Rollup merge of #100730 - CleanCut:diagnostics-rustc_monomorphize, r=davidtwco

Migrate rustc_monomorphize to use SessionDiagnostic

### Description

- Migrates diagnostics in `rustc_monomorphize` to use `SessionDiagnostic`
- Adds an `impl IntoDiagnosticArg for PathBuf`

### TODO / Help!
- [x] I'm having trouble figuring out how to apply an optional note. 😕  Help!?
  - Resolved. It was bad docs. Fixed in https://github.com/rust-lang/rustc-dev-guide/pull/1437/files
- [x] `errors:RecursionLimit` should be `#[fatal ...]`, but that doesn't exist so it's `#[error ...]` at the moment.
  - Maybe I can switch after this is merged in? --> https://github.com/rust-lang/rust/pull/100694
  - Or maybe I need to manually implement `SessionDiagnostic` instead of deriving it?
- [x] How does one go about converting an error inside of [a call to struct_span_lint_hir](8064a49508/compiler/rustc_monomorphize/src/collector.rs (L917-L927))?
- [x] ~What placeholder do you use in the fluent template to refer to the value in a vector? It seems like [this code](0b79f758c9/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs (L83-L114)) ought to have the answer (or something near it)...but I can't figure it out.~ You can't. Punted.

compiler/rustc_hir/src/errors.rs

@@ -0,0 +1,10 @@
+use crate::LangItem;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable)]
+pub struct LangItemError(pub LangItem);
+
+impl ToString for LangItemError {
+    fn to_string(&self) -> String {
+        format!("requires `{}` lang_item", self.0.name())
+    }
+}

compiler/rustc_hir/src/lang_items.rs

@@ -8,6 +8,7 @@
 //! * Functions called by the compiler itself.
 
 use crate::def_id::DefId;
+use crate::errors::LangItemError;
 use crate::{MethodKind, Target};
 
 use rustc_ast as ast;
@@ -115,9 +116,9 @@ macro_rules! language_item_table {
 
             /// Requires that a given `LangItem` was bound and returns the corresponding `DefId`.
             /// If it wasn't bound, e.g. due to a missing `#[lang = "<it.name()>"]`,
-            /// returns an error message as a string.
-            pub fn require(&self, it: LangItem) -> Result<DefId, String> {
-                self.items[it as usize].ok_or_else(|| format!("requires `{}` lang_item", it.name()))
+            /// returns an error encapsulating the `LangItem`.
+            pub fn require(&self, it: LangItem) -> Result<DefId, LangItemError> {
+                self.items[it as usize].ok_or_else(|| LangItemError(it))
             }
 
             /// Returns the [`DefId`]s of all lang items in a group.

compiler/rustc_hir/src/lib.rs

@@ -27,6 +27,7 @@ pub mod def;
 pub mod def_path_hash_map;
 pub mod definitions;
 pub mod diagnostic_items;
+pub mod errors;
 pub use rustc_span::def_id;
 mod hir;
 pub mod hir_id;