Add note regarding parent module containing use statement.

Author: Mark-Simulacrum

src/librustc_resolve/diagnostics.rs

@@ -838,6 +838,32 @@ trait Foo {
 
 fn foo<T>(x: T) {} // ok!
 ```
+
+Another case that causes this error is when a type is imported into a parent
+module. To fix this, you can follow the suggestion and use File directly or
+`use super::File;` which will import the types from the parent namespace. An
+example that causes this error is below:
+
+```compile_fail,E0412
+use std::fs::File;
+
+mod foo {
+    fn some_function(f: File) {}
+}
+```
+
+```
+use std::fs::File;
+
+mod foo {
+    // either
+    use super::File;
+    // or
+    // use std::fs::File;
+    fn foo(f: File) {}
+}
+# fn main() {} // don't insert it for us; that'll break imports
+```
 "##,
 
 E0415: r##"