Update #[coverage(..)] attribute error messages to match the current implementation

compiler/rustc_error_codes/src/error_codes/E0788.md

@@ -1,26 +1,24 @@
-A `#[coverage]` attribute was applied to something which does not show up
-in code coverage, or is too granular to be excluded from the coverage report.
+A `#[coverage(off|on)]` attribute was found in a position where it is not
+allowed.
 
-For now, this attribute can only be applied to function, method, and closure
-definitions. In the future, it may be added to statements, blocks, and
-expressions, and for the time being, using this attribute in those places
-will just emit an `unused_attributes` lint instead of this error.
+Coverage attributes can be applied to:
+- Function and method declarations that have a body, including trait methods
+  that have a default implementation.
+- Closure expressions, in situations where attributes can be applied to
+  expressions.
+- `impl` blocks (inherent or trait), and modules.
 
 Example of erroneous code:
 
 ```compile_fail,E0788
-#[coverage(off)]
-struct Foo;
-
-#[coverage(on)]
-const FOO: Foo = Foo;
+unsafe extern "C" {
+    #[coverage(off)]
+    fn foreign_fn();
+}
 ```
 
-`#[coverage(off)]` tells the compiler to not generate coverage instrumentation
-for a piece of code when the `-C instrument-coverage` flag is passed. Things
-like structs and consts are not coverable code, and thus cannot do anything
-with this attribute.
-
-If you wish to apply this attribute to all methods in an impl or module,
-manually annotate each method; it is not possible to annotate the entire impl
-with a `#[coverage]` attribute.
+When using the `-C instrument-coverage` flag, coverage attributes act as a
+hint to the compiler that it should instrument or not instrument the
+corresponding function or enclosed functions. The precise effect of applying
+a coverage attribute is not guaranteed and may change in future compiler
+versions.

compiler/rustc_passes/messages.ftl

@@ -112,9 +112,11 @@ passes_coroutine_on_non_closure =
     attribute should be applied to closures
     .label = not a closure
 
-passes_coverage_not_fn_or_closure =
-    attribute should be applied to a function definition or closure
-    .label = not a function or closure
+passes_coverage_attribute_not_allowed =
+    coverage attribute not allowed here
+    .not_fn_impl_mod = not a function, impl block, or module
+    .no_body = function has no body
+    .help = coverage attribute can be applied to a function (with body), impl block, or module
 
 passes_dead_codes =
     { $multiple ->

compiler/rustc_passes/src/check_attr.rs

@@ -432,21 +432,34 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
 
     /// Checks that `#[coverage(..)]` is applied to a function/closure/method,
     /// or to an impl block or module.
-    fn check_coverage(&self, attr: &Attribute, span: Span, target: Target) {
+    fn check_coverage(&self, attr: &Attribute, target_span: Span, target: Target) {
+        let mut not_fn_impl_mod = None;
+        let mut no_body = None;
+
         match target {
             Target::Fn
             | Target::Closure
             | Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent)
             | Target::Impl
-            | Target::Mod => {}
+            | Target::Mod => return,
+
+            // These are "functions", but they aren't allowed because they don't
+            // have a body, so the usual explanation would be confusing.
+            Target::Method(MethodKind::Trait { body: false }) | Target::ForeignFn => {
+                no_body = Some(target_span);
+            }
 
             _ => {
-                self.dcx().emit_err(errors::CoverageNotFnOrClosure {
-                    attr_span: attr.span,
-                    defn_span: span,
-                });
+                not_fn_impl_mod = Some(target_span);
             }
         }
+
+        self.dcx().emit_err(errors::CoverageAttributeNotAllowed {
+            attr_span: attr.span,
+            not_fn_impl_mod,
+            no_body,
+            help: (),
+        });
     }
 
     /// Checks that `#[optimize(..)]` is applied to a function/closure/method,

compiler/rustc_passes/src/errors.rs

@@ -71,13 +71,21 @@ pub(crate) struct InlineNotFnOrClosure {
     pub defn_span: Span,
 }
 
+/// "coverage attribute not allowed here"
 #[derive(Diagnostic)]
-#[diag(passes_coverage_not_fn_or_closure, code = E0788)]
-pub(crate) struct CoverageNotFnOrClosure {
+#[diag(passes_coverage_attribute_not_allowed, code = E0788)]
+pub(crate) struct CoverageAttributeNotAllowed {
     #[primary_span]
     pub attr_span: Span,
-    #[label]
-    pub defn_span: Span,
+    /// "not a function, impl block, or module"
+    #[label(passes_not_fn_impl_mod)]
+    pub not_fn_impl_mod: Option<Span>,
+    /// "function has no body"
+    #[label(passes_no_body)]
+    pub no_body: Option<Span>,
+    /// "coverage attribute can be applied to a function (with body), impl block, or module"
+    #[help]
+    pub help: (),
 }
 
 #[derive(Diagnostic)]

tests/ui/coverage-attr/allowed-positions.rs

@@ -0,0 +1,116 @@
+//! Tests where the `#[coverage(..)]` attribute can and cannot be used.
+
+//@ reference: attributes.coverage.allowed-positions
+
+#![feature(coverage_attribute)]
+#![feature(extern_types)]
+#![feature(impl_trait_in_assoc_type)]
+#![warn(unused_attributes)]
+#![coverage(off)]
+
+#[coverage(off)]
+mod submod {}
+
+#[coverage(off)] //~ ERROR coverage attribute not allowed here [E0788]
+type MyTypeAlias = ();
+
+#[coverage(off)] //~ ERROR [E0788]
+trait MyTrait {
+    #[coverage(off)] //~ ERROR [E0788]
+    const TRAIT_ASSOC_CONST: u32;
+
+    #[coverage(off)] //~ ERROR [E0788]
+    type TraitAssocType;
+
+    #[coverage(off)] //~ ERROR [E0788]
+    fn trait_method(&self);
+
+    #[coverage(off)]
+    fn trait_method_with_default(&self) {}
+
+    #[coverage(off)] //~ ERROR [E0788]
+    fn trait_assoc_fn();
+}
+
+#[coverage(off)]
+impl MyTrait for () {
+    const TRAIT_ASSOC_CONST: u32 = 0;
+
+    #[coverage(off)] //~ ERROR [E0788]
+    type TraitAssocType = Self;
+
+    #[coverage(off)]
+    fn trait_method(&self) {}
+    #[coverage(off)]
+    fn trait_method_with_default(&self) {}
+    #[coverage(off)]
+    fn trait_assoc_fn() {}
+}
+
+trait HasAssocType {
+    type T;
+    fn constrain_assoc_type() -> Self::T;
+}
+
+impl HasAssocType for () {
+    #[coverage(off)] //~ ERROR [E0788]
+    type T = impl Copy;
+    fn constrain_assoc_type() -> Self::T {}
+}
+
+#[coverage(off)] //~ ERROR [E0788]
+struct MyStruct {
+    #[coverage(off)] //~ ERROR [E0788]
+    field: u32,
+}
+
+#[coverage(off)]
+impl MyStruct {
+    #[coverage(off)]
+    fn method(&self) {}
+    #[coverage(off)]
+    fn assoc_fn() {}
+}
+
+extern "C" {
+    #[coverage(off)] //~ ERROR [E0788]
+    static X: u32;
+
+    #[coverage(off)] //~ ERROR [E0788]
+    type T;
+
+    #[coverage(off)] //~ ERROR [E0788]
+    fn foreign_fn();
+}
+
+#[coverage(off)]
+fn main() {
+    #[coverage(off)] //~ ERROR [E0788]
+    let _ = ();
+
+    // Currently not allowed on let statements, even if they bind to a closure.
+    // It might be nice to support this as a special case someday, but trying
+    // to define the precise boundaries of that special case might be tricky.
+    #[coverage(off)] //~ ERROR [E0788]
+    let _let_closure = || ();
+
+    // In situations where attributes can already be applied to expressions,
+    // the coverage attribute is allowed on closure expressions.
+    let _closure_tail_expr = {
+        #[coverage(off)]
+        || ()
+    };
+
+    // Applying attributes to arbitrary expressions requires an unstable
+    // feature, but if that feature were enabled then this would be allowed.
+    let _closure_expr = #[coverage(off)] || ();
+    //~^ ERROR attributes on expressions are experimental [E0658]
+
+    match () {
+        #[coverage(off)] //~ ERROR [E0788]
+        () => (),
+    }
+
+    #[coverage(off)] //~ ERROR [E0788]
+    return ();
+}