Remove suggestion from importing unstable items on stable channel

compiler/rustc_resolve/src/diagnostics.rs

@@ -1609,6 +1609,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             }
         }
 
+        suggestions.retain(|suggestion| suggestion.is_stable || self.tcx.sess.is_nightly_build());
         suggestions
     }
 

tests/ui/resolve/suggest-unresolve-nightly-149402.rs

@@ -0,0 +1,9 @@
+//@ only-nightly
+
+fn foo() {
+    let x = Vec::new();
+    x.push(Complete::Item { name: "hello" });
+    //~^ ERROR failed to resolve: use of undeclared type `Complete`
+}
+
+fn main() {}

tests/ui/resolve/suggest-unresolve-nightly-149402.stderr

@@ -0,0 +1,15 @@
+error[E0433]: failed to resolve: use of undeclared type `Complete`
+  --> $DIR/suggest-unresolve-nightly-149402.rs:5:12
+   |
+LL |     x.push(Complete::Item { name: "hello" });
+   |            ^^^^^^^^ use of undeclared type `Complete`
+   |
+help: there is an enum variant `std::ops::CoroutineState::Complete`; try using the variant's enum
+   |
+LL -     x.push(Complete::Item { name: "hello" });
+LL +     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/ui/resolve/suggest-unresolve-stable-149402.rs

@@ -0,0 +1,9 @@
+//@ only-stable
+
+fn foo() {
+    let x = Vec::new();
+    x.push(Complete::Item { name: "hello" });
+    //~^ ERROR failed to resolve: use of undeclared type `Complete`
+}
+
+fn main() {}

tests/ui/resolve/suggest-unresolve-stable-149402.stderr

@@ -0,0 +1,9 @@
+error[E0433]: failed to resolve: use of undeclared type `Complete`
+  --> $DIR/suggest-unresolve-stable-149402.rs:5:12
+   |
+LL |     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`.