Rollup merge of #126682 - Zalathar:coverage-attr, r=lcnr

coverage: Overhaul validation of the `#[coverage(..)]` attribute

This PR makes sweeping changes to how the (currently-unstable) coverage attribute is validated:
- Multiple coverage attributes on the same item/expression are now treated as an error.
- The attribute must always be `#[coverage(off)]` or `#[coverage(on)]`, and the error messages for this are more consistent.
  -  A trailing comma is still allowed after off/on, since that's part of the normal attribute syntax.
- Some places that silently ignored a coverage attribute now produce an error instead.
  - These cases were all clearly bugs.
- Some places that ignored a coverage attribute (with a warning) now produce an error instead.
  - These were originally added as lints, but I don't think it makes much sense to knowingly allow new attributes to be used in meaningless places.
  - Some of these errors might soon disappear, if it's easy to extend recursive coverage attributes to things like modules and impl blocks.

---

One of the goals of this PR is to lay a more solid foundation for making the coverage attribute recursive, so that it applies to all nested functions/closures instead of just the one it is directly attached to.

Fixes #126658.

This PR incorporates #126659, which adds more tests for validation of the coverage attribute.

`@rustbot` label +A-code-coverage

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -15,11 +15,7 @@ use rustc_span::{sym, Span};
 use rustc_target::spec::{abi, SanitizerSet};
 
 use crate::errors;
-use crate::target_features::from_target_feature;
-use crate::{
-    errors::{ExpectedCoverageSymbol, ExpectedUsedSymbol},
-    target_features::check_target_feature_trait_unsafe,
-};
+use crate::target_features::{check_target_feature_trait_unsafe, from_target_feature};
 
 fn linkage_by_name(tcx: TyCtxt<'_>, def_id: LocalDefId, name: &str) -> Linkage {
     use rustc_middle::mir::mono::Linkage::*;
@@ -139,7 +135,8 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                         // coverage on a smaller scope within an excluded larger scope.
                     }
                     Some(_) | None => {
-                        tcx.dcx().emit_err(ExpectedCoverageSymbol { span: attr.span });
+                        tcx.dcx()
+                            .span_delayed_bug(attr.span, "unexpected value of coverage attribute");
                     }
                 }
             }
@@ -174,7 +171,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                         codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED;
                     }
                     Some(_) => {
-                        tcx.dcx().emit_err(ExpectedUsedSymbol { span: attr.span });
+                        tcx.dcx().emit_err(errors::ExpectedUsedSymbol { span: attr.span });
                     }
                     None => {
                         // Unfortunately, unconditionally using `llvm.used` causes

compiler/rustc_codegen_ssa/src/errors.rs

@@ -564,13 +564,6 @@ pub struct UnknownArchiveKind<'a> {
     pub kind: &'a str,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_ssa_expected_coverage_symbol)]
-pub struct ExpectedCoverageSymbol {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_expected_used_symbol)]
 pub struct ExpectedUsedSymbol {