Support using #[unstable_feature_bound] on trait

compiler/rustc_passes/messages.ftl

@@ -669,8 +669,8 @@ passes_rustc_std_internal_symbol =
     .label = not a function or static
 
 passes_rustc_unstable_feature_bound =
-    attribute should be applied to `impl` or free function outside of any `impl` or trait
-    .label = not an `impl` or free function
+    attribute should be applied to `impl`, trait or free function
+    .label = not an `impl`, trait or free function
 
 passes_should_be_applied_to_fn =
     attribute should be applied to a function definition

compiler/rustc_passes/src/check_attr.rs

@@ -2294,7 +2294,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         match target {
             // FIXME(staged_api): There's no reason we can't support more targets here. We're just
             // being conservative to begin with.
-            Target::Fn | Target::Impl { .. } => {}
+            Target::Fn | Target::Impl { .. } | Target::Trait => {}
             Target::ExternCrate
             | Target::Use
             | Target::Static
@@ -2309,7 +2309,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             | Target::Struct
             | Target::Field
             | Target::Union
-            | Target::Trait
             | Target::TraitAlias
             | Target::Expression
             | Target::Statement

src/doc/rustc-dev-guide/src/stability.md

@@ -182,6 +182,11 @@ of the standard library raises it to a warning with
 `#![warn(deprecated_in_future)]`.
 
 ## unstable_feature_bound
-The `#[unstable_feature_bound(foo)]` attribute can be used together with `#[unstable]` attribute to mark an `impl` of stable type and stable trait as unstable. In std/core, an item annotated with `#[unstable_feature_bound(foo)]` can only be used by another item that is also annotated with `#[unstable_feature_bound(foo)]`. Outside of std/core, using an item with `#[unstable_feature_bound(foo)]` requires the feature to be enabled with `#![feature(foo)]` attribute on the crate. Currently, only `impl`s and free functions can be annotated with `#[unstable_feature_bound]`.
+The `#[unstable_feature_bound(foo)]` attribute can be used together with `#[unstable]` attribute to mark an `impl` of stable type and stable trait as unstable. In std/core, an item annotated with `#[unstable_feature_bound(foo)]` can only be used by another item that is also annotated with `#[unstable_feature_bound(foo)]`. Outside of std/core, using an item with `#[unstable_feature_bound(foo)]` requires the feature to be enabled with `#![feature(foo)]` attribute on the crate.
+
+Currently, the items that can be annotated with `#[unstable_feature_bound]` are:
+- `impl`
+- free function
+- trait
 
 [blog]: https://www.ralfj.de/blog/2018/07/19/const.html

tests/ui/unstable-feature-bound/unstable_feature_bound_on_trait.fail.stderr

@@ -0,0 +1,18 @@
+error: unstable feature `foo` is used without being enabled.
+  --> $DIR/unstable_feature_bound_on_trait.rs:28:5
+   |
+LL |     Foo::bar();
+   |     ^^^^^^^^^^
+   |
+   = help: The feature can be enabled by marking the current item with `#[unstable_feature_bound(foo)]`
+note: required by a bound in `Bar::bar`
+  --> $DIR/unstable_feature_bound_on_trait.rs:16:1
+   |
+LL | #[unstable_feature_bound(foo)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Bar::bar`
+...
+LL |     fn bar() {}
+   |        --- required by a bound in this associated function
+
+error: aborting due to 1 previous error
+

tests/ui/unstable-feature-bound/unstable_feature_bound_on_trait.rs

@@ -0,0 +1,33 @@
+//@ revisions: pass fail
+//@[pass] check-pass
+
+#![allow(internal_features)]
+#![feature(staged_api)]
+#![stable(feature = "a", since = "1.1.1" )]
+
+/// Test the behaviour of marking a trait with #[unstable_feature_bound].
+/// In this testcase, even though the trait method `bar` and the `struct Foo` are
+/// both stable, #[unstable_feature_bound] is still needed at the call site of Foo::bar().
+
+#[stable(feature = "a", since = "1.1.1" )]
+struct Foo;
+
+#[unstable(feature = "foo", issue = "none" )]
+#[unstable_feature_bound(foo)]
+trait Bar {
+    #[stable(feature = "a", since = "1.1.1" )]
+    fn bar() {}
+}
+
+#[unstable_feature_bound(foo)]
+impl Bar for Foo {
+}
+
+#[cfg_attr(pass, unstable_feature_bound(foo))]
+fn moo() {
+    Foo::bar();
+    //[fail]~^ ERROR: unstable feature `foo` is used without being enabled.
+}
+
+
+fn main() {}

tests/ui/unstable-feature-bound/unstable_inherent_method.rs

@@ -9,14 +9,14 @@
 pub trait Trait {
     #[unstable(feature = "feat", issue = "none" )]
     #[unstable_feature_bound(foo)]
-    //~^ ERROR: attribute should be applied to `impl` or free function outside of any `impl` or trait
+    //~^ ERROR: attribute should be applied to `impl`, trait or free function
     fn foo();
 }
 
 #[stable(feature = "a", since = "1.1.1" )]
 impl Trait for u8 {
     #[unstable_feature_bound(foo)]
-    //~^ ERROR: attribute should be applied to `impl` or free function outside of any `impl` or trait
+    //~^ ERROR: attribute should be applied to `impl`, trait or free function
     fn foo() {}
 }
 

tests/ui/unstable-feature-bound/unstable_inherent_method.stderr

@@ -1,20 +1,20 @@
-error: attribute should be applied to `impl` or free function outside of any `impl` or trait
+error: attribute should be applied to `impl`, trait or free function
   --> $DIR/unstable_inherent_method.rs:11:5
    |
 LL |     #[unstable_feature_bound(foo)]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |
 LL |     fn foo();
-   |     --------- not an `impl` or free function
+   |     --------- not an `impl`, trait or free function
 
-error: attribute should be applied to `impl` or free function outside of any `impl` or trait
+error: attribute should be applied to `impl`, trait or free function
   --> $DIR/unstable_inherent_method.rs:18:5
    |
 LL |     #[unstable_feature_bound(foo)]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |
 LL |     fn foo() {}
-   |     ----------- not an `impl` or free function
+   |     ----------- not an `impl`, trait or free function
 
 error: aborting due to 2 previous errors