Merge #9038

9038: Folding range for return types r=Veykril a=MozarellaMan

For issue #8957 

![return type fold](https://user-images.githubusercontent.com/48062697/119979082-5c62e100-bfb2-11eb-9729-1dea1ce74de1.gif)


Co-authored-by: Ayomide Bamidele <48062697+MozarellaMan@users.noreply.github.com>

crates/ide/src/folding_ranges.rs

@@ -19,6 +19,7 @@ pub enum FoldKind {
     Statics,
     Array,
     WhereClause,
+    ReturnType,
 }
 
 #[derive(Debug)]
@@ -131,6 +132,7 @@ fn fold_kind(kind: SyntaxKind) -> Option<FoldKind> {
         COMMENT => Some(FoldKind::Comment),
         ARG_LIST | PARAM_LIST => Some(FoldKind::ArgList),
         ARRAY_EXPR => Some(FoldKind::Array),
+        RET_TYPE => Some(FoldKind::ReturnType),
         ASSOC_ITEM_LIST
         | RECORD_FIELD_LIST
         | RECORD_PAT_FIELD_LIST
@@ -300,6 +302,7 @@ mod tests {
                 FoldKind::Statics => "statics",
                 FoldKind::Array => "array",
                 FoldKind::WhereClause => "whereclause",
+                FoldKind::ReturnType => "returntype",
             };
             assert_eq!(kind, &attr.unwrap());
         }
@@ -560,4 +563,18 @@ where
 "#,
         )
     }
+
+    #[test]
+    fn fold_return_type() {
+        check(
+            r#"
+fn foo()<fold returntype>-> (
+    bool,
+    bool,
+)</fold> { (true, true) }
+
+fn bar() -> (bool, bool) { (true, true) }
+            "#,
+        )
+    }
 }

crates/rust-analyzer/src/to_proto.rs

@@ -534,6 +534,7 @@ pub(crate) fn folding_range(
         | FoldKind::Consts
         | FoldKind::Statics
         | FoldKind::WhereClause
+        | FoldKind::ReturnType
         | FoldKind::Array => None,
     };