[RFC 2397] Initial implementation

compiler/rustc_feature/src/active.rs

@@ -374,6 +374,8 @@ declare_features! (
     (active, deprecated_safe, "1.61.0", Some(94978), None),
     /// Allows having using `suggestion` in the `#[deprecated]` attribute.
     (active, deprecated_suggestion, "1.61.0", Some(94785), None),
+    /// Controls errors in trait implementations.
+    (active, do_not_recommend, "1.67.0", Some(51992), None),
     /// Tells rustdoc to automatically generate `#[doc(cfg(...))]`.
     (active, doc_auto_cfg, "1.58.0", Some(43781), None),
     /// Allows `#[doc(cfg(...))]`.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -487,6 +487,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         experimental!(collapse_debuginfo)
     ),
 
+    // RFC 2397
+    gated!(do_not_recommend, Normal, template!(Word), WarnFollowing, experimental!(do_not_recommend)),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================

compiler/rustc_span/src/symbol.rs

@@ -613,6 +613,7 @@ symbols! {
         dispatch_from_dyn,
         div,
         div_assign,
+        do_not_recommend,
         doc,
         doc_alias,
         doc_auto_cfg,

src/test/ui/rfc-2397-do-not-recommend/feature-gate-do_not_recommend.rs

@@ -0,0 +1,21 @@
+#![feature(do_not_recommend)]
+
+pub trait Foo {
+}
+
+impl Foo for i32 {
+}
+
+pub trait Bar {
+}
+
+#[do_not_recommend]
+impl<T: Foo> Bar for T {
+}
+
+fn stuff<T: Bar>(_: T) {}
+
+fn main() {
+    stuff(1u8);
+    //~^ the trait bound `u8: Foo` is not satisfied
+}

src/test/ui/rfc-2397-do-not-recommend/feature-gate-do_not_recommend.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the trait bound `u8: Foo` is not satisfied
+  --> $DIR/feature-gate-do_not_recommend.rs:19:11
+   |
+LL |     stuff(1u8);
+   |     ----- ^^^ the trait `Foo` is not implemented for `u8`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Foo` is implemented for `i32`
+note: required for `u8` to implement `Bar`
+  --> $DIR/feature-gate-do_not_recommend.rs:13:14
+   |
+LL | impl<T: Foo> Bar for T {
+   |              ^^^     ^
+note: required by a bound in `stuff`
+  --> $DIR/feature-gate-do_not_recommend.rs:16:13
+   |
+LL | fn stuff<T: Bar>(_: T) {}
+   |             ^^^ required by this bound in `stuff`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2397-do-not-recommend/unstable-feature.rs

@@ -0,0 +1,7 @@
+#[do_not_recommend]
+//~^ ERROR the `#[do_not_recommend]` attribute is an experimental feature
+trait Foo {
+}
+
+fn main() {
+}

src/test/ui/rfc-2397-do-not-recommend/unstable-feature.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[do_not_recommend]` attribute is an experimental feature
+  --> $DIR/unstable-feature.rs:1:1
+   |
+LL | #[do_not_recommend]
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #51992 <https://github.com/rust-lang/rust/issues/51992> for more information
+   = help: add `#![feature(do_not_recommend)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.