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Emit pre-expansion feature gate warnings for negative impls and specialization #154527

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089c361
Emit pre-expansion feature gate warning for negative impls
fmease Mar 29, 2026
9a3ea7d
Emit pre-expansion feature gate warning for item modifier `default`
fmease Mar 28, 2026
8ba153b
Tidy up the feature gating code in AST passes
fmease Mar 28, 2026
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360 changes: 173 additions & 187 deletions compiler/rustc_ast_passes/src/feature_gate.rs
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9 changes: 8 additions & 1 deletion compiler/rustc_parse/src/parser/item.rs
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Expand Up @@ -248,10 +248,15 @@ 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 {
self.psess.gated_spans.gate(sym::min_specialization, span);
self.psess.gated_spans.ungate_last(sym::specialization, span);
}
Comment on lines +252 to +255
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@fmease fmease Mar 29, 2026 •
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The idea is: Enabling feat specialization should imply min_specialization (the latter only supports the specialization of associated functions, not associated constants & types however). That's how the post-expansion feature gate already works (it visits the AST).

For the soft pre-expansion feature gate we basically want the same thing. However, I don't want to pass a param to the item parser that tells it if the item is meant to be free, associated or foreign. Instead, all 3 fn item kinds are now part of min_specialization from a pre-expansion perspective if they're marked default which is absolutely fine (default on free & foreign items gets rejected later on during AST validation).

We ungate specialization here since later in rustc_ast_passes::feature_gate, a span gated behind specialization requires feature(specialization) … which is undesirable for assoc fns ofc. Finally, in AST passes we allow feature(specialization) to unlock spans gated behind specialization and ones behind min_specialization. This leads to the desired semantics.

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@nnethercote nnethercote Mar 30, 2026

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Seems like some/all of this information should be a comment in the code?

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,
Expand Down Expand Up @@ -603,6 +608,7 @@ 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 {
Expand Down Expand Up @@ -1015,6 +1021,7 @@ 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)) {
Expand Down
7 changes: 0 additions & 7 deletions tests/ui/auto-traits/ungated-impl.rs
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23 changes: 0 additions & 23 deletions tests/ui/auto-traits/ungated-impl.stderr
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11 changes: 2 additions & 9 deletions tests/ui/feature-gates/feature-gate-auto-traits.rs
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Original file line number Diff line number Diff line change
@@ -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
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@fmease fmease Mar 29, 2026 •
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A gate test for negative impls doesn't belong in a gate test file for auto traits. Likely a remnant of the time when negative impls and auto traits were part of the same feature, optin_builtin_traits.

pub unsafe auto trait AnotherAutoTrait {} //~ ERROR auto traits are experimental and possibly buggy

fn main() {}
16 changes: 8 additions & 8 deletions tests/ui/feature-gates/feature-gate-auto-traits.stderr
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@@ -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
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@nnethercote nnethercote Mar 30, 2026

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Is the marker types suggestion no longer true? That seems like useful information.

--> $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
Expand Down
12 changes: 12 additions & 0 deletions tests/ui/feature-gates/soft-feature-gate-negative_impls.rs
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@@ -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`.
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@nnethercote nnethercote Mar 30, 2026

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This comment confused me. Isn't this PR all about adding a proper pre-expansion feature gate? I guess it's not "proper" yet? I don't understand the difference.

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@fmease fmease Mar 30, 2026 •
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It's not "proper" yet as it's merely a warning for now (as backed by the MCP). Upgrading it to a hard error is the next step at some point in the future that's gonna be much harder to do (crater, fixing fallout downstream, T-lang nomination).

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Can you change "proper pre-expansion one" to "a pre-expansion hard error", or similar?

//@ 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() {}
14 changes: 14 additions & 0 deletions tests/ui/feature-gates/soft-feature-gate-negative_impls.stderr
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@@ -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

20 changes: 10 additions & 10 deletions tests/ui/impl-trait/precise-capturing/bound-modifiers.stderr
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Expand Up @@ -46,6 +46,16 @@ error[E0405]: cannot find trait `r#use` in this scope
LL | fn binder() -> impl Sized + for<'a> use<> {}
| ^^^ not found in this scope

error[E0658]: const trait impls are experimental
--> $DIR/bound-modifiers.rs:12:32
|
LL | fn constness() -> impl Sized + const use<> {}
| ^^^^^
|
= note: see issue #143874 <https://github.com/rust-lang/rust/issues/143874> for more information
= help: add `#![feature(const_trait_impl)]` 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]: `async` trait bounds are unstable
--> $DIR/bound-modifiers.rs:7:32
|
Expand All @@ -57,16 +67,6 @@ LL | fn asyncness() -> impl Sized + async use<> {}
= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
= help: use the desugared name of the async trait, such as `AsyncFn`

error[E0658]: const trait impls are experimental
--> $DIR/bound-modifiers.rs:12:32
|
LL | fn constness() -> impl Sized + const use<> {}
| ^^^^^
|
= note: see issue #143874 <https://github.com/rust-lang/rust/issues/143874> for more information
= help: add `#![feature(const_trait_impl)]` 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 10 previous errors

Some errors have detailed explanations: E0405, E0658.
Expand Down
3 changes: 2 additions & 1 deletion tests/ui/macros/stringify.rs
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Expand Up @@ -9,12 +9,13 @@
#![feature(const_trait_impl)]
#![feature(coroutines)]
#![feature(decl_macro)]
#![feature(macro_guard_matcher)]
#![feature(more_qualified_paths)]
#![feature(never_patterns)]
#![feature(specialization)]
#![feature(trait_alias)]
#![feature(try_blocks)]
#![feature(yeet_expr)]
#![feature(macro_guard_matcher)]
#![deny(unused_macros)]

// These macros force the use of AST pretty-printing by converting the input to
Expand Down
1 change: 1 addition & 0 deletions tests/ui/parser/trait-item-with-defaultness-pass.rs
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@@ -1,4 +1,5 @@
//@ check-pass
#![feature(specialization)]

fn main() {}

Expand Down
2 changes: 1 addition & 1 deletion tests/ui/specialization/defaultimpl/specialization-feature-gate-default.rs
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Expand Up @@ -4,7 +4,7 @@ trait Foo {
fn foo(&self);
}

default impl<T> Foo for T { //~ ERROR specialization is unstable
default impl<T> Foo for T { //~ ERROR specialization is experimental
fn foo(&self) {}
}

Expand Down
2 changes: 1 addition & 1 deletion tests/ui/specialization/defaultimpl/specialization-feature-gate-default.stderr
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@@ -1,4 +1,4 @@
error[E0658]: specialization is unstable
error[E0658]: specialization is experimental
--> $DIR/specialization-feature-gate-default.rs:7:1
|
LL | / default impl<T> Foo for T {
Expand Down
15 changes: 15 additions & 0 deletions tests/ui/specialization/feature-gate-specialization.rs
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@@ -0,0 +1,15 @@
// The gate for `default impl` is exercised in `defaultimpl/specialization-feature-gate-default.rs`.

trait Trait {
type Ty;
const CT: ();
fn fn_(&self);
}

impl<T> Trait for T {
default type Ty = (); //~ ERROR specialization is experimental
default const CT: () = (); //~ ERROR specialization is experimental
default fn fn_(&self) {} //~ ERROR specialization is experimental
}

fn main() {}
33 changes: 33 additions & 0 deletions tests/ui/specialization/feature-gate-specialization.stderr
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@@ -0,0 +1,33 @@
error[E0658]: specialization is experimental
--> $DIR/feature-gate-specialization.rs:10:5
|
LL | default type Ty = ();
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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]: specialization is experimental
--> $DIR/feature-gate-specialization.rs:11:5
|
LL | default const CT: () = ();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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]: specialization is experimental
--> $DIR/feature-gate-specialization.rs:12:5
|
LL | default fn fn_(&self) {}
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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 3 previous errors

For more information about this error, try `rustc --explain E0658`.
62 changes: 62 additions & 0 deletions tests/ui/specialization/soft-feature-gate-specialization.default.stderr
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@@ -0,0 +1,62 @@
warning: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:21:5
|
LL | default type Ty = ();
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:24:5
|
LL | default const CT: () = ();
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:40:1
|
LL | default impl Trait for () {}
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:27:5
|
LL | default fn fn_();
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:35:1
|
LL | default fn fn_() {}
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: 5 warnings emitted

38 changes: 38 additions & 0 deletions tests/ui/specialization/soft-feature-gate-specialization.min.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
warning: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:21:5
|
LL | default type Ty = ();
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:24:5
|
LL | default const CT: () = ();
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: specialization is experimental
--> $DIR/soft-feature-gate-specialization.rs:40:1
|
LL | default impl Trait for () {}
| ^^^^^^^
|
= note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
= help: add `#![feature(specialization)]` 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: 3 warnings emitted

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