Rollup merge of #118157 - Nadrieril:never_pat-feature-gate, r=compiler-errors

Add `never_patterns` feature gate

This PR adds the feature gate and most basic parsing for the experimental `never_patterns` feature. See the tracking issue (https://github.com/rust-lang/rust/issues/118155) for details on the experiment.

`@scottmcm` has agreed to be my lang-team liaison for this experiment.

2 files changed, 150 insertions, 0 deletions

diff --git a/tests/ui/pattern/never_patterns.rs b/tests/ui/pattern/never_patterns.rs
new file mode 100644
index 00000000000..e2e17e0e9a7
--- /dev/null
+++ b/tests/ui/pattern/never_patterns.rs
@@ -0,0 +1,99 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum Void {}
+
+fn main() {}
+
+// The classic use for empty types.
+fn safe_unwrap_result<T>(res: Result<T, Void>) {
+    let Ok(_x) = res;
+    // FIXME(never_patterns): These should be allowed
+    let (Ok(_x) | Err(!)) = &res;
+    //~^ ERROR: is not bound in all patterns
+    let (Ok(_x) | Err(&!)) = res.as_ref();
+    //~^ ERROR: is not bound in all patterns
+}
+
+// Check we only accept `!` where we want to.
+fn never_pattern_location(void: Void) {
+    // FIXME(never_patterns): Don't accept on a non-empty type.
+    match Some(0) {
+        None => {}
+        Some(!) => {}
+    }
+    // FIXME(never_patterns): Don't accept on an arbitrary type, even if there are no more branches.
+    match () {
+        () => {}
+        ! => {}
+    }
+    // FIXME(never_patterns): Don't accept even on an empty branch.
+    match None::<Void> {
+        None => {}
+        ! => {}
+    }
+    // FIXME(never_patterns): Let alone if the emptiness is behind a reference.
+    match None::<&Void> {
+        None => {}
+        ! => {}
+    }
+    // Participate in match ergonomics.
+    match &void {
+        ! => {}
+    }
+    match &&void {
+        ! => {}
+    }
+    match &&void {
+        &! => {}
+    }
+    match &None::<Void> {
+        None => {}
+        Some(!) => {}
+    }
+    match None::<&Void> {
+        None => {}
+        Some(!) => {}
+    }
+    // Accept on a composite empty type.
+    match None::<&(u32, Void)> {
+        None => {}
+        Some(&!) => {}
+    }
+    // Accept on an simple empty type.
+    match None::<Void> {
+        None => {}
+        Some(!) => {}
+    }
+    match None::<&Void> {
+        None => {}
+        Some(&!) => {}
+    }
+    match None::<&(u32, Void)> {
+        None => {}
+        Some(&(_, !)) => {}
+    }
+}
+
+fn never_and_bindings() {
+    let x: Result<bool, &(u32, Void)> = Ok(false);
+
+    // FIXME(never_patterns): Never patterns in or-patterns don't need to share the same bindings.
+    match x {
+        Ok(_x) | Err(&!) => {}
+        //~^ ERROR: is not bound in all patterns
+    }
+    let (Ok(_x) | Err(&!)) = x;
+        //~^ ERROR: is not bound in all patterns
+
+    // FIXME(never_patterns): A never pattern mustn't have bindings.
+    match x {
+        Ok(_) => {}
+        Err(&(_b, !)) => {}
+    }
+    match x {
+        Ok(_a) | Err(&(_b, !)) => {}
+        //~^ ERROR: is not bound in all patterns
+        //~| ERROR: is not bound in all patterns
+    }
+}
diff --git a/tests/ui/pattern/never_patterns.stderr b/tests/ui/pattern/never_patterns.stderr
new file mode 100644
index 00000000000..11e50debfd3
--- /dev/null
+++ b/tests/ui/pattern/never_patterns.stderr
@@ -0,0 +1,51 @@
+error[E0408]: variable `_x` is not bound in all patterns
+  --> $DIR/never_patterns.rs:12:19
+   |
+LL |     let (Ok(_x) | Err(!)) = &res;
+   |             --    ^^^^^^ pattern doesn't bind `_x`
+   |             |
+   |             variable not in all patterns
+
+error[E0408]: variable `_x` is not bound in all patterns
+  --> $DIR/never_patterns.rs:14:19
+   |
+LL |     let (Ok(_x) | Err(&!)) = res.as_ref();
+   |             --    ^^^^^^^ pattern doesn't bind `_x`
+   |             |
+   |             variable not in all patterns
+
+error[E0408]: variable `_x` is not bound in all patterns
+  --> $DIR/never_patterns.rs:83:18
+   |
+LL |         Ok(_x) | Err(&!) => {}
+   |            --    ^^^^^^^ pattern doesn't bind `_x`
+   |            |
+   |            variable not in all patterns
+
+error[E0408]: variable `_x` is not bound in all patterns
+  --> $DIR/never_patterns.rs:86:19
+   |
+LL |     let (Ok(_x) | Err(&!)) = x;
+   |             --    ^^^^^^^ pattern doesn't bind `_x`
+   |             |
+   |             variable not in all patterns
+
+error[E0408]: variable `_b` is not bound in all patterns
+  --> $DIR/never_patterns.rs:95:9
+   |
+LL |         Ok(_a) | Err(&(_b, !)) => {}
+   |         ^^^^^^         -- variable not in all patterns
+   |         |
+   |         pattern doesn't bind `_b`
+
+error[E0408]: variable `_a` is not bound in all patterns
+  --> $DIR/never_patterns.rs:95:18
+   |
+LL |         Ok(_a) | Err(&(_b, !)) => {}
+   |            --    ^^^^^^^^^^^^^ pattern doesn't bind `_a`
+   |            |
+   |            variable not in all patterns
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0408`.