macros: add helper functions for invalid attrs

Remove some duplicated code between both diagnostic derives by
introducing helper functions for reporting an error in case of a invalid
attribute.

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

compiler/rustc_macros/src/diagnostics/error.rs

@@ -1,11 +1,11 @@
 use proc_macro::{Diagnostic, Level, MultiSpan};
 use proc_macro2::TokenStream;
 use quote::quote;
-use syn;
+use syn::{spanned::Spanned, Attribute, Error as SynError, Meta, NestedMeta};
 
 #[derive(Debug)]
 pub(crate) enum SessionDiagnosticDeriveError {
-    SynError(syn::Error),
+    SynError(SynError),
     ErrorHandled,
 }
 
@@ -24,37 +24,109 @@ impl SessionDiagnosticDeriveError {
     }
 }
 
+impl From<SynError> for SessionDiagnosticDeriveError {
+    fn from(e: SynError) -> Self {
+        SessionDiagnosticDeriveError::SynError(e)
+    }
+}
+
+/// Helper function for use with `throw_*` macros - constraints `$f` to an `impl FnOnce`.
+pub(crate) fn _throw_err(
+    diag: Diagnostic,
+    f: impl FnOnce(Diagnostic) -> Diagnostic,
+) -> SessionDiagnosticDeriveError {
+    f(diag).emit();
+    SessionDiagnosticDeriveError::ErrorHandled
+}
+
+/// Returns an error diagnostic on span `span` with msg `msg`.
 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)`.
+///
+/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
 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, $f));
+        let diag = span_err($span, $msg);
+        return Err(crate::diagnostics::error::_throw_err(diag, $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
-}
+/// Returns an error diagnostic for an invalid attribute.
+pub(crate) fn invalid_attr(attr: &Attribute, meta: &Meta) -> Diagnostic {
+    let span = attr.span().unwrap();
+    let name = attr.path.segments.last().unwrap().ident.to_string();
+    let name = name.as_str();
 
-impl From<syn::Error> for SessionDiagnosticDeriveError {
-    fn from(e: syn::Error) -> Self {
-        SessionDiagnosticDeriveError::SynError(e)
+    match meta {
+        Meta::Path(_) => span_err(span, &format!("`#[{}]` is not a valid attribute", name)),
+        Meta::NameValue(_) => {
+            span_err(span, &format!("`#[{} = ...]` is not a valid attribute", name))
+        }
+        Meta::List(_) => span_err(span, &format!("`#[{}(...)]` is not a valid attribute", name)),
     }
 }
+
+/// Emit a error diagnostic for an invalid attribute (optionally performing additional decoration
+/// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+///
+/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+macro_rules! throw_invalid_attr {
+    ($attr:expr, $meta:expr) => {{ throw_invalid_attr!($attr, $meta, |diag| diag) }};
+    ($attr:expr, $meta:expr, $f:expr) => {{
+        let diag = crate::diagnostics::error::invalid_attr($attr, $meta);
+        return Err(crate::diagnostics::error::_throw_err(diag, $f));
+    }};
+}
+
+pub(crate) use throw_invalid_attr;
+
+/// Returns an error diagnostic for an invalid nested attribute.
+pub(crate) fn invalid_nested_attr(attr: &Attribute, nested: &NestedMeta) -> Diagnostic {
+    let name = attr.path.segments.last().unwrap().ident.to_string();
+    let name = name.as_str();
+
+    let span = nested.span().unwrap();
+    let meta = match nested {
+        syn::NestedMeta::Meta(meta) => meta,
+        syn::NestedMeta::Lit(_) => {
+            return span_err(span, &format!("`#[{}(\"...\")]` is not a valid attribute", name));
+        }
+    };
+
+    let span = meta.span().unwrap();
+    let nested_name = meta.path().segments.last().unwrap().ident.to_string();
+    let nested_name = nested_name.as_str();
+    match meta {
+        Meta::NameValue(..) => span_err(
+            span,
+            &format!("`#[{}({} = ...)]` is not a valid attribute", name, nested_name),
+        ),
+        Meta::Path(..) => {
+            span_err(span, &format!("`#[{}({})]` is not a valid attribute", name, nested_name))
+        }
+        Meta::List(..) => {
+            span_err(span, &format!("`#[{}({}(...))]` is not a valid attribute", name, nested_name))
+        }
+    }
+}
+
+/// Emit a error diagnostic for an invalid nested attribute (optionally performing additional
+/// decoration using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+///
+/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+macro_rules! throw_invalid_nested_attr {
+    ($attr:expr, $nested_attr:expr) => {{ throw_invalid_nested_attr!($attr, $nested_attr, |diag| diag) }};
+    ($attr:expr, $nested_attr:expr, $f:expr) => {{
+        let diag = crate::diagnostics::error::invalid_nested_attr($attr, $nested_attr);
+        return Err(crate::diagnostics::error::_throw_err(diag, $f));
+    }};
+}
+
+pub(crate) use throw_invalid_nested_attr;