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, 11 insertions, 8 deletions

diff --git a/compiler/rustc_lint/src/methods.rs b/compiler/rustc_lint/src/methods.rs
index 5f7f03480c0..313119637bc 100644
--- a/compiler/rustc_lint/src/methods.rs
+++ b/compiler/rustc_lint/src/methods.rs
@@ -90,14 +90,17 @@ fn lint_cstring_as_ptr(
         if cx.tcx.is_diagnostic_item(sym::Result, def.did()) {
             if let ty::Adt(adt, _) = substs.type_at(0).kind() {
                 if cx.tcx.is_diagnostic_item(sym::cstring_type, adt.did()) {
-                    cx.struct_span_lint(TEMPORARY_CSTRING_AS_PTR, as_ptr_span, |diag| {
-                        diag.build(fluent::lint::cstring_ptr)
-                            .span_label(as_ptr_span, fluent::lint::as_ptr_label)
-                            .span_label(unwrap.span, fluent::lint::unwrap_label)
-                            .note(fluent::lint::note)
-                            .help(fluent::lint::help)
-                            .emit();
-                    });
+                    cx.struct_span_lint(
+                        TEMPORARY_CSTRING_AS_PTR,
+                        as_ptr_span,
+                        fluent::lint::cstring_ptr,
+                        |diag| {
+                            diag.span_label(as_ptr_span, fluent::lint::as_ptr_label)
+                                .span_label(unwrap.span, fluent::lint::unwrap_label)
+                                .note(fluent::lint::note)
+                                .help(fluent::lint::help)
+                        },
+                    );
                 }
             }
         }