add test for rust-lang#71450

Author: surechen

tests/ui/lint/use-redundant/use-redundant-issue-71450.rs

@@ -0,0 +1,45 @@
+//@ check-pass
+
+#![warn(unused_imports)]
+
+mod foo {
+    use std::fmt;
+
+    pub struct String;
+
+    impl String {
+        pub fn new() -> String {
+            String{}
+        }
+    }
+
+    impl fmt::Display for String {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            write!(f, "String")
+        }
+    }
+}
+
+fn main() {
+
+    {
+        use std::string::String; //~ WARNING the item `String` is imported redundantly
+        // 'String' from 'std::string::String'.
+        let s = String::new();
+        println!("{}", s);
+    }
+
+    {
+        // 'String' from 'std::string::String'.
+        let s = String::new();
+        println!("{}", s);
+    }
+
+    {
+        use foo::*;
+        // 'String' from 'foo::String'.
+        let s = String::new();
+        println!("{}", s);
+    }
+
+}

tests/ui/lint/use-redundant/use-redundant-issue-71450.stderr

@@ -0,0 +1,17 @@
+warning: the item `String` is imported redundantly
+  --> $DIR/use-redundant-issue-71450.rs:26:13
+   |
+LL |         use std::string::String;
+   |             ^^^^^^^^^^^^^^^^^^^
+  --> $SRC_DIR/std/src/prelude/mod.rs:LL:COL
+   |
+   = note: the item `String` is already defined here
+   |
+note: the lint level is defined here
+  --> $DIR/use-redundant-issue-71450.rs:3:9
+   |
+LL | #![warn(unused_imports)]
+   |         ^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+