3 files changed, 77 insertions, 31 deletions

diff --git a/crates/ra_assists/Cargo.toml b/crates/ra_assists/Cargo.toml
index 635d87611..02966bbda 100644
--- a/crates/ra_assists/Cargo.toml
+++ b/crates/ra_assists/Cargo.toml
@@ -10,6 +10,7 @@ once_cell = "1.0.1"
 join_to_string = "0.1.3"
 itertools = "0.8.0"
 arrayvec = "0.4.10"
+rustc-hash = "1.0.1"
 
 ra_syntax = { path = "../ra_syntax" }
 ra_text_edit = { path = "../ra_text_edit" }
diff --git a/crates/ra_assists/src/assists/move_bounds.rs b/crates/ra_assists/src/assists/move_bounds.rs
index 6fd2fb72b..671826013 100644
--- a/crates/ra_assists/src/assists/move_bounds.rs
+++ b/crates/ra_assists/src/assists/move_bounds.rs
@@ -3,10 +3,9 @@ use ra_syntax::{
     ast::{self, AstNode, NameOwner, TypeBoundsOwner},
     SyntaxElement,
     SyntaxKind::*,
-    TextRange,
 };
 
-use crate::{ast_builder::Make, Assist, AssistCtx, AssistId};
+use crate::{ast_builder::Make, ast_editor::AstEditor, Assist, AssistCtx, AssistId};
 
 pub(crate) fn move_bounds_to_where_clause(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
     let type_param_list = ctx.node_at_offset::<ast::TypeParamList>()?;
@@ -36,23 +35,23 @@ pub(crate) fn move_bounds_to_where_clause(mut ctx: AssistCtx<impl HirDatabase>)
         AssistId("move_bounds_to_where_clause"),
         "move_bounds_to_where_clause",
         |edit| {
-            let type_params = type_param_list.type_params().collect::<Vec<_>>();
-
-            for param in &type_params {
-                if let Some(bounds) = param.type_bound_list() {
-                    let colon = param
-                        .syntax()
-                        .children_with_tokens()
-                        .find(|it| it.kind() == COLON)
-                        .unwrap();
-                    let start = colon.text_range().start();
-                    let end = bounds.syntax().text_range().end();
-                    edit.delete(TextRange::from_to(start, end));
-                }
-            }
-
-            let predicates = type_params.iter().filter_map(build_predicate);
-            let where_clause = Make::<ast::WhereClause>::from_predicates(predicates);
+            let new_params = type_param_list
+                .type_params()
+                .filter(|it| it.type_bound_list().is_some())
+                .map(|type_param| {
+                    let without_bounds =
+                        AstEditor::new(type_param.clone()).remove_bounds().ast().clone();
+                    (type_param, without_bounds)
+                });
+
+            let mut ast_editor = AstEditor::new(type_param_list.clone());
+            ast_editor.replace_descendants(new_params);
+            ast_editor.into_text_edit(edit.text_edit_builder());
+
+            let where_clause = {
+                let predicates = type_param_list.type_params().filter_map(build_predicate);
+                Make::<ast::WhereClause>::from_predicates(predicates)
+            };
 
             let to_insert = match anchor.prev_sibling_or_token() {
                 Some(ref elem) if elem.kind() == WHITESPACE => {
@@ -68,7 +67,7 @@ pub(crate) fn move_bounds_to_where_clause(mut ctx: AssistCtx<impl HirDatabase>)
     ctx.build()
 }
 
-fn build_predicate(param: &ast::TypeParam) -> Option<ast::WherePred> {
+fn build_predicate(param: ast::TypeParam) -> Option<ast::WherePred> {
     let path = Make::<ast::Path>::from_name(param.name()?);
     let predicate = Make::<ast::WherePred>::from(path, param.type_bound_list()?.bounds());
     Some(predicate)
diff --git a/crates/ra_assists/src/ast_editor.rs b/crates/ra_assists/src/ast_editor.rs
index 55c0aa59f..4e253f0a4 100644
--- a/crates/ra_assists/src/ast_editor.rs
+++ b/crates/ra_assists/src/ast_editor.rs
@@ -1,11 +1,13 @@
 use std::{iter, ops::RangeInclusive};
 
 use arrayvec::ArrayVec;
+use rustc_hash::FxHashMap;
 
 use ra_fmt::leading_indent;
 use ra_syntax::{
-    algo::{insert_children, replace_children},
-    ast, AstNode, Direction, InsertPosition, SyntaxElement,
+    algo,
+    ast::{self, TypeBoundsOwner},
+    AstNode, Direction, InsertPosition, NodeOrToken, SyntaxElement,
     SyntaxKind::*,
     T,
 };
@@ -27,26 +29,55 @@ impl<N: AstNode> AstEditor<N> {
     }
 
     pub fn into_text_edit(self, builder: &mut TextEditBuilder) {
-        // FIXME: compute a more fine-grained diff here.
-        // If *you* know a nice algorithm to compute diff between two syntax
-        // tree, tell me about it!
-        builder.replace(
-            self.original_ast.syntax().text_range(),
-            self.ast().syntax().text().to_string(),
-        );
+        // FIXME: this is both horrible inefficient and gives larger than
+        // necessary diff. I bet there's a cool algorithm to diff trees properly.
+        go(builder, self.original_ast.syntax().clone().into(), self.ast().syntax().clone().into());
+
+        fn go(buf: &mut TextEditBuilder, lhs: SyntaxElement, rhs: SyntaxElement) {
+            if lhs.kind() == rhs.kind() && lhs.text_range().len() == rhs.text_range().len() {
+                if match (&lhs, &rhs) {
+                    (NodeOrToken::Node(lhs), NodeOrToken::Node(rhs)) => lhs.text() == rhs.text(),
+                    (NodeOrToken::Token(lhs), NodeOrToken::Token(rhs)) => lhs.text() == rhs.text(),
+                    _ => false,
+                } {
+                    return;
+                }
+            }
+            if let (Some(lhs), Some(rhs)) = (lhs.as_node(), rhs.as_node()) {
+                if lhs.children_with_tokens().count() == rhs.children_with_tokens().count() {
+                    for (lhs, rhs) in lhs.children_with_tokens().zip(rhs.children_with_tokens()) {
+                        go(buf, lhs, rhs)
+                    }
+                    return;
+                }
+            }
+            buf.replace(lhs.text_range(), rhs.to_string())
+        }
     }
 
     pub fn ast(&self) -> &N {
         &self.ast
     }
 
+    pub fn replace_descendants<T: AstNode>(
+        &mut self,
+        replacement_map: impl Iterator<Item = (T, T)>,
+    ) -> &mut Self {
+        let map = replacement_map
+            .map(|(from, to)| (from.syntax().clone().into(), to.syntax().clone().into()))
+            .collect::<FxHashMap<_, _>>();
+        let new_syntax = algo::replace_descendants(self.ast.syntax(), &map);
+        self.ast = N::cast(new_syntax).unwrap();
+        self
+    }
+
     #[must_use]
     fn insert_children(
         &self,
         position: InsertPosition<SyntaxElement>,
         mut to_insert: impl Iterator<Item = SyntaxElement>,
     ) -> N {
-        let new_syntax = insert_children(self.ast().syntax(), position, &mut to_insert);
+        let new_syntax = algo::insert_children(self.ast().syntax(), position, &mut to_insert);
         N::cast(new_syntax).unwrap()
     }
 
@@ -56,7 +87,7 @@ impl<N: AstNode> AstEditor<N> {
         to_delete: RangeInclusive<SyntaxElement>,
         mut to_insert: impl Iterator<Item = SyntaxElement>,
     ) -> N {
-        let new_syntax = replace_children(self.ast().syntax(), to_delete, &mut to_insert);
+        let new_syntax = algo::replace_children(self.ast().syntax(), to_delete, &mut to_insert);
         N::cast(new_syntax).unwrap()
     }
 
@@ -240,3 +271,18 @@ impl AstEditor<ast::FnDef> {
         self.ast = self.replace_children(replace_range, to_insert.into_iter())
     }
 }
+
+impl AstEditor<ast::TypeParam> {
+    pub fn remove_bounds(&mut self) -> &mut Self {
+        let colon = match self.ast.colon_token() {
+            Some(it) => it,
+            None => return self,
+        };
+        let end = match self.ast.type_bound_list() {
+            Some(it) => it.syntax().clone().into(),
+            None => colon.clone().into(),
+        };
+        self.ast = self.replace_children(RangeInclusive::new(colon.into(), end), iter::empty());
+        self
+    }
+}