3 files changed, 42 insertions, 0 deletions

diff --git a/crates/ra_syntax/Cargo.toml b/crates/ra_syntax/Cargo.toml
index d3a8b516a..724c38e17 100644
--- a/crates/ra_syntax/Cargo.toml
+++ b/crates/ra_syntax/Cargo.toml
@@ -11,6 +11,7 @@ repository = "https://github.com/rust-analyzer/rust-analyzer"
 itertools = "0.8.0"
 rowan = "0.6.1"
 rustc_lexer = "0.1.0"
+rustc-hash = "1.0.1"
 
 # ideally, `serde` should be enabled by `ra_lsp_server`, but we enable it here
 # to reduce number of compilations
diff --git a/crates/ra_syntax/src/algo.rs b/crates/ra_syntax/src/algo.rs
index 7ee5aa85b..f0ed96a17 100644
--- a/crates/ra_syntax/src/algo.rs
+++ b/crates/ra_syntax/src/algo.rs
@@ -3,6 +3,7 @@ pub mod visit;
 use std::ops::RangeInclusive;
 
 use itertools::Itertools;
+use rustc_hash::FxHashMap;
 
 use crate::{
     AstNode, Direction, NodeOrToken, SyntaxElement, SyntaxNode, SyntaxNodePtr, TextRange, TextUnit,
@@ -123,6 +124,37 @@ pub fn replace_children(
     with_children(parent, new_children)
 }
 
+/// Replaces descendants in the node, according to the mapping.
+///
+/// This is a type-unsafe low-level editing API, if you need to use it, prefer
+/// to create a type-safe abstraction on top of it instead.
+pub fn replace_descendants(
+    parent: &SyntaxNode,
+    map: &FxHashMap<SyntaxElement, SyntaxElement>,
+) -> SyntaxNode {
+    //  FIXME: this could be made much faster.
+    let new_children = parent.children_with_tokens().map(|it| go(map, it)).collect::<Box<[_]>>();
+    return with_children(parent, new_children);
+
+    fn go(
+        map: &FxHashMap<SyntaxElement, SyntaxElement>,
+        element: SyntaxElement,
+    ) -> NodeOrToken<rowan::GreenNode, rowan::GreenToken> {
+        if let Some(replacement) = map.get(&element) {
+            return match replacement {
+                NodeOrToken::Node(it) => NodeOrToken::Node(it.green().clone()),
+                NodeOrToken::Token(it) => NodeOrToken::Token(it.green().clone()),
+            };
+        }
+        match element {
+            NodeOrToken::Token(it) => NodeOrToken::Token(it.green().clone()),
+            NodeOrToken::Node(it) => {
+                NodeOrToken::Node(replace_descendants(&it, map).green().clone())
+            }
+        }
+    }
+}
+
 fn with_children(
     parent: &SyntaxNode,
     new_children: Box<[NodeOrToken<rowan::GreenNode, rowan::GreenToken>]>,
diff --git a/crates/ra_syntax/src/ast/extensions.rs b/crates/ra_syntax/src/ast/extensions.rs
index d3a375f87..5f7e9f5b1 100644
--- a/crates/ra_syntax/src/ast/extensions.rs
+++ b/crates/ra_syntax/src/ast/extensions.rs
@@ -373,6 +373,15 @@ impl ast::LifetimeParam {
     }
 }
 
+impl ast::TypeParam {
+    pub fn colon_token(&self) -> Option<SyntaxToken> {
+        self.syntax()
+            .children_with_tokens()
+            .filter_map(|it| it.into_token())
+            .find(|it| it.kind() == T![:])
+    }
+}
+
 impl ast::WherePred {
     pub fn lifetime_token(&self) -> Option<SyntaxToken> {
         self.syntax()