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_passes/messages.ftl

@@ -103,18 +103,9 @@ passes_continue_labeled_block =
     .label = labeled blocks cannot be `continue`'d
     .block_label = labeled block the `continue` points to
 
-passes_coverage_fn_defn =
-    `#[coverage]` may only be applied to function definitions
-
-passes_coverage_ignored_function_prototype =
-    `#[coverage]` is ignored on function prototypes
-
-passes_coverage_not_coverable =
-    `#[coverage]` must be applied to coverable code
-    .label = not coverable code
-
-passes_coverage_propagate =
-    `#[coverage]` does not propagate into items and must be applied to the contained functions directly
+passes_coverage_not_fn_or_closure =
+    attribute should be applied to a function definition or closure
+    .label = not a function or closure
 
 passes_dead_codes =
     { $multiple ->

compiler/rustc_passes/src/check_attr.rs

@@ -122,7 +122,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     self.check_diagnostic_on_unimplemented(attr.span, hir_id, target)
                 }
                 [sym::inline] => self.check_inline(hir_id, attr, span, target),
-                [sym::coverage] => self.check_coverage(hir_id, attr, span, target),
+                [sym::coverage] => self.check_coverage(attr, span, target),
                 [sym::non_exhaustive] => self.check_non_exhaustive(hir_id, attr, span, target),
                 [sym::marker] => self.check_marker(hir_id, attr, span, target),
                 [sym::target_feature] => {
@@ -369,47 +369,15 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
-    /// Checks if a `#[coverage]` is applied directly to a function
-    fn check_coverage(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) -> bool {
+    /// Checks that `#[coverage(..)]` is applied to a function or closure.
+    fn check_coverage(&self, attr: &Attribute, span: Span, target: Target) -> bool {
         match target {
-            // #[coverage] on function is fine
+            // #[coverage(..)] on function is fine
             Target::Fn
             | Target::Closure
             | Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent) => true,
-
-            // function prototypes can't be covered
-            Target::Method(MethodKind::Trait { body: false }) | Target::ForeignFn => {
-                self.tcx.emit_node_span_lint(
-                    UNUSED_ATTRIBUTES,
-                    hir_id,
-                    attr.span,
-                    errors::IgnoredCoverageFnProto,
-                );
-                true
-            }
-
-            Target::Mod | Target::ForeignMod | Target::Impl | Target::Trait => {
-                self.tcx.emit_node_span_lint(
-                    UNUSED_ATTRIBUTES,
-                    hir_id,
-                    attr.span,
-                    errors::IgnoredCoveragePropagate,
-                );
-                true
-            }
-
-            Target::Expression | Target::Statement | Target::Arm => {
-                self.tcx.emit_node_span_lint(
-                    UNUSED_ATTRIBUTES,
-                    hir_id,
-                    attr.span,
-                    errors::IgnoredCoverageFnDefn,
-                );
-                true
-            }
-
             _ => {
-                self.dcx().emit_err(errors::IgnoredCoverageNotCoverable {
+                self.dcx().emit_err(errors::CoverageNotFnOrClosure {
                     attr_span: attr.span,
                     defn_span: span,
                 });

compiler/rustc_passes/src/errors.rs

@@ -60,21 +60,9 @@ pub struct InlineNotFnOrClosure {
     pub defn_span: Span,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(passes_coverage_ignored_function_prototype)]
-pub struct IgnoredCoverageFnProto;
-
-#[derive(LintDiagnostic)]
-#[diag(passes_coverage_propagate)]
-pub struct IgnoredCoveragePropagate;
-
-#[derive(LintDiagnostic)]
-#[diag(passes_coverage_fn_defn)]
-pub struct IgnoredCoverageFnDefn;
-
 #[derive(Diagnostic)]
-#[diag(passes_coverage_not_coverable, code = E0788)]
-pub struct IgnoredCoverageNotCoverable {
+#[diag(passes_coverage_not_fn_or_closure, code = E0788)]
+pub struct CoverageNotFnOrClosure {
     #[primary_span]
     pub attr_span: Span,
     #[label]