Move part of the target checking for #[may_dangle] to the parser

compiler/rustc_attr_parsing/src/attributes/semantics.rs

@@ -1,11 +1,17 @@
 use rustc_feature::AttributeStability;
+use rustc_hir::target::GenericParamKind;
 
 use super::prelude::*;
 
 pub(crate) struct MayDangleParser;
 impl NoArgsAttributeParser for MayDangleParser {
     const PATH: &[Symbol] = &[sym::may_dangle];
-    const ALLOWED_TARGETS: AllowedTargets<'_> = AllowedTargets::AllowList(ALL_TARGETS); //FIXME Still checked fully in `check_attr.rs`
+    const ALLOWED_TARGETS: AllowedTargets<'_> = AllowedTargets::AllowList(&[
+        Allow(Target::GenericParam { kind: GenericParamKind::Type, has_default: false }),
+        Allow(Target::GenericParam { kind: GenericParamKind::Type, has_default: true }),
+        Allow(Target::GenericParam { kind: GenericParamKind::Lifetime, has_default: false }),
+        Allow(Target::GenericParam { kind: GenericParamKind::Lifetime, has_default: true }),
+    ]);
     const STABILITY: AttributeStability = unstable!(dropck_eyepatch);
     const CREATE: fn(span: Span) -> AttributeKind = AttributeKind::MayDangle;
 }

compiler/rustc_attr_parsing/src/target_checking.rs

@@ -416,6 +416,7 @@ pub(crate) fn allowed_targets_applied(
 
     // ensure a consistent order
     target_strings.sort();
+    target_strings.dedup();
 
     // If there is now only 1 target left, show that as the only possible target
     let only_target = target_strings.len() == 1;

compiler/rustc_passes/src/check_attr.rs

@@ -24,8 +24,9 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalModDefId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{
-    self as hir, Attribute, CRATE_HIR_ID, Constness, FnSig, ForeignItem, GenericParamKind, HirId,
-    Item, ItemKind, MethodKind, Node, ParamName, Target, TraitItem, find_attr,
+    self as hir, Attribute, CRATE_HIR_ID, Constness, FnSig, ForeignItem, GenericParam,
+    GenericParamKind, HirId, Item, ItemKind, MethodKind, Node, ParamName, Target, TraitItem,
+    find_attr,
 };
 use rustc_macros::Diagnostic;
 use rustc_middle::hir::nested_filter;
@@ -1121,12 +1122,18 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
 
     /// Checks if `#[may_dangle]` is applied to a lifetime or type generic parameter in `Drop` impl.
     fn check_may_dangle(&self, hir_id: HirId, attr_span: Span) {
-        if let hir::Node::GenericParam(param) = self.tcx.hir_node(hir_id)
-            && matches!(
-                param.kind,
-                hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. }
-            )
-            && matches!(param.source, hir::GenericParamSource::Generics)
+        let hir::Node::GenericParam(
+            param @ GenericParam {
+                kind: hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. },
+                ..
+            },
+        ) = self.tcx.hir_node(hir_id)
+        else {
+            self.dcx().delayed_bug("Checked in attr parser");
+            return;
+        };
+
+        if matches!(param.source, hir::GenericParamSource::Generics)
             && let parent_hir_id = self.tcx.parent_hir_id(hir_id)
             && let hir::Node::Item(item) = self.tcx.hir_node(parent_hir_id)
             && let hir::ItemKind::Impl(impl_) = item.kind

tests/ui/attributes/may_dangle.rs

@@ -12,7 +12,7 @@ unsafe impl<'a, #[may_dangle] T, const N: usize> NotDrop for Implee2<'a, T, N> {
 //~^ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
 
 unsafe impl<'a, T, #[may_dangle] const N: usize> Drop for Implee1<'a, T, N> {
-    //~^ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+    //~^ ERROR attribute cannot be used on
     fn drop(&mut self) {}
 }
 
@@ -39,15 +39,15 @@ mod fake {
     }
 }
 
-#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+#[may_dangle] //~ ERROR attribute cannot be used on
 struct Dangling;
 
-#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+#[may_dangle] //~ ERROR attribute cannot be used on
 impl NotDrop for () {
 }
 
-#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+#[may_dangle] //~ ERROR attribute cannot be used on
 fn main() {
-    #[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+    #[may_dangle] //~ ERROR attribute cannot be used on
     let () = ();
 }

tests/ui/attributes/may_dangle.stderr

@@ -1,44 +1,54 @@
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
-  --> $DIR/may_dangle.rs:8:13
-   |
-LL | unsafe impl<#[may_dangle] 'a, T, const N: usize> NotDrop for Implee1<'a, T, N> {}
-   |             ^^^^^^^^^^^^^
-
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
-  --> $DIR/may_dangle.rs:11:17
-   |
-LL | unsafe impl<'a, #[may_dangle] T, const N: usize> NotDrop for Implee2<'a, T, N> {}
-   |                 ^^^^^^^^^^^^^
-
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+error: `#[may_dangle]` attribute cannot be used on const parameters
   --> $DIR/may_dangle.rs:14:20
    |
 LL | unsafe impl<'a, T, #[may_dangle] const N: usize> Drop for Implee1<'a, T, N> {
    |                    ^^^^^^^^^^^^^
+   |
+   = help: `#[may_dangle]` can be applied to lifetime parameters and type parameters
 
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+error: `#[may_dangle]` attribute cannot be used on structs
   --> $DIR/may_dangle.rs:42:1
    |
 LL | #[may_dangle]
    | ^^^^^^^^^^^^^
+   |
+   = help: `#[may_dangle]` can be applied to lifetime parameters and type parameters
 
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+error: `#[may_dangle]` attribute cannot be used on trait impl blocks
   --> $DIR/may_dangle.rs:45:1
    |
 LL | #[may_dangle]
    | ^^^^^^^^^^^^^
+   |
+   = help: `#[may_dangle]` can be applied to lifetime parameters and type parameters
 
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+error: `#[may_dangle]` attribute cannot be used on functions
   --> $DIR/may_dangle.rs:49:1
    |
 LL | #[may_dangle]
    | ^^^^^^^^^^^^^
+   |
+   = help: `#[may_dangle]` can be applied to lifetime parameters and type parameters
 
-error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+error: `#[may_dangle]` attribute cannot be used on statements
   --> $DIR/may_dangle.rs:51:5
    |
 LL |     #[may_dangle]
    |     ^^^^^^^^^^^^^
+   |
+   = help: `#[may_dangle]` can be applied to lifetime parameters and type parameters
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:8:13
+   |
+LL | unsafe impl<#[may_dangle] 'a, T, const N: usize> NotDrop for Implee1<'a, T, N> {}
+   |             ^^^^^^^^^^^^^
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:11:17
+   |
+LL | unsafe impl<'a, #[may_dangle] T, const N: usize> NotDrop for Implee2<'a, T, N> {}
+   |                 ^^^^^^^^^^^^^
 
 error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
   --> $DIR/may_dangle.rs:36:17