Emit pre-expansion feature gate warning for negative impls

Author: fmease

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -227,8 +227,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                         &self,
                         negative_impls,
                         span.to(of_trait.trait_ref.path.span),
-                        "negative trait bounds are not fully implemented; \
-                         use marker types for now"
+                        "negative impls are experimental"
                     );
                 }
 
@@ -644,6 +643,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all_legacy_dont_use!(decl_macro, "`macro` is experimental");
     gate_all_legacy_dont_use!(try_blocks, "`try` blocks are unstable");
     gate_all_legacy_dont_use!(auto_traits, "`auto` traits are unstable");
+    gate_all_legacy_dont_use!(negative_impls, "negative impls are experimental");
 
     visit::walk_crate(&mut visitor, krate);
 }

compiler/rustc_parse/src/parser/item.rs

@@ -603,6 +603,7 @@ impl<'a> Parser<'a> {
     fn parse_polarity(&mut self) -> ast::ImplPolarity {
         // Disambiguate `impl !Trait for Type { ... }` and `impl ! { ... }` for the never type.
         if self.check(exp!(Bang)) && self.look_ahead(1, |t| t.can_begin_type()) {
+            self.psess.gated_spans.gate(sym::negative_impls, self.token.span);
             self.bump(); // `!`
             ast::ImplPolarity::Negative(self.prev_token.span)
         } else {

tests/ui/auto-traits/ungated-impl.rs

@@ -1,12 +1,5 @@
-// Test that default and negative trait implementations are gated by
-// `auto_traits` feature gate
+auto trait DummyAutoTrait {} //~ ERROR auto traits are experimental and possibly buggy
 
-struct DummyStruct;
-
-auto trait AutoDummyTrait {}
-//~^ ERROR auto traits are experimental and possibly buggy
-
-impl !AutoDummyTrait for DummyStruct {}
-//~^ ERROR negative trait bounds are not fully implemented; use marker types for now
+pub unsafe auto trait AnotherAutoTrait {} //~ ERROR auto traits are experimental and possibly buggy
 
 fn main() {}

tests/ui/auto-traits/ungated-impl.stderr

@@ -1,21 +1,21 @@
 error[E0658]: auto traits are experimental and possibly buggy
-  --> $DIR/feature-gate-auto-traits.rs:6:1
+  --> $DIR/feature-gate-auto-traits.rs:1:1
    |
-LL | auto trait AutoDummyTrait {}
+LL | auto trait DummyAutoTrait {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
    = help: add `#![feature(auto_traits)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: negative trait bounds are not fully implemented; use marker types for now
-  --> $DIR/feature-gate-auto-traits.rs:9:6
+error[E0658]: auto traits are experimental and possibly buggy
+  --> $DIR/feature-gate-auto-traits.rs:3:1
    |
-LL | impl !AutoDummyTrait for DummyStruct {}
-   |      ^^^^^^^^^^^^^^^
+LL | pub unsafe auto trait AnotherAutoTrait {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
-   = help: add `#![feature(negative_impls)]` to the crate attributes to enable
+   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
+   = help: add `#![feature(auto_traits)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors

tests/ui/feature-gates/feature-gate-auto-traits.rs

@@ -0,0 +1,12 @@
+// For historical reasons, negative impls don't have a proper pre-expansion feature gate.
+// We're now at least issuing a *warning* for those that only exist before macro expansion.
+// FIXME(#154045): Turn their post-expansion feature gate into a proper pre-expansion one.
+//                 As part of this, move these test cases into `feature-gate-negative_impls.rs`.
+//@ check-pass
+
+#[cfg(false)]
+impl !Trait for () {}
+//~^ WARN negative impls are experimental
+//~| WARN unstable syntax can change at any point in the future
+
+fn main() {}

tests/ui/feature-gates/feature-gate-auto-traits.stderr

@@ -0,0 +1,14 @@
+warning: negative impls are experimental
+  --> $DIR/soft-feature-gate-negative_impls.rs:8:6
+   |
+LL | impl !Trait for () {}
+   |      ^
+   |
+   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
+   = help: add `#![feature(negative_impls)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 1 warning emitted
+

tests/ui/feature-gates/soft-feature-gate-negative_impls.rs

@@ -1,3 +1,5 @@
 trait MyTrait {}
-impl !MyTrait for u32 {} //~ ERROR negative trait bounds are not fully implemented
+
+impl !MyTrait for u32 {} //~ ERROR negative impls are experimental
+
 fn main() {}

tests/ui/feature-gates/soft-feature-gate-negative_impls.stderr

@@ -1,5 +1,5 @@
-error[E0658]: negative trait bounds are not fully implemented; use marker types for now
-  --> $DIR/feature-gate-negative_impls.rs:2:6
+error[E0658]: negative impls are experimental
+  --> $DIR/feature-gate-negative_impls.rs:3:6
    |
 LL | impl !MyTrait for u32 {}
    |      ^^^^^^^^