Remove suggestion from importing unstable items on stable channel

compiler/rustc_resolve/src/diagnostics.rs

@@ -1610,6 +1610,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             }
         }
 
+        suggestions.retain(|suggestion| suggestion.is_stable || self.tcx.sess.is_nightly_build());
         suggestions
     }
 

tests/run-make/issue-149402-suggest-unresolve/foo.rs

@@ -0,0 +1,6 @@
+fn foo() {
+    let x = Vec::new();
+    x.push(Complete::Item { name: "hello" });
+}
+
+fn main() {}

tests/run-make/issue-149402-suggest-unresolve/nightly.err

@@ -0,0 +1,18 @@
+error[E0433]: failed to resolve: use of undeclared type `Complete`
+ --> foo.rs:3:12
+  |
+3 |     x.push(Complete::Item { name: "hello" });
+  |            ^^^^^^^^ use of undeclared type `Complete`
+  |
+help: there is an enum variant `core::ops::CoroutineState::Complete` and 1 other; try using the variant's enum
+  |
+3 -     x.push(Complete::Item { name: "hello" });
+3 +     x.push(core::ops::CoroutineState::Item { name: "hello" });
+  |
+3 -     x.push(Complete::Item { name: "hello" });
+3 +     x.push(std::ops::CoroutineState::Item { name: "hello" });
+  |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0433`.

tests/run-make/issue-149402-suggest-unresolve/output.diff

@@ -0,0 +1,16 @@
+@@ -3,6 +3,15 @@
+   |
+ 3 |     x.push(Complete::Item { name: "hello" });
+   |            ^^^^^^^^ use of undeclared type `Complete`
++  |
++help: there is an enum variant `core::ops::CoroutineState::Complete` and 1 other; try using the variant's enum
++  |
++3 -     x.push(Complete::Item { name: "hello" });
++3 +     x.push(core::ops::CoroutineState::Item { name: "hello" });
++  |
++3 -     x.push(Complete::Item { name: "hello" });
++3 +     x.push(std::ops::CoroutineState::Item { name: "hello" });
++  |
+
+ error: aborting due to 1 previous error
+

tests/run-make/issue-149402-suggest-unresolve/rmake.rs

@@ -0,0 +1,29 @@
+//! Check that unstable name-resolution suggestions are omitted on stable.
+//!
+//! Regression test for <https://github.com/rust-lang/rust/issues/149402>.
+//!
+//@ only-nightly
+//@ needs-target-std
+
+use run_make_support::{diff, rustc, similar};
+
+fn main() {
+    let stable_like = rustc()
+        .env("RUSTC_BOOTSTRAP", "-1")
+        .edition("2024")
+        .input("foo.rs")
+        .run_fail()
+        .stderr_utf8();
+
+    assert!(!stable_like.contains("CoroutineState::Complete"));
+    diff().expected_file("stable.err").actual_text("stable_like", &stable_like).run();
+
+    let nightly = rustc().edition("2024").input("foo.rs").run_fail().stderr_utf8();
+
+    assert!(nightly.contains("CoroutineState::Complete"));
+    diff().expected_file("nightly.err").actual_text("nightly", &nightly).run();
+
+    let stderr_diff =
+        similar::TextDiff::from_lines(&stable_like, &nightly).unified_diff().to_string();
+    diff().expected_file("output.diff").actual_text("diff", stderr_diff).run();
+}

tests/run-make/issue-149402-suggest-unresolve/stable.err

@@ -0,0 +1,9 @@
+error[E0433]: failed to resolve: use of undeclared type `Complete`
+ --> foo.rs:3:12
+  |
+3 |     x.push(Complete::Item { name: "hello" });
+  |            ^^^^^^^^ use of undeclared type `Complete`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0433`.