Merge #4175

4175: Introduce HirDisplay method for rendering source code & use it in add_function assist r=flodiebold a=TimoFreiberg

Next feature for #3639.

So far the only change in the new `HirDisplay` method is that paths are qualified, but more changes will be necessary (omitting the function name from function types, returning an error instead of printing `"{unknown}"`, probably more).

Is that approach okay?

Co-authored-by: Timo Freiberg <timo.freiberg@gmail.com>

crates/ra_hir_ty/src/tests/display_source_code.rs

@@ -0,0 +1,23 @@
+use super::displayed_source_at_pos;
+use crate::test_db::TestDB;
+use ra_db::fixture::WithFixture;
+
+#[test]
+fn qualify_path_to_submodule() {
+    let (db, pos) = TestDB::with_position(
+        r#"
+//- /main.rs
+
+mod foo {
+    pub struct Foo;
+}
+
+fn bar() {
+    let foo: foo::Foo = foo::Foo;
+    foo<|>
+}
+
+"#,
+    );
+    assert_eq!("foo::Foo", displayed_source_at_pos(&db, pos));
+}