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, 7 deletions

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 4781585b82c..a3489226f62 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -35,7 +35,9 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{self, Lock, Lrc, ReadGuard, RwLock, WorkerLocal};
 use rustc_data_structures::vec_map::VecMap;
-use rustc_errors::{DecorateLint, ErrorGuaranteed, LintDiagnosticBuilder, MultiSpan};
+use rustc_errors::{
+    DecorateLint, DiagnosticBuilder, DiagnosticMessage, ErrorGuaranteed, MultiSpan,
+};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LOCAL_CRATE};
@@ -2857,7 +2859,9 @@ impl<'tcx> TyCtxt<'tcx> {
         span: impl Into<MultiSpan>,
         decorator: impl for<'a> DecorateLint<'a, ()>,
     ) {
-        self.struct_span_lint_hir(lint, hir_id, span, |diag| decorator.decorate_lint(diag))
+        self.struct_span_lint_hir(lint, hir_id, span, decorator.msg(), |diag| {
+            decorator.decorate_lint(diag)
+        })
     }
 
     pub fn struct_span_lint_hir(
@@ -2865,10 +2869,13 @@ impl<'tcx> TyCtxt<'tcx> {
         lint: &'static Lint,
         hir_id: HirId,
         span: impl Into<MultiSpan>,
-        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
+        msg: impl Into<DiagnosticMessage>,
+        decorate: impl for<'a, 'b> FnOnce(
+            &'b mut DiagnosticBuilder<'a, ()>,
+        ) -> &'b mut DiagnosticBuilder<'a, ()>,
     ) {
         let (level, src) = self.lint_level_at_node(lint, hir_id);
-        struct_lint_level(self.sess, lint, level, src, Some(span.into()), decorate);
+        struct_lint_level(self.sess, lint, level, src, Some(span.into()), msg, decorate);
     }
 
     /// Emit a lint from a lint struct (some type that implements `DecorateLint`, typically
@@ -2879,17 +2886,20 @@ impl<'tcx> TyCtxt<'tcx> {
         id: HirId,
         decorator: impl for<'a> DecorateLint<'a, ()>,
     ) {
-        self.struct_lint_node(lint, id, |diag| decorator.decorate_lint(diag))
+        self.struct_lint_node(lint, id, decorator.msg(), |diag| decorator.decorate_lint(diag))
     }
 
     pub fn struct_lint_node(
         self,
         lint: &'static Lint,
         id: HirId,
-        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
+        msg: impl Into<DiagnosticMessage>,
+        decorate: impl for<'a, 'b> FnOnce(
+            &'b mut DiagnosticBuilder<'a, ()>,
+        ) -> &'b mut DiagnosticBuilder<'a, ()>,
     ) {
         let (level, src) = self.lint_level_at_node(lint, id);
-        struct_lint_level(self.sess, lint, level, src, None, decorate);
+        struct_lint_level(self.sess, lint, level, src, None, msg, decorate);
     }
 
     pub fn in_scope_traits(self, id: HirId) -> Option<&'tcx [TraitCandidate]> {