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, 7 insertions, 15 deletions

diff --git a/compiler/rustc_mir_transform/src/check_unsafety.rs b/compiler/rustc_mir_transform/src/check_unsafety.rs
index beff19a3ab2..4730be1244b 100644
--- a/compiler/rustc_mir_transform/src/check_unsafety.rs
+++ b/compiler/rustc_mir_transform/src/check_unsafety.rs
@@ -489,21 +489,20 @@ fn unsafety_check_result<'tcx>(
 
 fn report_unused_unsafe(tcx: TyCtxt<'_>, kind: UnusedUnsafe, id: HirId) {
     let span = tcx.sess.source_map().guess_head_span(tcx.hir().span(id));
-    tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, |lint| {
-        let msg = "unnecessary `unsafe` block";
-        let mut db = lint.build(msg);
-        db.span_label(span, msg);
+    let msg = "unnecessary `unsafe` block";
+    tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, msg, |lint| {
+        lint.span_label(span, msg);
         match kind {
             UnusedUnsafe::Unused => {}
             UnusedUnsafe::InUnsafeBlock(id) => {
-                db.span_label(
+                lint.span_label(
                     tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
                     "because it's nested under this `unsafe` block",
                 );
             }
         }
 
-        db.emit();
+        lint
     });
 }
 
@@ -543,15 +542,8 @@ pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                 UNSAFE_OP_IN_UNSAFE_FN,
                 lint_root,
                 source_info.span,
-                |lint| {
-                    lint.build(&format!(
-                        "{} is unsafe and requires unsafe block (error E0133)",
-                        description,
-                    ))
-                    .span_label(source_info.span, description)
-                    .note(note)
-                    .emit();
-                },
+                format!("{} is unsafe and requires unsafe block (error E0133)", description,),
+                |lint| lint.span_label(source_info.span, description).note(note),
             ),
         }
     }