Move diagnostic::on_const target check

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_const.rs

@@ -1,8 +1,10 @@
+use rustc_errors::Diagnostic;
 use rustc_hir::attrs::diagnostic::Directive;
+use rustc_session::lint::builtin::MISPLACED_DIAGNOSTIC_ATTRIBUTES;
 
 use crate::attributes::diagnostic::*;
 use crate::attributes::prelude::*;
-
+use crate::errors::DiagnosticOnConstOnlyForTraitImpls;
 #[derive(Default)]
 pub(crate) struct OnConstParser {
     span: Option<Span>,
@@ -21,6 +23,21 @@ impl<S: Stage> AttributeParser<S> for OnConstParser {
 
             let span = cx.attr_span;
             this.span = Some(span);
+
+            // FIXME(mejrs) no constness field on `Target`,
+            // so non-constness is still checked in check_attr.rs
+            if !matches!(cx.target, Target::Impl { of_trait: true }) {
+                let target_span = cx.target_span;
+                cx.emit_dyn_lint(
+                    MISPLACED_DIAGNOSTIC_ATTRIBUTES,
+                    move |dcx, level| {
+                        DiagnosticOnConstOnlyForTraitImpls { target_span }.into_diag(dcx, level)
+                    },
+                    span,
+                );
+                return;
+            }
+
             let mode = Mode::DiagnosticOnConst;
 
             let Some(items) = parse_list(cx, args, mode) else { return };
@@ -32,7 +49,8 @@ impl<S: Stage> AttributeParser<S> for OnConstParser {
         },
     )];
 
-    //FIXME Still checked in `check_attr.rs`
+    // "Allowed" on all targets; noop on anything but non-const trait impls;
+    // this linted on in parser.
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(ALL_TARGETS);
 
     fn finalize(self, _cx: &FinalizeContext<'_, '_, S>) -> Option<AttributeKind> {

compiler/rustc_attr_parsing/src/errors.rs

@@ -252,3 +252,10 @@ pub(crate) struct DocUnknownAny {
 #[derive(Diagnostic)]
 #[diag("expected boolean for `#[doc(auto_cfg = ...)]`")]
 pub(crate) struct DocAutoCfgWrongLiteral;
+
+#[derive(Diagnostic)]
+#[diag("`#[diagnostic::on_const]` can only be applied to trait impls")]
+pub(crate) struct DiagnosticOnConstOnlyForTraitImpls {
+    #[label("not a trait impl")]
+    pub target_span: Span,
+}

compiler/rustc_passes/src/check_attr.rs

@@ -54,13 +54,6 @@
 #[diag("`#[diagnostic::on_unimplemented]` can only be applied to trait definitions")]
 struct DiagnosticOnUnimplementedOnlyForTraits;
 
-#[derive(Diagnostic)]
-#[diag("`#[diagnostic::on_const]` can only be applied to trait impls")]
-struct DiagnosticOnConstOnlyForTraitImpls {
-    #[label("not a trait impl")]
-    item_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("`#[diagnostic::on_const]` can only be applied to non-const trait impls")]
 struct DiagnosticOnConstOnlyForNonConstTraitImpls {
@@ -587,7 +580,7 @@ fn check_diagnostic_on_unimplemented(
         }
     }
 
-    /// Checks if `#[diagnostic::on_const]` is applied to a trait impl
+    /// Checks if `#[diagnostic::on_const]` is applied to a on-const trait impl
     fn check_diagnostic_on_const(
         &self,
         attr_span: Span,
@@ -595,6 +588,8 @@ fn check_diagnostic_on_const(
         target: Target,
         item: Option<ItemLike<'_>>,
     ) {
+        // We only check the non-constness here. A diagnostic for use
+        // on not-trait impl items is issued during attribute parsing.
         if target == (Target::Impl { of_trait: true }) {
             match item.unwrap() {
                 ItemLike::Item(it) => match it.expect_impl().constness {
@@ -613,16 +608,9 @@ fn check_diagnostic_on_const(
                 ItemLike::ForeignItem => {}
             }
         }
-        let item_span = self.tcx.hir_span(hir_id);
-        self.tcx.emit_node_span_lint(
-            MISPLACED_DIAGNOSTIC_ATTRIBUTES,
-            hir_id,
-            attr_span,
-            DiagnosticOnConstOnlyForTraitImpls { item_span },
-        );
-
-        // We don't check the validity of generic args here...whose generics would that be, anyway?
-        // The traits' or the impls'?
+        // FIXME(#155570) Can we do something with generic args here?
+        // regardless, we don't check the validity of generic args here
+        // ...whose generics would that be, anyway? The traits' or the impls'?
     }
 
     /// Checks if `#[diagnostic::on_move]` is applied to an ADT definition

tests/ui/diagnostic_namespace/on_const/misplaced_attr.stderr

@@ -1,18 +1,3 @@
-error: `#[diagnostic::on_const]` can only be applied to trait impls
-  --> $DIR/misplaced_attr.rs:4:1
-   |
-LL | #[diagnostic::on_const(message = "tadaa", note = "boing")]
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-LL |
-LL | pub struct Foo;
-   | -------------- not a trait impl
-   |
-note: the lint level is defined here
-  --> $DIR/misplaced_attr.rs:2:9
-   |
-LL | #![deny(misplaced_diagnostic_attributes)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error: `#[diagnostic::on_const]` can only be applied to non-const trait impls
   --> $DIR/misplaced_attr.rs:8:1
    |
@@ -21,24 +6,45 @@ LL | #[diagnostic::on_const(message = "tadaa", note = "boing")]
 LL |
 LL | impl const PartialEq for Foo {
    | ---------------------------- this is a const trait impl
+   |
+note: the lint level is defined here
+  --> $DIR/misplaced_attr.rs:2:9
+   |
+LL | #![deny(misplaced_diagnostic_attributes)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `#[diagnostic::on_const]` can only be applied to trait impls
-  --> $DIR/misplaced_attr.rs:16:1
+  --> $DIR/misplaced_attr.rs:4:1
    |
 LL | #[diagnostic::on_const(message = "tadaa", note = "boing")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |
-LL | impl Foo {
-   | -------- not a trait impl
+LL | pub struct Foo;
+   | --------------- not a trait impl
+
+error: `#[diagnostic::on_const]` can only be applied to trait impls
+  --> $DIR/misplaced_attr.rs:16:1
+   |
+LL |   #[diagnostic::on_const(message = "tadaa", note = "boing")]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | / impl Foo {
+LL | |     fn eq(&self, _other: &Foo) -> bool {
+LL | |         true
+LL | |     }
+LL | | }
+   | |_- not a trait impl
 
 error: `#[diagnostic::on_const]` can only be applied to trait impls
   --> $DIR/misplaced_attr.rs:25:5
    |
-LL |     #[diagnostic::on_const(message = "tadaa", note = "boing")]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |       #[diagnostic::on_const(message = "tadaa", note = "boing")]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |
-LL |     fn partial_cmp(&self, other: &Foo) -> Option<std::cmp::Ordering> {
-   |     ---------------------------------------------------------------- not a trait impl
+LL | /     fn partial_cmp(&self, other: &Foo) -> Option<std::cmp::Ordering> {
+LL | |         None
+LL | |     }
+   | |_____- not a trait impl
 
 error: aborting due to 4 previous errors