Check that #[may_dangle] is properly applied

It's only valid when applied to a type or lifetime parameter
in `Drop` trait implementation.

compiler/rustc_passes/messages.ftl

@@ -444,6 +444,9 @@ passes_macro_export_on_decl_macro =
 passes_macro_use =
     `#[{$name}]` only has an effect on `extern crate` and modules
 
+passes_may_dangle =
+    `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+
 passes_maybe_string_interpolation = you might have meant to use string interpolation in this string literal
 passes_missing_const_err =
     attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`

compiler/rustc_passes/src/check_attr.rs

@@ -189,6 +189,7 @@ fn check_attributes(
                 [sym::collapse_debuginfo, ..] => self.check_collapse_debuginfo(attr, span, target),
                 [sym::must_not_suspend, ..] => self.check_must_not_suspend(attr, span, target),
                 [sym::must_use, ..] => self.check_must_use(hir_id, attr, target),
+                [sym::may_dangle, ..] => self.check_may_dangle(hir_id, attr),
                 [sym::rustc_pass_by_value, ..] => self.check_pass_by_value(attr, span, target),
                 [sym::rustc_allow_incoherent_impl, ..] => {
                     self.check_allow_incoherent_impl(attr, span, target)
@@ -255,7 +256,6 @@ fn check_attributes(
                     | sym::cfg_attr
                     // need to be fixed
                     | sym::cfi_encoding // FIXME(cfi_encoding)
-                    | sym::may_dangle // FIXME(dropck_eyepatch)
                     | sym::pointee // FIXME(derive_smart_pointer)
                     | sym::omit_gdb_pretty_printer_section // FIXME(omit_gdb_pretty_printer_section)
                     | sym::used // handled elsewhere to restrict to static items
@@ -1373,6 +1373,27 @@ fn check_must_not_suspend(&self, attr: &Attribute, span: Span, target: Target) {
         }
     }
 
+    /// 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: &Attribute) {
+        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 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
+            && let Some(trait_) = impl_.of_trait
+            && let Some(def_id) = trait_.trait_def_id()
+            && self.tcx.is_lang_item(def_id, hir::LangItem::Drop)
+        {
+            return;
+        }
+
+        self.dcx().emit_err(errors::InvalidMayDangle { attr_span: attr.span });
+    }
+
     /// Checks if `#[cold]` is applied to a non-function.
     fn check_cold(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) {
         match target {

compiler/rustc_passes/src/errors.rs

@@ -737,6 +737,13 @@ pub struct NonExportedMacroInvalidAttrs {
     pub attr_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_may_dangle)]
+pub struct InvalidMayDangle {
+    #[primary_span]
+    pub attr_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(passes_unused_duplicate)]
 pub struct UnusedDuplicate {

tests/ui/attributes/may_dangle.rs

@@ -0,0 +1,53 @@
+#![feature(dropck_eyepatch)]
+
+struct Implee1<'a, T, const N: usize>(&'a T);
+struct Implee2<'a, T, const N: usize>(&'a T);
+struct Implee3<'a, T, const N: usize>(&'a T);
+trait NotDrop {}
+
+unsafe impl<#[may_dangle] 'a, T, const N: usize> NotDrop for Implee1<'a, T, N> {}
+//~^ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+
+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
+    fn drop(&mut self) {}
+}
+
+// Ok, lifetime param in a `Drop` impl.
+unsafe impl<#[may_dangle] 'a, T, const N: usize> Drop for Implee2<'a, T, N> {
+    fn drop(&mut self) {}
+}
+
+// Ok, type param in a `Drop` impl.
+unsafe impl<'a, #[may_dangle] T, const N: usize> Drop for Implee3<'a, T, N> {
+    fn drop(&mut self) {}
+}
+
+// Check that this check is not textual.
+mod fake {
+    trait Drop {
+        fn drop(&mut self);
+    }
+    struct Implee<T>(T);
+
+    unsafe impl<#[may_dangle] T> Drop for Implee<T> {
+        //~^ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+        fn drop(&mut self) {}
+    }
+}
+
+#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+struct Dangling;
+
+#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+impl NotDrop for () {
+}
+
+#[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+fn main() {
+    #[may_dangle] //~ ERROR must be applied to a lifetime or type generic parameter in `Drop` impl
+    let () = ();
+}

tests/ui/attributes/may_dangle.stderr

@@ -0,0 +1,50 @@
+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:14:20
+   |
+LL | unsafe impl<'a, T, #[may_dangle] const N: usize> Drop 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:42:1
+   |
+LL | #[may_dangle]
+   | ^^^^^^^^^^^^^
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:45:1
+   |
+LL | #[may_dangle]
+   | ^^^^^^^^^^^^^
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:49:1
+   |
+LL | #[may_dangle]
+   | ^^^^^^^^^^^^^
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:51:5
+   |
+LL |     #[may_dangle]
+   |     ^^^^^^^^^^^^^
+
+error: `#[may_dangle]` must be applied to a lifetime or type generic parameter in `Drop` impl
+  --> $DIR/may_dangle.rs:36:17
+   |
+LL |     unsafe impl<#[may_dangle] T> Drop for Implee<T> {
+   |                 ^^^^^^^^^^^^^
+
+error: aborting due to 8 previous errors
+