Rollup merge of #132668 - ehuss:yield-gate-2024, r=davidtwco

Feature gate yield expressions not in 2024

This changes it so that yield expressions are no longer allowed in the 2024 edition without a feature gate. We are currently only reserving the `gen` keyword in the 2024 edition, and not allowing anything else to be implicitly enabled by the edition.

In practice this doesn't have a significant difference since yield expressions can't really be used outside of coroutines or gen blocks, which have their own feature gates. However, it does affect what is accepted pre-expansion, and I would feel more comfortable not allowing yield expressions.

I believe the stabilization process for gen blocks or coroutines will not need to check the edition here, so this shouldn't ever be needed.

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -523,9 +523,18 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         "consider removing `for<...>`"
     );
     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
-    for &span in spans.get(&sym::yield_expr).iter().copied().flatten() {
-        if !span.at_least_rust_2024() {
-            gate!(&visitor, coroutines, span, "yield syntax is experimental");
+    // yield can be enabled either by `coroutines` or `gen_blocks`
+    if let Some(spans) = spans.get(&sym::yield_expr) {
+        for span in spans {
+            if (!visitor.features.coroutines() && !span.allows_unstable(sym::coroutines))
+                && (!visitor.features.gen_blocks() && !span.allows_unstable(sym::gen_blocks))
+            {
+                #[allow(rustc::untranslatable_diagnostic)]
+                // Don't know which of the two features to include in the
+                // error message, so I am arbitrarily picking one.
+                feature_err(&visitor.sess, sym::coroutines, *span, "yield syntax is experimental")
+                    .emit();
+            }
         }
     }
     gate_all!(gen_blocks, "gen blocks are experimental");

tests/ui/feature-gates/feature-gate-coroutines.e2024.stderr

@@ -8,6 +8,46 @@ LL |     yield true;
    = help: add `#![feature(coroutines)]` 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    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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]: yield syntax is experimental
+  --> $DIR/feature-gate-coroutines.rs:10:16
+   |
+LL |     let _ = || yield true;
+   |                ^^^^^^^^^^
+   |
+   = note: see issue #43122 <https://github.com/rust-lang/rust/issues/43122> for more information
+   = help: add `#![feature(coroutines)]` 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]: yield syntax is experimental
+  --> $DIR/feature-gate-coroutines.rs:18:5
+   |
+LL |     yield;
+   |     ^^^^^
+   |
+   = note: see issue #43122 <https://github.com/rust-lang/rust/issues/43122> for more information
+   = help: add `#![feature(coroutines)]` 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]: yield syntax is experimental
+  --> $DIR/feature-gate-coroutines.rs:19:5
+   |
+LL |     yield 0;
+   |     ^^^^^^^
+   |
+   = note: see issue #43122 <https://github.com/rust-lang/rust/issues/43122> for more information
+   = help: add `#![feature(coroutines)]` 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]: yield syntax is experimental
+  --> $DIR/feature-gate-coroutines.rs:5:5
+   |
+LL |     yield true;
+   |     ^^^^^^^^^^
+   |
+   = note: see issue #43122 <https://github.com/rust-lang/rust/issues/43122> for more information
+   = help: add `#![feature(coroutines)]` 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: `yield` can only be used in `#[coroutine]` closures, or `gen` blocks
   --> $DIR/feature-gate-coroutines.rs:5:5
    |
@@ -46,7 +86,7 @@ error[E0627]: yield expression outside of coroutine literal
 LL |     yield true;
    |     ^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: aborting due to 9 previous errors
 
 Some errors have detailed explanations: E0627, E0658.
 For more information about an error, try `rustc --explain E0627`.

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

@@ -4,17 +4,17 @@
 fn main() {
     yield true; //~ ERROR yield syntax is experimental
                 //~^ ERROR yield expression outside of coroutine literal
-                //[none]~^^ ERROR yield syntax is experimental
+                //~^^ ERROR yield syntax is experimental
                 //~^^^ ERROR `yield` can only be used
 
     let _ = || yield true; //~ ERROR yield syntax is experimental
-    //[none]~^ ERROR yield syntax is experimental
+    //~^ ERROR yield syntax is experimental
     //~^^ ERROR `yield` can only be used
 }
 
 #[cfg(FALSE)]
 fn foo() {
     // Ok in 2024 edition
-    yield; //[none]~ ERROR yield syntax is experimental
-    yield 0; //[none]~ ERROR yield syntax is experimental
+    yield; //~ ERROR yield syntax is experimental
+    yield 0; //~ ERROR yield syntax is experimental
 }