Emit pre-expansion feature gate warning for item modifier default

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -224,7 +224,7 @@ fn visit_item(&mut self, i: &'a ast::Item) {
                 }
 
                 if let ast::Defaultness::Default(_) = of_trait.defaultness {
-                    gate!(self, specialization, i.span, "specialization is unstable");
+                    gate!(self, specialization, i.span, "specialization is experimental");
                 }
             }
 
@@ -450,7 +450,7 @@ fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
                 self.features.specialization() || (is_fn && self.features.min_specialization()),
                 sym::specialization,
                 i.span,
-                "specialization is unstable"
+                "specialization is experimental"
             );
         }
         visit::walk_assoc_item(self, i, ctxt)
@@ -615,10 +615,21 @@ macro_rules! soft_gate_all_legacy_dont_use {
     soft_gate_all_legacy_dont_use!(box_patterns, "box pattern syntax is experimental");
     soft_gate_all_legacy_dont_use!(decl_macro, "`macro` is experimental");
     soft_gate_all_legacy_dont_use!(negative_impls, "negative impls are experimental");
+    soft_gate_all_legacy_dont_use!(specialization, "specialization is experimental");
     soft_gate_all_legacy_dont_use!(trait_alias, "trait aliases are experimental");
     soft_gate_all_legacy_dont_use!(try_blocks, "`try` blocks are unstable");
     // tidy-alphabetical-end
 
+    for &span in spans.get(&sym::min_specialization).into_iter().flatten() {
+        if !visitor.features.specialization()
+            && !visitor.features.min_specialization()
+            && !span.allows_unstable(sym::specialization)
+            && !span.allows_unstable(sym::min_specialization)
+        {
+            feature_warn(visitor.sess, sym::specialization, span, "specialization is experimental");
+        }
+    }
+
     // -----------------------------------------------------------------------------
 
     visit::walk_crate(&mut visitor, krate);

compiler/rustc_parse/src/parser/item.rs

@@ -248,10 +248,18 @@ fn parse_item_kind(
             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,
@@ -1016,6 +1024,7 @@ fn parse_defaultness(&mut self) -> Defaultness {
         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)) {

tests/ui/macros/stringify.rs

@@ -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

tests/ui/parser/trait-item-with-defaultness-pass.rs

@@ -1,4 +1,5 @@
 //@ check-pass
+#![feature(specialization)]
 
 fn main() {}
 

tests/ui/specialization/feature-gate-specialization.rs

@@ -0,0 +1,21 @@
+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
+}
+
+trait OtherTrait {
+    fn fn_();
+}
+
+default impl<T> OtherTrait for T { //~ ERROR specialization is experimental
+    fn fn_() {}
+}
+
+fn main() {}

tests/ui/specialization/feature-gate-specialization.stderr

@@ -0,0 +1,45 @@
+error[E0658]: specialization is experimental
+  --> $DIR/feature-gate-specialization.rs:8:5
+   |
+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:9:5
+   |
+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:10:5
+   |
+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[E0658]: specialization is experimental
+  --> $DIR/feature-gate-specialization.rs:17:1
+   |
+LL | / default impl<T> OtherTrait for T {
+LL | |     fn fn_() {}
+LL | | }
+   | |_^
+   |
+   = 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 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/specialization/soft-feature-gate-specialization.default.stderr

@@ -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
+

tests/ui/specialization/soft-feature-gate-specialization.min.stderr

@@ -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
+

tests/ui/specialization/soft-feature-gate-specialization.rs

@@ -0,0 +1,44 @@
+// For historical reasons, item modifier `default` doesn'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-specialization.rs`.
+//
+// Moreover, `specialization` implies `min_specialization` similar to the post-expansion gate.
+//
+// However, while we only gate `default` *associated* functions only behind `min_specialization` OR
+// `specialization` in the post-expansion case, in the pre-expansion case we gate all kinds of
+// functions (free, assoc, foreign) behind `min_specialization` OR `specialization` if marked with
+// `default` for simplicity of implementation. Ultimately it doesn't matter since we later reject
+// `default` on anything other than impls & impl assoc items during semantic analysis.
+//
+//@ revisions: default min full
+//@ check-pass
+#![cfg_attr(min, feature(min_specialization))]
+#![cfg_attr(full, feature(specialization))]
+
+#[cfg(false)]
+impl Trait for () {
+    default type Ty = ();
+    //[default,min]~^ WARN specialization is experimental
+    //[default,min]~| WARN unstable syntax can change at any point in the future
+    default const CT: () = ();
+    //[default,min]~^ WARN specialization is experimental
+    //[default,min]~| WARN unstable syntax can change at any point in the future
+    default fn fn_();
+    //[default]~^ WARN specialization is experimental
+    //[default]~| WARN unstable syntax can change at any point in the future
+}
+
+// While free ty/ct/fn items marked `default` are
+// semantically malformed we still need to gate the keyword!
+#[cfg(false)]
+default fn fn_() {}
+//[default]~^ WARN specialization is experimental
+//[default]~| WARN unstable syntax can change at any point in the future
+
+#[cfg(false)]
+default impl Trait for () {}
+//[default,min]~^ WARN specialization is experimental
+//[default,min]~| WARN unstable syntax can change at any point in the future
+
+fn main() {}

tests/ui/specialization/specialization-feature-gate-default.rs

@@ -1,13 +0,0 @@
-// Check that specialization must be ungated to use the `default` keyword
-
-// gate-test-specialization
-
-trait Foo {
-    fn foo(&self);
-}
-
-impl<T> Foo for T {
-    default fn foo(&self) {} //~ ERROR specialization is unstable
-}
-
-fn main() {}

tests/ui/specialization/specialization-feature-gate-default.stderr

@@ -1,13 +0,0 @@
-error[E0658]: specialization is unstable
-  --> $DIR/specialization-feature-gate-default.rs:10:5
-   |
-LL |     default fn foo(&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 1 previous error
-
-For more information about this error, try `rustc --explain E0658`.

tests/ui/track-diagnostics/track6.rs

@@ -12,7 +12,7 @@ pub trait Foo {
 
 impl<T> Foo for T {
     default fn bar() {}
-    //~^ ERROR specialization is unstable
+    //~^ ERROR specialization is experimental
     //~| NOTE created at
 }
 

tests/ui/track-diagnostics/track6.stderr

@@ -1,4 +1,4 @@
-error[E0658]: specialization is unstable
+error[E0658]: specialization is experimental
   --> $DIR/track6.rs:LL:CC
    |
 LL |     default fn bar() {}