Rollup merge of #154475 - fmease:soft-gate-box-struct-field-pat, r=Kivooeo

Emit a pre-expansion feature gate warning for `box`'ed struct field patterns

While the following code triggers a feature gate *warning*:

```rs
fn f() {
    #[cfg(false)]
    let box x; //~ WARN box pattern syntax is experimental
}
```

the code below does not (on stable & main):

```rs
fn f() {
    #[cfg(false)]
    let Struct { box x };
}
```

This is an oversight as both are part of the unstable feature `box_patterns` (that isn't properly gated pre expansion for historical reasons). Of course, both forms lead to a feature gate error *post expansion*.

This is a bug fix and doesn't need any input from T-compiler or T-lang. For context, emitting warnings in these cases is legitimized by [MCP 535](https://github.com/rust-lang/compiler-team/issues/535)[^1].

Part of #154045.

[^1]: In case you're wondering why the MCP talks about a *lint* even though the feature gate warnings as seen today don't reference any lint by name, read #154045 (comment).

Author: RalfJung

compiler/rustc_parse/src/parser/pat.rs

@@ -1749,6 +1749,9 @@ impl<'a> Parser<'a> {
         } else {
             // Parsing a pattern of the form `(box) (ref) (mut) fieldname`.
             let is_box = self.eat_keyword(exp!(Box));
+            if is_box {
+                self.psess.gated_spans.gate(sym::box_patterns, self.prev_token.span);
+            }
             let boxed_span = self.token.span;
             let mutability = self.parse_mutability();
             let by_ref = self.parse_byref();

tests/ui/auto-traits/pre-cfg.rs

@@ -1,4 +1,9 @@
 fn main() {
     let box x = Box::new('c'); //~ ERROR box pattern syntax is experimental
-    println!("x: {}", x);
+    let _: char = x;
+
+    struct Packet { x: Box<i32> }
+
+    let Packet { box x } = Packet { x: Box::new(0) }; //~ ERROR box pattern syntax is experimental
+    let _: i32 = x;
 }

tests/ui/feature-gates/feature-gate-box_patterns.rs

@@ -8,6 +8,16 @@ LL |     let box x = Box::new('c');
    = help: add `#![feature(box_patterns)]` 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    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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
+error[E0658]: box pattern syntax is experimental
+  --> $DIR/feature-gate-box_patterns.rs:7:18
+   |
+LL |     let Packet { box x } = Packet { x: Box::new(0) };
+   |                  ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` 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
 
 For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-box_patterns.stderr

@@ -0,0 +1,12 @@
+// 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.
+//                 As part of this, move these test cases into `feature-gate-auto-traits.rs`.
+//@ check-pass
+
+#[cfg(false)]
+auto trait Foo {}
+//~^ WARN `auto` traits are unstable
+//~| WARN unstable syntax can change at any point in the future
+
+fn main() {}

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

@@ -1,5 +1,5 @@
 warning: `auto` traits are unstable
-  --> $DIR/pre-cfg.rs:4:1
+  --> $DIR/soft-feature-gate-auto_traits.rs:8:1
    |
 LL | auto trait Foo {}
    | ^^^^

tests/ui/auto-traits/pre-cfg.stderr …tes/soft-feature-gate-auto_traits.stderrtests/ui/auto-traits/pre-cfg.stderr renamed to tests/ui/feature-gates/soft-feature-gate-auto_traits.stderr tests/ui/auto-traits/pre-cfg.stderr renamed to tests/ui/feature-gates/soft-feature-gate-auto_traits.stderr

@@ -0,0 +1,17 @@
+// 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.
+//                 As part of this, move these test cases into `feature-gate-box_patterns.rs`.
+//@ check-pass
+
+fn main() {
+    #[cfg(false)]
+    let box x;
+    //~^ WARN box pattern syntax is experimental
+    //~| WARN unstable syntax can change at any point in the future
+
+    #[cfg(false)]
+    let Packet { box x };
+    //~^ WARN box pattern syntax is experimental
+    //~| WARN unstable syntax can change at any point in the future
+}

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

@@ -0,0 +1,26 @@
+warning: box pattern syntax is experimental
+  --> $DIR/soft-feature-gate-box_patterns.rs:9:9
+   |
+LL |     let box x;
+   |         ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` 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: box pattern syntax is experimental
+  --> $DIR/soft-feature-gate-box_patterns.rs:14:18
+   |
+LL |     let Packet { box x };
+   |                  ^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` 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: 2 warnings emitted
+

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

@@ -0,0 +1,17 @@
+// 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.
+//                 As part of this, move these test cases into `feature-gate-decl_macro.rs`.
+//@ check-pass
+
+#[cfg(false)]
+macro make() {}
+//~^ WARN `macro` is experimental
+//~| WARN unstable syntax can change at any point in the future
+
+#[cfg(false)]
+macro create { () => {} }
+//~^ WARN `macro` is experimental
+//~| WARN unstable syntax can change at any point in the future
+
+fn main() {}

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

@@ -0,0 +1,26 @@
+warning: `macro` is experimental
+  --> $DIR/soft-feature-gate-decl_macro.rs:8:1
+   |
+LL | macro make() {}
+   | ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` 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: `macro` is experimental
+  --> $DIR/soft-feature-gate-decl_macro.rs:13:1
+   |
+LL | macro create { () => {} }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` 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: 2 warnings emitted
+