new chapter with examples of diagnostic translation PRs

src/SUMMARY.md

@@ -149,6 +149,7 @@
     - [Error codes](./diagnostics/error-codes.md)
     - [Diagnostic items](./diagnostics/diagnostic-items.md)
     - [`ErrorGuaranteed`](./diagnostics/error-guaranteed.md)
+    - [Making diagnostics translatable](diagnostics/making-diagnostics-translatable.md)
 
 # MIR to Binaries
 

src/diagnostics/making-diagnostics-translatable.md

@@ -0,0 +1,195 @@
+# Making diagnostics translatable
+
+<!-- toc -->
+
+There is an ongoing effort to make diagnostics translatable,
+and the coordination happens [on GitHub].
+
+This is intended to be a gentle guide to help with this effort,
+with the use of examples.
+
+> A more detailed explanation can be found in [the announcement of the initiative].
+> However,
+> note that some of the important details are now outdated.
+>
+> Reference documentation can be found in these chapters:
+> - [Diagnostic structs](diagnostic-structs.md)
+> - [Translation system](translation.md)
+
+## General comments
+
+For a single diagnostic, 3 files need to be modified:
+
+- One file where the diagnostic is emitted,
+  typically by calling [Session::struct_span_err].
+- One file where the type representing the migrated diagnostic,
+  either a `struct` or an `enum`, will be added.
+  This is typically in a module named `errors` in the relevant compiler crate.
+- One file where the actual text of the diagnostic is located,
+  located in a file named `messages.ftl`,
+  at the root of the relevant rustc crate.
+
+## Simple example
+
+*This uses [issue #108727] for demonstration*
+
+Suppose you have the following code:
+
+```rust
+ecx.struct_span_err(span, "proc-macro derive produced unparseable tokens").emit();
+```
+
+> Note that `ecx.struct_span_err` indirectly calls [Session::struct_span_err].
+
+Referring to the [general comments](#general-comments) section above,
+follow these changes:
+
+- Replace the code above with this:
+
+  ```rust
+  ecx.sess.emit_err(errors::ProcMacroDeriveTokens { span });
+  ```
+
+- Create the type introduced above, `errors::ProcMacroDeriveTokens`,
+  in `src/errors.rs` (relative to crate directory root):
+
+  ```rust
+  #[derive(Diagnostic)]
+  #[diag(expand_proc_macro_derive_tokens)]
+  pub struct ProcMacroDeriveTokens {
+      #[primary_span]
+      pub span: Span,
+  }
+  ```
+
+- Create the actual text of the diagnostic in `compiler/rustc_expand/messages.ftl`:
+
+  ```fluent
+  expand_proc_macro_derive_tokens =
+      proc-macro derive produced unparseable tokens
+  ```
+
+Once that is done, a PR may be submitted,
+and the following comment on the PR page will label the PR accordingly,
+as well as alert the right people to review it:
+
+```
+@rustbot label +A-translation
+r? rust-lang/diagnostics
+```
+
+## Example with variable interpolation
+
+*This uses [issue #108436] for demonstration*
+
+Suppose you have the following code:
+
+```rust
+let mut err = ecx.struct_span_err(span, "proc macro panicked");
+if let Some(s) = e.as_str() {
+    err.help(&format!("message: {}", s));
+}
+err.emit()
+```
+
+> Note that `ecx.struct_span_err` indirectly calls [Session::struct_span_err].
+
+- Replace the code above with this:
+
+  ```rust
+  ecx.sess.emit_err(errors::ProcMacroPanicked {
+      span,
+      message: e
+          .as_str()
+          .map(|message| errors::ProcMacroPanickedHelp { message: message.into() }),
+  })
+  ```
+
+- Create the type introduced above, `errors::ProcMacroPanickedHelp`,
+  in `src/errors.rs` (relative to crate directory root):
+
+  ```rust
+  #[derive(Diagnostic)]
+  #[diag(expand_proc_macro_panicked)]
+  pub(crate) struct ProcMacroPanicked {
+      #[primary_span]
+      pub span: Span,
+      #[subdiagnostic]
+      pub message: Option<ProcMacroPanickedHelp>,
+  }
+
+  #[derive(Subdiagnostic)]
+  #[help(expand_help)]
+  pub(crate) struct ProcMacroPanickedHelp {
+      pub message: String,
+  }
+  ```
+
+- Create the actual text of the diagnostic in `messages.ftl`
+  (also at crate directory root):
+
+  ```fluent
+  expand_proc_macro_panicked =
+      proc macro panicked
+      .help = message: {$message}
+  ```
+
+## Example with a macro, `struct_span_err!`
+
+*This uses [issue #108373] for demonstration*
+
+Suppose you have the following code:
+
+```rust
+let mut diag = struct_span_err!(
+    tcx.sess,
+    generics_where_clauses_span.unwrap_or(main_span),
+    E0646,
+    "`main` function is not allowed to have a `where` clause"
+);
+if let Some(generics_where_clauses_span) = generics_where_clauses_span {
+    diag.span_label(generics_where_clauses_span, "`main` cannot have a `where` clause");
+}
+diag.emit();
+```
+
+> Note that `struct_span_err!` indirectly calls [Session::struct_span_err_with_code].
+
+- Replace the code above with this:
+
+  ```rust
+  tcx.sess.emit_err(errors::WhereClauseOnMain {
+      span: generics_where_clauses_span.unwrap_or(main_span),
+      generics_span: generics_where_clauses_span,
+  });
+  ```
+
+- Create the type introduced above, `errors::WhereClauseOnMain`,
+  in `src/errors.rs` (relative to crate directory root):
+
+  ```rust
+  #[derive(Diagnostic)]
+  #[diag(hir_analysis_where_clause_on_main, code = "E0646")]
+  pub(crate) struct WhereClauseOnMain {
+      #[primary_span]
+      pub span: Span,
+      #[label]
+      pub generics_span: Option<Span>,
+  }
+  ```
+
+- Create the actual text of the diagnostic in `messages.ftl`:
+
+  ```fluent
+  hir_analysis_where_clause_on_main =
+      `main` function is not allowed to have a `where` clause
+      .label = `main` cannot have a `where` clause
+  ```
+
+[Session::struct_span_err]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_session/session/struct.Session.html#method.struct_span_err
+[Session::struct_span_err_with_code]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_session/session/struct.Session.html#method.struct_span_err_with_code
+[the announcement of the initiative]: https://blog.rust-lang.org/inside-rust/2022/08/16/diagnostic-effort.html#manually-implementing-sessiondiagnostic
+[on GitHub]: https://github.com/rust-lang/rust/issues/100717
+[issue #108373]: https://github.com/rust-lang/rust/pull/108373
+[issue #108436]: https://github.com/rust-lang/rust/pull/108436
+[issue #108727]: https://github.com/rust-lang/rust/pull/108727