Rollup merge of rust-lang#106645 - c410-f3r:rfc-2397-1, r=oli-obk

[RFC 2397] Initial implementation

cc rust-lang#51992

Because of previous experiences where ppl didn't have the time to review large PRs (or any at all), the implementation of this feature will be delivered in small chunks to hopefully make things faster.

In this initial PR, only the attribute is being declared and gated with ordinary tests.

Author: Yuki Okushi

Diff for: 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(...))]`.

Diff for: 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:
     // ==========================================================================

Diff for: 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,

Diff for: 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
+}

Diff for: 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`.

Diff for: 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() {
+}

Diff for: 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`.