens().unwrap_or_else(|v| v.to_compile_error())
+            });
+
+            quote! {
+                match self {
+                    #variants_
+                }
+            }
+        };
+
+        let ret = structure.gen_impl(quote! {
+            gen impl rustc_errors::AddSubdiagnostic for @Self {
+                fn add_to_diagnostic(self, #diag: &mut rustc_errors::Diagnostic) {
+                    use rustc_errors::{Applicability, IntoDiagnosticArg};
+                    #implementation
+                }
+            }
+        });
+        ret
+    }
+}
+
+struct SessionSubdiagnosticDeriveBuilder<'a> {
+    /// The identifier to use for the generated `DiagnosticBuilder` instance.
+    diag: &'a syn::Ident,
+
+    /// Info for the current variant (or the type if not an enum).
+    variant: &'a VariantInfo<'a>,
+    /// Span for the entire type.
+    span: proc_macro::Span,
+
+    /// Store a map of field name to its corresponding field. This is built on construction of the
+    /// derive builder.
+    fields: HashMap<String, TokenStream>,
+
+    /// Subdiagnostic kind of the type/variant.
+    kind: Option<(SubdiagnosticKind, proc_macro::Span)>,
+
+    /// Slug of the subdiagnostic - corresponds to the Fluent identifier for the message - from the
+    /// `#[kind(slug = "...")]` attribute on the type or variant.
+    slug: Option<(String, proc_macro::Span)>,
+    /// If a suggestion, the code to suggest as a replacement - from the `#[kind(code = "...")]`
+    /// attribute on the type or variant.
+    code: Option<(TokenStream, proc_macro::Span)>,
+
+    /// Identifier for the binding to the `#[primary_span]` field.
+    span_field: Option<(proc_macro2::Ident, proc_macro::Span)>,
+    /// If a suggestion, the identifier for the binding to the `#[applicability]` field or a
+    /// `rustc_errors::Applicability::*` variant directly.
+    applicability: Option<(TokenStream, proc_macro::Span)>,
+}
+
+impl<'a> HasFieldMap for SessionSubdiagnosticDeriveBuilder<'a> {
+    fn get_field_binding(&self, field: &String) -> Option<&TokenStream> {
+        self.fields.get(field)
+    }
+}
+
+impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
+    fn identify_kind(&mut self) -> Result<(), SessionDiagnosticDeriveError> {
+        for attr in self.variant.ast().attrs {
+            let span = attr.span().unwrap();
+
+            let name = attr.path.segments.last().unwrap().ident.to_string();
+            let name = name.as_str();
+
+            let meta = attr.parse_meta()?;
+            let kind = match meta {
+                Meta::Path(_) => throw_span_err!(
+                    span,
+                    &format!("`#[{}]` is not a valid `SessionSubdiagnostic` attribute", name)
+                ),
+                Meta::NameValue(_) => throw_span_err!(
+                    span,
+                    &format!("`#[{} = ...]` is not a valid `SessionSubdiagnostic` attribute", name)
+                ),
+                Meta::List(MetaList { nested, .. }) => {
+                    for attr in nested {
+                        let meta = match attr {
+                            syn::NestedMeta::Meta(meta) => meta,
+                            syn::NestedMeta::Lit(_) => throw_span_err!(
+                                span,
+                                &format!(
+                                    "`#[{}(\"...\")]` is not a valid `SessionSubdiagnostic` attribute",
+                                    name
+                                )
+                            ),
+                        };
+
+                        let span = meta.span().unwrap();
+                        let nested_name = meta.path().segments.last().unwrap().ident.to_string();
+                        let nested_name = nested_name.as_str();
+
+                        match meta {
+                            Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                                match nested_name {
+                                    "code" => {
+                                        let formatted_str = self.build_format(&s.value(), s.span());
+                                        self.code.set_once((formatted_str, span));
+                                    }
+                                    "slug" => self.slug.set_once((s.value(), span)),
+                                    "applicability" => {
+                                        let value = match Applicability::from_str(&s.value()) {
+                                            Ok(v) => v,
+                                            Err(()) => {
+                                                span_err(span, "invalid applicability").emit();
+                                                Applicability::Unspecified
+                                            }
+                                        };
+                                        self.applicability.set_once((quote! { #value }, span));
+                                    }
+                                    other => throw_span_err!(
+                                        span,
+                                        &format!(
+                                            "`#[{}({} = ...)]` is not a valid `SessionSubdiagnostic` attribute",
+                                            name, other
+                                        )
+                                    ),
+                                }
+                            }
+                            Meta::NameValue(..) => throw_span_err!(
+                                span,
+                                &format!(
+                                    "`#[{}({} = ...)]` is not a valid `SessionSubdiagnostic` attribute",
+                                    name, nested_name
+                                ),
+                                |diag| diag.help("value must be a string")
+                            ),
+                            Meta::Path(..) => throw_span_err!(
+                                span,
+                                &format!(
+                                    "`#[{}({})]` is not a valid `SessionSubdiagnostic` attribute",
+                                    name, nested_name
+                                )
+                            ),
+                            Meta::List(..) => throw_span_err!(
+                                span,
+                                &format!(
+                                    "`#[{}({}(...))]` is not a valid `SessionSubdiagnostic` attribute",
+                                    name, nested_name
+                                )
+                            ),
+                        }
+                    }
+
+                    let Ok(kind) = SubdiagnosticKind::from_str(name) else {
+                        throw_span_err!(
+                            span,
+                            &format!(
+                                "`#[{}(...)]` is not a valid `SessionSubdiagnostic` attribute",
+                                name
+                            )
+                        );
+                    };
+                    kind
+                }
+            };
+
+            if matches!(
+                kind,
+                SubdiagnosticKind::Label | SubdiagnosticKind::Help | SubdiagnosticKind::Note
+            ) && self.code.is_some()
+            {
+                throw_span_err!(
+                    span,
+                    &format!("`code` is not a valid nested attribute of a `{}` attribute", name)
+                );
+            }
+
+            if self.slug.is_none() {
+                throw_span_err!(
+                    span,
+                    &format!("`slug` must be set in a `#[{}(...)]` attribute", name)
+                );
+            }
+
+            self.kind.set_once((kind, span));
+        }
+
+        Ok(())
+    }
+
+    fn generate_field_code(
+        &mut self,
+        binding: &BindingInfo<'_>,
+        is_suggestion: bool,
+    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+        let ast = binding.ast();
+
+        let option_ty = option_inner_ty(&ast.ty);
+        let info = FieldInfo {
+            vis: &ast.vis,
+            binding: binding,
+            ty: option_ty.unwrap_or(&ast.ty),
+            span: &ast.span(),
+        };
+
+        for attr in &ast.attrs {
+            let name = attr.path.segments.last().unwrap().ident.to_string();
+            let name = name.as_str();
+            let span = attr.span().unwrap();
+
+            let meta = attr.parse_meta()?;
+            match meta {
+                Meta::Path(_) => match name {
+                    "primary_span" => {
+                        report_error_if_not_applied_to_span(attr, &info)?;
+                        self.span_field.set_once((binding.binding.clone(), span));
+                        return Ok(quote! {});
+                    }
+                    "applicability" if is_suggestion => {
+                        report_error_if_not_applied_to_applicability(attr, &info)?;
+                        let binding = binding.binding.clone();
+                        self.applicability.set_once((quote! { #binding }, span));
+                        return Ok(quote! {});
+                    }
+                    "applicability" => {
+                        span_err(span, "`#[applicability]` is only valid on suggestions").emit();
+                        return Ok(quote! {});
+                    }
+                    "skip_arg" => {
+                        return Ok(quote! {});
+                    }
+                    other => span_err(
+                        span,
+                        &format!(
+                            "`#[{}]` is not a valid `SessionSubdiagnostic` field attribute",
+                            other
+                        ),
+                    )
+                    .emit(),
+                },
+                Meta::NameValue(_) => span_err(
+                    span,
+                    &format!(
+                        "`#[{} = ...]` is not a valid `SessionSubdiagnostic` field attribute",
+                        name
+                    ),
+                )
+                .emit(),
+                Meta::List(_) => span_err(
+                    span,
+                    &format!(
+                        "`#[{}(...)]` is not a valid `SessionSubdiagnostic` field attribute",
+                        name
+                    ),
+                )
+                .emit(),
+            }
+        }
+
+        let ident = ast.ident.as_ref().unwrap();
+
+        let diag = &self.diag;
+        let generated = quote! {
+            #diag.set_arg(
+                stringify!(#ident),
+                #binding.into_diagnostic_arg()
+            );
+        };
+
+        if option_ty.is_none() {
+            Ok(quote! { #generated })
+        } else {
+            Ok(quote! {
+                if let Some(#binding) = #binding {
+                    #generated
+                }
+            })
+        }
+    }
+
+    fn into_tokens(&mut self) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+        self.identify_kind()?;
+        let Some(kind) = self.kind.map(|(kind, _)| kind) else {
+            throw_span_err!(
+                self.variant.ast().ident.span().unwrap(),
+                "subdiagnostic kind not specified"
+            );
+        };
+
+        let is_suggestion = matches!(kind, SubdiagnosticKind::Suggestion(_));
+
+        let mut args = TokenStream::new();
+        for binding in self.variant.bindings() {
+            let arg = self
+                .generate_field_code(binding, is_suggestion)
+                .unwrap_or_else(|v| v.to_compile_error());
+            args.extend(arg);
+        }
+
+        // Missing slug errors will already have been reported.
+        let slug = self.slug.as_ref().map(|(slug, _)| &**slug).unwrap_or("missing-slug");
+        let code = match self.code.as_ref() {
+            Some((code, _)) => Some(quote! { #code }),
+            None if is_suggestion => {
+                span_err(self.span, "suggestion without `code = \"...\"`").emit();
+                Some(quote! { /* macro error */ "..." })
+            }
+            None => None,
+        };
+
+        let span_field = self.span_field.as_ref().map(|(span, _)| span);
+        let applicability = match self.applicability.clone() {
+            Some((applicability, _)) => Some(applicability),
+            None if is_suggestion => {
+                span_err(self.span, "suggestion without `applicability`").emit();
+                Some(quote! { rustc_errors::Applicability::Unspecified })
+            }
+            None => None,
+        };
+
+        let diag = &self.diag;
+        let name = format_ident!("{}{}", if span_field.is_some() { "span_" } else { "" }, kind);
+        let message = quote! { rustc_errors::DiagnosticMessage::fluent(#slug) };
+        let call = if matches!(kind, SubdiagnosticKind::Suggestion(..)) {
+            if let Some(span) = span_field {
+                quote! { #diag.#name(#span, #message, #code, #applicability); }
+            } else {
+                span_err(self.span, "suggestion without `#[primary_span]` field").emit();
+                quote! { unreachable!(); }
+            }
+        } else if matches!(kind, SubdiagnosticKind::Label) {
+            if let Some(span) = span_field {
+                quote! { #diag.#name(#span, #message); }
+            } else {
+                span_err(self.span, "label without `#[primary_span]` field").emit();
+                quote! { unreachable!(); }
+            }
+        } else {
+            if let Some(span) = span_field {
+                quote! { #diag.#name(#span, #message); }
+            } else {
+                quote! { #diag.#name(#message); }
+            }
+        };
+
+        Ok(quote! {
+            #call
+            #args
+        })
+    }
+}

compiler/rustc_macros/src/diagnostics/utils.rs

@@ -0,0 +1,221 @@
+use crate::diagnostics::error::{span_err, throw_span_err, SessionDiagnosticDeriveError};
+use proc_macro::Span;
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use std::collections::BTreeSet;
+use syn::{spanned::Spanned, Attribute, Meta, Type, Visibility};
+use synstructure::BindingInfo;
+
+/// Checks whether the type name of `ty` matches `name`.
+///
+/// Given some struct at `a::b::c::Foo`, this will return true for `c::Foo`, `b::c::Foo`, or
+/// `a::b::c::Foo`. This reasonably allows qualified names to be used in the macro.
+pub(crate) fn type_matches_path(ty: &Type, name: &[&str]) -> bool {
+    if let Type::Path(ty) = ty {
+        ty.path
+            .segments
+            .iter()
+            .map(|s| s.ident.to_string())
+            .rev()
+            .zip(name.iter().rev())
+            .all(|(x, y)| &x.as_str() == y)
+    } else {
+        false
+    }
+}
+
+/// Reports an error if the field's type is not `Applicability`.
+fn report_error_if_not_applied_to_ty(
+    attr: &Attribute,
+    info: &FieldInfo<'_>,
+    path: &[&str],
+    ty_name: &str,
+) -> Result<(), SessionDiagnosticDeriveError> {
+    if !type_matches_path(&info.ty, path) {
+        let name = attr.path.segments.last().unwrap().ident.to_string();
+        let name = name.as_str();
+        let meta = attr.parse_meta()?;
+
+        throw_span_err!(
+            attr.span().unwrap(),
+            &format!(
+                "the `#[{}{}]` attribute can only be applied to fields of type `{}`",
+                name,
+                match meta {
+                    Meta::Path(_) => "",
+                    Meta::NameValue(_) => " = ...",
+                    Meta::List(_) => "(...)",
+                },
+                ty_name
+            )
+        );
+    }
+
+    Ok(())
+}
+
+/// Reports an error if the field's type is not `Applicability`.
+pub(crate) fn report_error_if_not_applied_to_applicability(
+    attr: &Attribute,
+    info: &FieldInfo<'_>,
+) -> Result<(), SessionDiagnosticDeriveError> {
+    report_error_if_not_applied_to_ty(
+        attr,
+        info,
+        &["rustc_errors", "Applicability"],
+        "Applicability",
+    )
+}
+
+/// Reports an error if the field's type is not `Span`.
+pub(crate) fn report_error_if_not_applied_to_span(
+    attr: &Attribute,
+    info: &FieldInfo<'_>,
+) -> Result<(), SessionDiagnosticDeriveError> {
+    report_error_if_not_applied_to_ty(attr, info, &["rustc_span", "Span"], "Span")
+}
+
+/// If `ty` is an Option, returns `Some(inner type)`, otherwise returns `None`.
+pub(crate) fn option_inner_ty(ty: &Type) -> Option<&Type> {
+    if type_matches_path(ty, &["std", "option", "Option"]) {
+        if let Type::Path(ty_path) = ty {
+            let path = &ty_path.path;
+            let ty = path.segments.iter().last().unwrap();
+            if let syn::PathArguments::AngleBracketed(bracketed) = &ty.arguments {
+                if bracketed.args.len() == 1 {
+                    if let syn::GenericArgument::Type(ty) = &bracketed.args[0] {
+                        return Some(ty);
+                    }
+                }
+            }
+        }
+    }
+    None
+}
+
+/// Field information passed to the builder. Deliberately omits attrs to discourage the
+/// `generate_*` methods from walking the attributes themselves.
+pub(crate) struct FieldInfo<'a> {
+    pub(crate) vis: &'a Visibility,
+    pub(crate) binding: &'a BindingInfo<'a>,
+    pub(crate) ty: &'a Type,
+    pub(crate) span: &'a proc_macro2::Span,
+}
+
+pub(crate) trait SetOnce<T> {
+    fn set_once(&mut self, value: T);
+}
+
+impl<T> SetOnce<(T, Span)> for Option<(T, Span)> {
+    fn set_once(&mut self, (value, span): (T, Span)) {
+        match self {
+            None => {
+                *self = Some((value, span));
+            }
+            Some((_, prev_span)) => {
+                span_err(span, "specified multiple times")
+                    .span_note(*prev_span, "previously specified here")
+                    .emit();
+            }
+        }
+    }
+}
+
+pub(crate) trait HasFieldMap {
+    fn get_field_binding(&self, field: &String) -> Option<&TokenStream>;
+
+    /// In the strings in the attributes supplied to this macro, we want callers to be able to
+    /// reference fields in the format string. For example:
+    ///
+    /// ```ignore (not-usage-example)
+    /// /// Suggest `==` when users wrote `===`.
+    /// #[suggestion(slug = "parser-not-javascript-eq", code = "{lhs} == {rhs}")]
+    /// struct NotJavaScriptEq {
+    ///     #[primary_span]
+    ///     span: Span,
+    ///     lhs: Ident,
+    ///     rhs: Ident,
+    /// }
+    /// ```
+    ///
+    /// We want to automatically pick up that `{lhs}` refers `self.lhs` and `{rhs}` refers to
+    /// `self.rhs`, then generate this call to `format!`:
+    ///
+    /// ```ignore (not-usage-example)
+    /// format!("{lhs} == {rhs}", lhs = self.lhs, rhs = self.rhs)
+    /// ```
+    ///
+    /// This function builds the entire call to `format!`.
+    fn build_format(&self, input: &str, span: proc_macro2::Span) -> TokenStream {
+        // This set is used later to generate the final format string. To keep builds reproducible,
+        // the iteration order needs to be deterministic, hence why we use a `BTreeSet` here
+        // instead of a `HashSet`.
+        let mut referenced_fields: BTreeSet<String> = BTreeSet::new();
+
+        // At this point, we can start parsing the format string.
+        let mut it = input.chars().peekable();
+
+        // Once the start of a format string has been found, process the format string and spit out
+        // the referenced fields. Leaves `it` sitting on the closing brace of the format string, so
+        // the next call to `it.next()` retrieves the next character.
+        while let Some(c) = it.next() {
+            if c == '{' && *it.peek().unwrap_or(&'\0') != '{' {
+                let mut eat_argument = || -> Option<String> {
+                    let mut result = String::new();
+                    // Format specifiers look like:
+                    //
+                    //   format   := '{' [ argument ] [ ':' format_spec ] '}' .
+                    //
+                    // Therefore, we only need to eat until ':' or '}' to find the argument.
+                    while let Some(c) = it.next() {
+                        result.push(c);
+                        let next = *it.peek().unwrap_or(&'\0');
+                        if next == '}' {
+                            break;
+                        } else if next == ':' {
+                            // Eat the ':' character.
+                            assert_eq!(it.next().unwrap(), ':');
+                            break;
+                        }
+                    }
+                    // Eat until (and including) the matching '}'
+                    while it.next()? != '}' {
+                        continue;
+                    }
+                    Some(result)
+                };
+
+                if let Some(referenced_field) = eat_argument() {
+                    referenced_fields.insert(referenced_field);
+                }
+            }
+        }
+
+        // At this point, `referenced_fields` contains a set of the unique fields that were
+        // referenced in the format string. Generate the corresponding "x = self.x" format
+        // string parameters:
+        let args = referenced_fields.into_iter().map(|field: String| {
+            let field_ident = format_ident!("{}", field);
+            let value = match self.get_field_binding(&field) {
+                Some(value) => value.clone(),
+                // This field doesn't exist. Emit a diagnostic.
+                None => {
+                    span_err(
+                        span.unwrap(),
+                        &format!("`{}` doesn't refer to a field on this type", field),
+                    )
+                    .emit();
+                    quote! {
+                        "{#field}"
+                    }
+                }
+            };
+            quote! {
+                #field_ident = #value
+            }
+        });
+        quote! {
+            format!(#input #(,#args)*)
+        }
+    }
+}

compiler/rustc_macros/src/lib.rs

@@ -8,12 +8,12 @@ use synstructure::decl_derive;
 
 use proc_macro::TokenStream;
 
+mod diagnostics;
 mod hash_stable;
 mod lift;
 mod newtype;
 mod query;
 mod serialize;
-mod session_diagnostic;
 mod symbols;
 mod type_foldable;
 
@@ -76,7 +76,7 @@ decl_derive!(
         suggestion,
         suggestion_short,
         suggestion_hidden,
-        suggestion_verbose)] => session_diagnostic::session_diagnostic_derive
+        suggestion_verbose)] => diagnostics::session_diagnostic_derive
 );
 decl_derive!(
     [SessionSubdiagnostic, attributes(
@@ -91,5 +91,5 @@ decl_derive!(
         // field attributes
         skip_arg,
         primary_span,
-        applicability)] => session_diagnostic::session_subdiagnostic_derive
+        applicability)] => diagnostics::session_subdiagnostic_derive
 );