Auto merge of #13167 - iDawer:exhaustive_patterns, r=Veykril

feat: Implement `feature(exhaustive_patterns)` from unstable Rust

Closes #12753

Recognize Rust's unstable `#![feature(exhaustive_patterns)]` (RFC 1872). Allow omitting visibly uninhabited variants from `match` expressions when the feature is on.

This adjusts match checking to the current implementation of the postponed RFC 1872 in rustc.

crates/ide-diagnostics/src/handlers/missing_match_arms.rs

@@ -947,6 +947,50 @@ fn f() {
         );
     }
 
+    mod rust_unstable {
+        use super::*;
+
+        #[test]
+        fn rfc_1872_exhaustive_patterns() {
+            check_diagnostics_no_bails(
+                r"
+//- minicore: option, result
+#![feature(exhaustive_patterns)]
+enum Void {}
+fn test() {
+    match None::<!> { None => () }
+    match Result::<u8, !>::Ok(2) { Ok(_) => () }
+    match Result::<u8, Void>::Ok(2) { Ok(_) => () }
+    match (2, loop {}) {}
+    match Result::<!, !>::Ok(loop {}) {}
+    match (&loop {}) {} // https://github.com/rust-lang/rust/issues/50642#issuecomment-388234919
+    //    ^^^^^^^^^^ error: missing match arm: type `&!` is non-empty
+}",
+            );
+        }
+
+        #[test]
+        fn rfc_1872_private_uninhabitedness() {
+            check_diagnostics_no_bails(
+                r"
+//- minicore: option
+//- /lib.rs crate:lib
+#![feature(exhaustive_patterns)]
+pub struct PrivatelyUninhabited { private_field: Void }
+enum Void {}
+fn test_local(x: Option<PrivatelyUninhabited>) {
+    match x {}
+} //      ^ error: missing match arm: `None` not covered
+//- /main.rs crate:main deps:lib
+#![feature(exhaustive_patterns)]
+fn test(x: Option<lib::PrivatelyUninhabited>) {
+    match x {}
+    //    ^ error: missing match arm: `None` and `Some(_)` not covered
+}",
+            );
+        }
+    }
+
     mod false_negatives {
         //! The implementation of match checking here is a work in progress. As we roll this out, we
         //! prefer false negatives to false positives (ideally there would be no false positives). This