Rollup merge of #125258 - compiler-errors:static-if-no-lt, r=nnethercote

Resolve elided lifetimes in assoc const to static if no other lifetimes are in scope

Implements the change to elided lifetime resolution in *associated consts* subject to FCP here: https://github.com/rust-lang/rust/issues/125190#issue-2301532282

Specifically, walk the enclosing lifetime ribs in an associated const, and if we find no other lifetimes, then resolve to `'static`.

Also make it work for traits, but don't lint -- just give a hard error in that case.

compiler/rustc_lint/messages.ftl

@@ -14,6 +14,7 @@ lint_associated_const_elided_lifetime = {$elided ->
         *[false] `'_` cannot be used here
     }
     .suggestion = use the `'static` lifetime
+    .note = cannot automatically infer `'static` because of other lifetimes in scope
 
 lint_async_fn_in_trait = use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
     .note = you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`

compiler/rustc_lint/src/context/diagnostics.rs

@@ -319,11 +319,20 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::UnusedQualifications { removal_span } => {
             lints::UnusedQualifications { removal_span }.decorate_lint(diag);
         }
-        BuiltinLintDiag::AssociatedConstElidedLifetime { elided, span: lt_span } => {
+        BuiltinLintDiag::AssociatedConstElidedLifetime {
+            elided,
+            span: lt_span,
+            lifetimes_in_scope,
+        } => {
             let lt_span = if elided { lt_span.shrink_to_hi() } else { lt_span };
             let code = if elided { "'static " } else { "'static" };
-            lints::AssociatedConstElidedLifetime { span: lt_span, code, elided }
-                .decorate_lint(diag);
+            lints::AssociatedConstElidedLifetime {
+                span: lt_span,
+                code,
+                elided,
+                lifetimes_in_scope,
+            }
+            .decorate_lint(diag);
         }
         BuiltinLintDiag::RedundantImportVisibility { max_vis, span: vis_span, import_vis } => {
             lints::RedundantImportVisibility { span: vis_span, help: (), max_vis, import_vis }

compiler/rustc_lint/src/lints.rs

@@ -2873,6 +2873,8 @@ pub struct AssociatedConstElidedLifetime {
 
     pub code: &'static str,
     pub elided: bool,
+    #[note]
+    pub lifetimes_in_scope: MultiSpan,
 }
 
 #[derive(LintDiagnostic)]