Auto merge of #101986 - WaffleLapkin:move_lint_note_to_the_bottom, r=estebank

Move lint level source explanation to the bottom

So, uhhhhh

r? `@estebank`

## User-facing change

"note: `#[warn(...)]` on by default" and such are moved to the bottom of the diagnostic:
```diff
-   = note: `#[warn(unsupported_calling_conventions)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #87678 <https://github.com/rust-lang/rust/issues/87678>
+   = note: `#[warn(unsupported_calling_conventions)]` on by default
```

Why warning is enabled is the least important thing, so it shouldn't be the first note the user reads, IMO.

## Developer-facing change

`struct_span_lint` and similar methods have a different signature.

Before: `..., impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>)`
After: `..., impl Into<DiagnosticMessage>, impl for<'a, 'b> FnOnce(&'b mut DiagnosticBuilder<'a, ()>) -> &'b mut DiagnosticBuilder<'a, ()>`

The reason for this is that `struct_span_lint` needs to edit the diagnostic _after_ `decorate` closure is called. This also makes lint code a little bit nicer in my opinion.

Another option is to use `impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>) -> DiagnosticBuilder<'a, ()>` altough I don't _really_ see reasons to do `let lint = lint.build(message)` everywhere.

## Subtle problem

By moving the message outside of the closure (that may not be called if the lint is disabled) `format!(...)` is executed earlier, possibly formatting `Ty` which may call a query that trims paths that crashes the compiler if there were no warnings...

I don't think it's that big of a deal, considering that we move from `format!(...)` to `fluent` (which is lazy by-default) anyway, however this required adding a workaround which is unfortunate.

## P.S.

I'm sorry, I do not how to make this PR smaller/easier to review. Changes to the lint API affect SO MUCH 😢

1 files changed, 12 insertions, 8 deletions

diff --git a/compiler/rustc_hir_analysis/src/check_unused.rs b/compiler/rustc_hir_analysis/src/check_unused.rs
index 1d23ed92921..922833f8580 100644
--- a/compiler/rustc_hir_analysis/src/check_unused.rs
+++ b/compiler/rustc_hir_analysis/src/check_unused.rs
@@ -29,14 +29,18 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
             continue;
         }
         let hir::ItemKind::Use(path, _) = item.kind else { unreachable!() };
-        tcx.struct_span_lint_hir(lint::builtin::UNUSED_IMPORTS, item.hir_id(), path.span, |lint| {
-            let msg = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(path.span) {
-                format!("unused import: `{}`", snippet)
-            } else {
-                "unused import".to_owned()
-            };
-            lint.build(&msg).emit();
-        });
+        let msg = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(path.span) {
+            format!("unused import: `{}`", snippet)
+        } else {
+            "unused import".to_owned()
+        };
+        tcx.struct_span_lint_hir(
+            lint::builtin::UNUSED_IMPORTS,
+            item.hir_id(),
+            path.span,
+            msg,
+            |lint| lint,
+        );
     }
 
     unused_crates_lint(tcx);