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, 30 insertions, 25 deletions

diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index ea6da299b8f..6e373e41b4c 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -2010,30 +2010,35 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         tcx.check_stability(item.def_id, Some(hir_ref_id), span, None);
 
         if let Some(variant_def_id) = variant_resolution {
-            tcx.struct_span_lint_hir(AMBIGUOUS_ASSOCIATED_ITEMS, hir_ref_id, span, |lint| {
-                let mut err = lint.build("ambiguous associated item");
-                let mut could_refer_to = |kind: DefKind, def_id, also| {
-                    let note_msg = format!(
-                        "`{}` could{} refer to the {} defined here",
-                        assoc_ident,
-                        also,
-                        kind.descr(def_id)
-                    );
-                    err.span_note(tcx.def_span(def_id), &note_msg);
-                };
+            tcx.struct_span_lint_hir(
+                AMBIGUOUS_ASSOCIATED_ITEMS,
+                hir_ref_id,
+                span,
+                "ambiguous associated item",
+                |lint| {
+                    let mut could_refer_to = |kind: DefKind, def_id, also| {
+                        let note_msg = format!(
+                            "`{}` could{} refer to the {} defined here",
+                            assoc_ident,
+                            also,
+                            kind.descr(def_id)
+                        );
+                        lint.span_note(tcx.def_span(def_id), &note_msg);
+                    };
 
-                could_refer_to(DefKind::Variant, variant_def_id, "");
-                could_refer_to(kind, item.def_id, " also");
+                    could_refer_to(DefKind::Variant, variant_def_id, "");
+                    could_refer_to(kind, item.def_id, " also");
 
-                err.span_suggestion(
-                    span,
-                    "use fully-qualified syntax",
-                    format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
-                    Applicability::MachineApplicable,
-                );
+                    lint.span_suggestion(
+                        span,
+                        "use fully-qualified syntax",
+                        format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
+                        Applicability::MachineApplicable,
+                    );
 
-                err.emit();
-            });
+                    lint
+                },
+            );
         }
         Ok((ty, kind, item.def_id))
     }
@@ -3079,15 +3084,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     BARE_TRAIT_OBJECTS,
                     self_ty.hir_id,
                     self_ty.span,
+                    msg,
                     |lint| {
-                        let mut diag = lint.build(msg);
-                        diag.multipart_suggestion_verbose(
+                        lint.multipart_suggestion_verbose(
                             "use `dyn`",
                             sugg,
                             Applicability::MachineApplicable,
                         );
-                        self.maybe_lint_blanket_trait_impl(&self_ty, &mut diag);
-                        diag.emit();
+                        self.maybe_lint_blanket_trait_impl(&self_ty, lint);
+                        lint
                     },
                 );
             }