From 4ca56d28884678d89c753c03de9f0c7544d018e6 Mon Sep 17 00:00:00 2001
From: Maybe Waffle <waffle.lapkin@gmail.com>
Date: Thu, 27 Jan 2022 21:25:51 +0300
Subject: [PATCH] Check that `#[rustc_must_implement_one_of]` is applied to a
 trait

---
 compiler/rustc_passes/src/check_attr.rs       | 23 +++++++++++++++++++
 .../rustc_must_implement_one_of_misuse.rs     |  8 +++++++
 .../rustc_must_implement_one_of_misuse.stderr | 20 +++++++++++++++-
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index c3d5bae32e681..ec4cefa3537f9 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -76,6 +76,9 @@ impl CheckAttrVisitor<'_> {
                 sym::inline => self.check_inline(hir_id, attr, span, target),
                 sym::non_exhaustive => self.check_non_exhaustive(hir_id, attr, span, target),
                 sym::marker => self.check_marker(hir_id, attr, span, target),
+                sym::rustc_must_implement_one_of => {
+                    self.check_rustc_must_implement_one_of(attr, span, target)
+                }
                 sym::target_feature => self.check_target_feature(hir_id, attr, span, target),
                 sym::track_caller => {
                     self.check_track_caller(hir_id, &attr.span, attrs, span, target)
@@ -477,6 +480,26 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
+    /// Checks if the `#[rustc_must_implement_one_of]` attribute on a `target` is valid. Returns `true` if valid.
+    fn check_rustc_must_implement_one_of(
+        &self,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+    ) -> bool {
+        match target {
+            Target::Trait => true,
+            _ => {
+                self.tcx
+                    .sess
+                    .struct_span_err(attr.span, "attribute can only be applied to a trait")
+                    .span_label(*span, "not a trait")
+                    .emit();
+                false
+            }
+        }
+    }
+
     /// Checks if the `#[target_feature]` attribute on `item` is valid. Returns `true` if valid.
     fn check_target_feature(
         &self,
diff --git a/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.rs b/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.rs
index 1089e5f9c4ab9..d9de6d5edb9b4 100644
--- a/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.rs
+++ b/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.rs
@@ -35,4 +35,12 @@ trait Tr5 {
     fn b(); //~ This function doesn't have a default implementation
 }
 
+#[rustc_must_implement_one_of(abc, xyz)]
+//~^ attribute can only be applied to a trait
+fn function() {}
+
+#[rustc_must_implement_one_of(abc, xyz)]
+//~^ attribute can only be applied to a trait
+struct Struct {}
+
 fn main() {}
diff --git a/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.stderr b/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.stderr
index 74a6dc8fec97c..bc28dc2c4f40d 100644
--- a/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.stderr
+++ b/src/test/ui/traits/default-method/rustc_must_implement_one_of_misuse.stderr
@@ -4,6 +4,24 @@ error: malformed `rustc_must_implement_one_of` attribute input
 LL | #[rustc_must_implement_one_of]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[rustc_must_implement_one_of(function1, function2, ...)]`
 
+error: attribute can only be applied to a trait
+  --> $DIR/rustc_must_implement_one_of_misuse.rs:38:1
+   |
+LL | #[rustc_must_implement_one_of(abc, xyz)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | fn function() {}
+   | ---------------- not a trait
+
+error: attribute can only be applied to a trait
+  --> $DIR/rustc_must_implement_one_of_misuse.rs:42:1
+   |
+LL | #[rustc_must_implement_one_of(abc, xyz)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | struct Struct {}
+   | ---------------- not a trait
+
 error: Function not found in this trait
   --> $DIR/rustc_must_implement_one_of_misuse.rs:3:31
    |
@@ -78,5 +96,5 @@ note: required by this annotation
 LL | #[rustc_must_implement_one_of(a, b)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: aborting due to 11 previous errors