Add MCP 563 syntax

Author: SoniEx2

compiler/rustc_feature/src/builtin_attrs.rs

@@ -540,7 +540,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
 
     gated!(fundamental, Normal, template!(Word), WarnFollowing, experimental!(fundamental)),
     gated!(
-        may_dangle, Normal, template!(Word), WarnFollowing, dropck_eyepatch,
+        may_dangle, Normal, template!(Word, List: "borrow"), WarnFollowing, dropck_eyepatch,
         "`may_dangle` has unstable semantics and may be removed in the future",
     ),
 

compiler/rustc_span/src/symbol.rs

@@ -425,6 +425,7 @@ symbols! {
         black_box,
         block,
         bool,
+        borrow,
         borrowck_graphviz_format,
         borrowck_graphviz_postflow,
         box_free,

tests/ui/dropck/mcp563-dropck-eyepatch-syntax.rs

@@ -0,0 +1,26 @@
+#![feature(dropck_eyepatch)]
+
+// This test checks the `#[may_dangle]` attribute syntax.
+
+struct Foo<T>(*const T);
+
+// No error: "drops" bound
+unsafe impl<#[may_dangle] T> Drop for Foo<T> {
+    fn drop(&mut self) { }
+}
+
+struct Bar<T>(*const T);
+
+// No error: "borrows" bound
+unsafe impl<#[may_dangle(borrow)] T> Drop for Bar<T> {
+    fn drop(&mut self) { }
+}
+
+struct Baz<T>(*const T);
+
+// Error: invalid syntax
+unsafe impl<#[may_dangle(invalid)] T> Drop for Baz<T> {
+    fn drop(&mut self) { }
+}
+
+pub fn main() { }