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, 36 insertions, 31 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 2773b61e9ba..8f87a7fdeba 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -14,7 +14,7 @@ use crate::infer::TyCtxtInferExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, Obligation, ObligationCause};
 use hir::def::DefKind;
-use rustc_errors::{FatalError, MultiSpan};
+use rustc_errors::{DelayDm, FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
@@ -164,37 +164,42 @@ fn lint_object_unsafe_trait(
 ) {
     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
     // It's also hard to get a use site span, so we use the method definition span.
-    tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
-        let mut err = lint.build(&format!(
-            "the trait `{}` cannot be made into an object",
-            tcx.def_path_str(trait_def_id)
-        ));
-        let node = tcx.hir().get_if_local(trait_def_id);
-        let mut spans = MultiSpan::from_span(span);
-        if let Some(hir::Node::Item(item)) = node {
-            spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
-            spans.push_span_label(span, format!("...because {}", violation.error_msg()));
-        } else {
-            spans.push_span_label(
-                span,
-                format!(
-                    "the trait cannot be made into an object because {}",
-                    violation.error_msg()
-                ),
+    tcx.struct_span_lint_hir(
+        WHERE_CLAUSES_OBJECT_SAFETY,
+        hir::CRATE_HIR_ID,
+        span,
+        DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
+        |err| {
+            let node = tcx.hir().get_if_local(trait_def_id);
+            let mut spans = MultiSpan::from_span(span);
+            if let Some(hir::Node::Item(item)) = node {
+                spans.push_span_label(
+                    item.ident.span,
+                    "this trait cannot be made into an object...",
+                );
+                spans.push_span_label(span, format!("...because {}", violation.error_msg()));
+            } else {
+                spans.push_span_label(
+                    span,
+                    format!(
+                        "the trait cannot be made into an object because {}",
+                        violation.error_msg()
+                    ),
+                );
+            };
+            err.span_note(
+                spans,
+                "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
+                call to be resolvable dynamically; for more information visit \
+                <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
             );
-        };
-        err.span_note(
-            spans,
-            "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
-             call to be resolvable dynamically; for more information visit \
-             <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
-        );
-        if node.is_some() {
-            // Only provide the help if its a local trait, otherwise it's not
-            violation.solution(&mut err);
-        }
-        err.emit();
-    });
+            if node.is_some() {
+                // Only provide the help if its a local trait, otherwise it's not
+                violation.solution(err);
+            }
+            err
+        },
+    );
 }
 
 fn sized_trait_bound_spans<'tcx>(