Rollup merge of rust-lang#154527 - fmease:more-soft-gates, r=nnethercote

Emit pre-expansion feature gate warnings for negative impls and specialization

Follow up to rust-lang#154475; part of rust-lang#154045.

This shouldn't need any extra input from T-compiler or T-lang since it's legitimized by [MCP 535](rust-lang/compiler-team#535).

However, I have a feeling that negative impls & specialization behind "`#[cfg(feature = "nightly")]`" are more prevalent in the ecosystem compared to e.g., auto traits & box patterns, so these new warnings will probably hit a bunch of users. In any case, it's only a warning for now not an error so e.g., running crater "in deny mode" or nominating for a T-lang meeting discussion would be disproportionate (esp. since T-lang [has confirmed in the past](rust-lang#116393 (comment)) that this is a T-compiler matter).

Author: JonathanBrouwer

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -248,10 +248,18 @@ impl<'a> Parser<'a> {
             self.parse_use_item()?
         } else if self.check_fn_front_matter(check_pub, case) {
             // FUNCTION ITEM
+            let defaultness = def_();
+            if let Defaultness::Default(span) = defaultness {
+                // Default functions should only require feature `min_specialization`. We remove the
+                // `specialization` tag again as such spans *require* feature `specialization` to be
+                // enabled. In a later stage, we make `specialization` imply `min_specialization`.
+                self.psess.gated_spans.gate(sym::min_specialization, span);
+                self.psess.gated_spans.ungate_last(sym::specialization, span);
+            }
             let (ident, sig, generics, contract, body) =
                 self.parse_fn(attrs, fn_parse_mode, lo, vis, case)?;
             ItemKind::Fn(Box::new(Fn {
-                defaultness: def_(),
+                defaultness,
                 ident,
                 sig,
                 generics,
@@ -603,6 +611,7 @@ 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 {
@@ -1015,6 +1024,7 @@ impl<'a> Parser<'a> {
         if self.check_keyword(exp!(Default))
             && self.look_ahead(1, |t| t.is_non_raw_ident_where(|i| i.name != kw::As))
         {
+            self.psess.gated_spans.gate(sym::specialization, self.token.span);
             self.bump(); // `default`
             Defaultness::Default(self.prev_token_uninterpolated_span())
         } else if self.eat_keyword(exp!(Final)) {

compiler/rustc_parse/src/parser/item.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.rs

@@ -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/auto-traits/ungated-impl.stderr

@@ -1,6 +1,6 @@
-// For historical reasons, auto traits 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.
+// For historical reasons, auto traits don't have an erroring pre-expansion feature gate.
+// We're now at least issuing a warning for those that only exist before macro expansion.
+// FIXME(#154045): Turn this pre-expansion warning into an error and remove the post-expansion gate.
 //                 As part of this, move these test cases into `feature-gate-auto-traits.rs`.
 //@ check-pass
 

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

@@ -1,6 +1,6 @@
-// For historical reasons, box patterns 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.
+// For historical reasons, box patterns don't have an erroring pre-expansion feature gate.
+// We're now at least issuing a warning for those that only exist before macro expansion.
+// FIXME(#154045): Turn this pre-expansion warning into an error and remove the post-expansion gate.
 //                 As part of this, move these test cases into `feature-gate-box_patterns.rs`.
 //@ check-pass
 

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

@@ -1,6 +1,6 @@
-// For historical reasons, decl macros 2.0 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.
+// For historical reasons, decl macros 2.0 don't have an erroring pre-expansion feature gate.
+// We're now at least issuing a warning for those that only exist before macro expansion.
+// FIXME(#154045): Turn this pre-expansion warning into an error and remove the post-expansion gate.
 //                 As part of this, move these test cases into `feature-gate-decl_macro.rs`.
 //@ check-pass
 

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

@@ -0,0 +1,12 @@
+// For historical reasons, negative impls don't have an erroring pre-expansion feature gate.
+// We're now at least issuing a warning for those that only exist before macro expansion.
+// FIXME(#154045): Turn this pre-expansion warning into an error and remove the post-expansion gate.
+//                 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() {}