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, 17 insertions, 17 deletions

diff --git a/compiler/rustc_lint/src/traits.rs b/compiler/rustc_lint/src/traits.rs
index df1587c5948..078465bdce6 100644
--- a/compiler/rustc_lint/src/traits.rs
+++ b/compiler/rustc_lint/src/traits.rs
@@ -100,15 +100,18 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
                 if trait_predicate.trait_ref.self_ty().is_impl_trait() {
                     continue;
                 }
-                cx.struct_span_lint(DROP_BOUNDS, span, |lint| {
-                    let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else {
-                        return
-                    };
-                    lint.build(fluent::lint::drop_trait_constraints)
-                        .set_arg("predicate", predicate)
-                        .set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
-                        .emit();
-                });
+                let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else {
+                    continue;
+                };
+                cx.struct_span_lint(
+                    DROP_BOUNDS,
+                    span,
+                    fluent::lint::drop_trait_constraints,
+                    |lint| {
+                        lint.set_arg("predicate", predicate)
+                            .set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
+                    },
+                );
             }
         }
     }
@@ -119,14 +122,11 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
         };
         for bound in &bounds[..] {
             let def_id = bound.trait_ref.trait_def_id();
-            if cx.tcx.lang_items().drop_trait() == def_id {
-                cx.struct_span_lint(DYN_DROP, bound.span, |lint| {
-                    let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else {
-                        return
-                    };
-                    lint.build(fluent::lint::drop_glue)
-                        .set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
-                        .emit();
+            if cx.tcx.lang_items().drop_trait() == def_id
+                && let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop)
+            {
+                cx.struct_span_lint(DYN_DROP, bound.span, fluent::lint::drop_glue, |lint| {
+                    lint.set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
                 });
             }
         }