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, 55 insertions, 57 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/method/prelude2021.rs b/compiler/rustc_hir_analysis/src/check/method/prelude2021.rs
index 392695cca68..ca4cdf5a0d0 100644
--- a/compiler/rustc_hir_analysis/src/check/method/prelude2021.rs
+++ b/compiler/rustc_hir_analysis/src/check/method/prelude2021.rs
@@ -82,14 +82,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 prelude_or_array_lint,
                 self_expr.hir_id,
                 self_expr.span,
+                format!("trait method `{}` will become ambiguous in Rust 2021", segment.ident.name),
                 |lint| {
                     let sp = self_expr.span;
 
-                    let mut lint = lint.build(&format!(
-                        "trait method `{}` will become ambiguous in Rust 2021",
-                        segment.ident.name
-                    ));
-
                     let derefs = "*".repeat(pick.autoderefs);
 
                     let autoref = match pick.autoref_or_ptr_adjustment {
@@ -133,7 +129,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
 
-                    lint.emit();
+                    lint
                 },
             );
         } else {
@@ -143,6 +139,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 prelude_or_array_lint,
                 call_expr.hir_id,
                 call_expr.span,
+                format!("trait method `{}` will become ambiguous in Rust 2021", segment.ident.name),
                 |lint| {
                     let sp = call_expr.span;
                     let trait_name = self.trait_path_or_bare_name(
@@ -151,11 +148,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         pick.item.container_id(self.tcx),
                     );
 
-                    let mut lint = lint.build(&format!(
-                        "trait method `{}` will become ambiguous in Rust 2021",
-                        segment.ident.name
-                    ));
-
                     let (self_adjusted, precise) = self.adjust_expr(pick, self_expr, sp);
                     if precise {
                         let args = args
@@ -202,7 +194,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
 
-                    lint.emit();
+                    lint
                 },
             );
         }
@@ -257,15 +249,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
-        self.tcx.struct_span_lint_hir(RUST_2021_PRELUDE_COLLISIONS, expr_id, span, |lint| {
-            // "type" refers to either a type or, more likely, a trait from which
-            // the associated function or method is from.
-            let container_id = pick.item.container_id(self.tcx);
-            let trait_path = self.trait_path_or_bare_name(span, expr_id, container_id);
-            let trait_generics = self.tcx.generics_of(container_id);
-
-            let trait_name =
-                if trait_generics.params.len() <= trait_generics.has_self as usize {
+        self.tcx.struct_span_lint_hir(
+            RUST_2021_PRELUDE_COLLISIONS,
+            expr_id,
+            span,
+            format!(
+                "trait-associated function `{}` will become ambiguous in Rust 2021",
+                method_name.name
+            ),
+            |lint| {
+                // "type" refers to either a type or, more likely, a trait from which
+                // the associated function or method is from.
+                let container_id = pick.item.container_id(self.tcx);
+                let trait_path = self.trait_path_or_bare_name(span, expr_id, container_id);
+                let trait_generics = self.tcx.generics_of(container_id);
+
+                let trait_name = if trait_generics.params.len() <= trait_generics.has_self as usize
+                {
                     trait_path
                 } else {
                     let counts = trait_generics.own_counts();
@@ -282,44 +282,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     )
                 };
 
-            let mut lint = lint.build(&format!(
-                "trait-associated function `{}` will become ambiguous in Rust 2021",
-                method_name.name
-            ));
-
-            let mut self_ty_name = self_ty_span
-                .find_ancestor_inside(span)
-                .and_then(|span| self.sess().source_map().span_to_snippet(span).ok())
-                .unwrap_or_else(|| self_ty.to_string());
-
-            // Get the number of generics the self type has (if an Adt) unless we can determine that
-            // the user has written the self type with generics already which we (naively) do by looking
-            // for a "<" in `self_ty_name`.
-            if !self_ty_name.contains('<') {
-                if let Adt(def, _) = self_ty.kind() {
-                    let generics = self.tcx.generics_of(def.did());
-                    if !generics.params.is_empty() {
-                        let counts = generics.own_counts();
-                        self_ty_name += &format!(
-                            "<{}>",
-                            std::iter::repeat("'_")
-                                .take(counts.lifetimes)
-                                .chain(std::iter::repeat("_").take(counts.types + counts.consts))
-                                .collect::<Vec<_>>()
-                                .join(", ")
-                        );
+                let mut self_ty_name = self_ty_span
+                    .find_ancestor_inside(span)
+                    .and_then(|span| self.sess().source_map().span_to_snippet(span).ok())
+                    .unwrap_or_else(|| self_ty.to_string());
+
+                // Get the number of generics the self type has (if an Adt) unless we can determine that
+                // the user has written the self type with generics already which we (naively) do by looking
+                // for a "<" in `self_ty_name`.
+                if !self_ty_name.contains('<') {
+                    if let Adt(def, _) = self_ty.kind() {
+                        let generics = self.tcx.generics_of(def.did());
+                        if !generics.params.is_empty() {
+                            let counts = generics.own_counts();
+                            self_ty_name += &format!(
+                                "<{}>",
+                                std::iter::repeat("'_")
+                                    .take(counts.lifetimes)
+                                    .chain(
+                                        std::iter::repeat("_").take(counts.types + counts.consts)
+                                    )
+                                    .collect::<Vec<_>>()
+                                    .join(", ")
+                            );
+                        }
                     }
                 }
-            }
-            lint.span_suggestion(
-                span,
-                "disambiguate the associated function",
-                format!("<{} as {}>::{}", self_ty_name, trait_name, method_name.name,),
-                Applicability::MachineApplicable,
-            );
-
-            lint.emit();
-        });
+                lint.span_suggestion(
+                    span,
+                    "disambiguate the associated function",
+                    format!("<{} as {}>::{}", self_ty_name, trait_name, method_name.name,),
+                    Applicability::MachineApplicable,
+                );
+
+                lint
+            },
+        );
     }
 
     fn trait_path_or_bare_name(