13 files changed, 569 insertions, 196 deletions

diff --git a/Cargo.lock b/Cargo.lock
index e5729f968..3be73d66d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -903,8 +903,10 @@ version = "0.1.0"
 name = "ra_assists"
 version = "0.1.0"
 dependencies = [
+ "arrayvec 0.4.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "itertools 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "join_to_string 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "lazy_static 1.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "ra_db 0.1.0",
  "ra_fmt 0.1.0",
  "ra_hir 0.1.0",
diff --git a/crates/ra_assists/Cargo.toml b/crates/ra_assists/Cargo.toml
index d4056a349..29d9ceb59 100644
--- a/crates/ra_assists/Cargo.toml
+++ b/crates/ra_assists/Cargo.toml
@@ -5,8 +5,10 @@ version = "0.1.0"
 authors = ["rust-analyzer developers"]
 
 [dependencies]
+lazy_static = "1.3.0"
 join_to_string = "0.1.3"
 itertools = "0.8.0"
+arrayvec = "0.4.10"
 
 ra_syntax = { path = "../ra_syntax" }
 ra_text_edit = { path = "../ra_text_edit" }
diff --git a/crates/ra_assists/src/add_missing_impl_members.rs b/crates/ra_assists/src/add_missing_impl_members.rs
index c82447b84..17c2af899 100644
--- a/crates/ra_assists/src/add_missing_impl_members.rs
+++ b/crates/ra_assists/src/add_missing_impl_members.rs
@@ -1,14 +1,9 @@
-use std::fmt::Write;
-
-use crate::{Assist, AssistId, AssistCtx};
+use crate::{Assist, AssistId, AssistCtx, ast_editor::{AstEditor, AstBuilder}};
 
 use hir::db::HirDatabase;
-use ra_syntax::{SmolStr, SyntaxKind, TextRange, TextUnit, TreeArc};
-use ra_syntax::ast::{self, AstNode, AstToken, FnDef, ImplItem, ImplItemKind, NameOwner};
+use ra_syntax::{SmolStr, TreeArc};
+use ra_syntax::ast::{self, AstNode, FnDef, ImplItem, ImplItemKind, NameOwner};
 use ra_db::FilePosition;
-use ra_fmt::{leading_indent, reindent};
-
-use itertools::Itertools;
 
 enum AddMissingImplMembersMode {
     DefaultMethodsOnly,
@@ -76,48 +71,35 @@ fn add_missing_impl_members_inner(
     }
 
     ctx.add_action(AssistId(assist_id), label, |edit| {
-        let (parent_indent, indent) = {
-            // FIXME: Find a way to get the indent already used in the file.
-            // Now, we copy the indent of first item or indent with 4 spaces relative to impl block
-            const DEFAULT_INDENT: &str = "    ";
-            let first_item = impl_item_list.impl_items().next();
-            let first_item_indent =
-                first_item.and_then(|i| leading_indent(i.syntax())).map(ToOwned::to_owned);
-            let impl_block_indent = leading_indent(impl_node.syntax()).unwrap_or_default();
-
-            (
-                impl_block_indent.to_owned(),
-                first_item_indent.unwrap_or_else(|| impl_block_indent.to_owned() + DEFAULT_INDENT),
-            )
-        };
-
-        let changed_range = {
-            let children = impl_item_list.syntax().children_with_tokens();
-            let last_whitespace =
-                children.filter_map(|it| ast::Whitespace::cast(it.as_token()?)).last();
-
-            last_whitespace.map(|w| w.syntax().range()).unwrap_or_else(|| {
-                let in_brackets = impl_item_list.syntax().range().end() - TextUnit::of_str("}");
-                TextRange::from_to(in_brackets, in_brackets)
-            })
-        };
-
-        let func_bodies = format!("\n{}", missing_fns.into_iter().map(build_func_body).join("\n"));
-        let trailing_whitespace = format!("\n{}", parent_indent);
-        let func_bodies = reindent(&func_bodies, &indent) + &trailing_whitespace;
-
-        let replaced_text_range = TextUnit::of_str(&func_bodies);
-
-        edit.replace(changed_range, func_bodies);
-        // FIXME: place the cursor on the first unimplemented?
-        edit.set_cursor(
-            changed_range.start() + replaced_text_range - TextUnit::of_str(&trailing_whitespace),
-        );
+        let n_existing_items = impl_item_list.impl_items().count();
+        let fns = missing_fns.into_iter().map(add_body_and_strip_docstring).collect::<Vec<_>>();
+
+        let mut ast_editor = AstEditor::new(impl_item_list);
+        if n_existing_items == 0 {
+            ast_editor.make_multiline();
+        }
+        ast_editor.append_functions(fns.iter().map(|it| &**it));
+        let first_new_item = ast_editor.ast().impl_items().nth(n_existing_items).unwrap();
+        let cursor_poisition = first_new_item.syntax().range().start();
+        ast_editor.into_text_edit(edit.text_edit_builder());
+
+        edit.set_cursor(cursor_poisition);
     });
 
     ctx.build()
 }
 
+fn add_body_and_strip_docstring(fn_def: &ast::FnDef) -> TreeArc<ast::FnDef> {
+    let mut ast_editor = AstEditor::new(fn_def);
+    if fn_def.body().is_none() {
+        ast_editor.set_body(&AstBuilder::<ast::Block>::single_expr(
+            &AstBuilder::<ast::Expr>::unimplemented(),
+        ));
+    }
+    ast_editor.strip_attrs_and_docs();
+    ast_editor.ast().to_owned()
+}
+
 /// Given an `ast::ImplBlock`, resolves the target trait (the one being
 /// implemented) to a `ast::TraitDef`.
 fn resolve_target_trait_def(
@@ -134,22 +116,6 @@ fn resolve_target_trait_def(
     }
 }
 
-fn build_func_body(def: &ast::FnDef) -> String {
-    let mut buf = String::new();
-
-    for child in def.syntax().children_with_tokens() {
-        match (child.prev_sibling_or_token().map(|c| c.kind()), child.kind()) {
-            (_, SyntaxKind::SEMI) => buf.push_str(" {\n    unimplemented!()\n}"),
-            (_, SyntaxKind::ATTR) | (_, SyntaxKind::COMMENT) => {}
-            (Some(SyntaxKind::ATTR), SyntaxKind::WHITESPACE)
-            | (Some(SyntaxKind::COMMENT), SyntaxKind::WHITESPACE) => {}
-            _ => write!(buf, "{}", child).unwrap(),
-        };
-    }
-
-    buf.trim_end().to_string()
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -170,7 +136,7 @@ struct S;
 
 impl Foo for S {
     fn bar(&self) {}
-    <|>
+<|>
 }",
             "
 trait Foo {
@@ -183,12 +149,9 @@ struct S;
 
 impl Foo for S {
     fn bar(&self) {}
-    fn foo(&self) {
-        unimplemented!()
-    }
-    fn baz(&self) {
-        unimplemented!()
-    }<|>
+    <|>fn foo(&self) { unimplemented!() }
+    fn baz(&self) { unimplemented!() }
+
 }",
         );
     }
@@ -208,7 +171,7 @@ struct S;
 
 impl Foo for S {
     fn bar(&self) {}
-    <|>
+<|>
 }",
             "
 trait Foo {
@@ -221,9 +184,8 @@ struct S;
 
 impl Foo for S {
     fn bar(&self) {}
-    fn foo(&self) {
-        unimplemented!()
-    }<|>
+    <|>fn foo(&self) { unimplemented!() }
+
 }",
         );
     }
@@ -240,9 +202,7 @@ impl Foo for S { <|> }",
 trait Foo { fn foo(&self); }
 struct S;
 impl Foo for S {
-    fn foo(&self) {
-        unimplemented!()
-    }<|>
+    <|>fn foo(&self) { unimplemented!() }
 }",
         );
     }
@@ -259,9 +219,7 @@ impl Foo for S {}<|>",
 trait Foo { fn foo(&self); }
 struct S;
 impl Foo for S {
-    fn foo(&self) {
-        unimplemented!()
-    }<|>
+    <|>fn foo(&self) { unimplemented!() }
 }",
         )
     }
@@ -292,35 +250,6 @@ impl Foo for S { <|> }",
     }
 
     #[test]
-    fn test_indented_impl_block() {
-        check_assist(
-            add_missing_impl_members,
-            "
-trait Foo {
-    fn valid(some: u32) -> bool;
-}
-struct S;
-
-mod my_mod {
-    impl crate::Foo for S { <|> }
-}",
-            "
-trait Foo {
-    fn valid(some: u32) -> bool;
-}
-struct S;
-
-mod my_mod {
-    impl crate::Foo for S {
-        fn valid(some: u32) -> bool {
-            unimplemented!()
-        }<|>
-    }
-}",
-        )
-    }
-
-    #[test]
     fn test_with_docstring_and_attrs() {
         check_assist(
             add_missing_impl_members,
@@ -342,9 +271,7 @@ trait Foo {
 }
 struct S;
 impl Foo for S {
-    fn foo(&self) {
-        unimplemented!()
-    }<|>
+    <|>fn foo(&self) { unimplemented!() }
 }"#,
         )
     }
@@ -367,7 +294,7 @@ trait Foo {
 }
 struct S;
 impl Foo for S {
-    fn valid(some: u32) -> bool { false }<|>
+    <|>fn valid(some: u32) -> bool { false }
 }",
         )
     }
diff --git a/crates/ra_assists/src/assist_ctx.rs b/crates/ra_assists/src/assist_ctx.rs
index e80e35738..f46de61eb 100644
--- a/crates/ra_assists/src/assist_ctx.rs
+++ b/crates/ra_assists/src/assist_ctx.rs
@@ -161,6 +161,10 @@ impl AssistBuilder {
         self.target = Some(target)
     }
 
+    pub(crate) fn text_edit_builder(&mut self) -> &mut TextEditBuilder {
+        &mut self.edit
+    }
+
     fn build(self) -> AssistAction {
         AssistAction {
             edit: self.edit.finish(),
diff --git a/crates/ra_assists/src/ast_editor.rs b/crates/ra_assists/src/ast_editor.rs
new file mode 100644
index 000000000..6854294ae
--- /dev/null
+++ b/crates/ra_assists/src/ast_editor.rs
@@ -0,0 +1,333 @@
+use std::{iter, ops::RangeInclusive};
+
+use arrayvec::ArrayVec;
+use ra_text_edit::TextEditBuilder;
+use ra_syntax::{AstNode, TreeArc, ast, SyntaxKind::*, SyntaxElement, SourceFile, InsertPosition, Direction};
+use ra_fmt::leading_indent;
+
+pub struct AstEditor<N: AstNode> {
+    original_ast: TreeArc<N>,
+    ast: TreeArc<N>,
+}
+
+impl<N: AstNode> AstEditor<N> {
+    pub fn new(node: &N) -> AstEditor<N> {
+        AstEditor { original_ast: node.to_owned(), ast: node.to_owned() }
+    }
+
+    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().range(), self.ast().syntax().text().to_string());
+    }
+
+    pub fn ast(&self) -> &N {
+        &*self.ast
+    }
+
+    #[must_use]
+    fn insert_children<'a>(
+        &self,
+        position: InsertPosition<SyntaxElement<'_>>,
+        to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+    ) -> TreeArc<N> {
+        let new_syntax = self.ast().syntax().insert_children(position, to_insert);
+        N::cast(&new_syntax).unwrap().to_owned()
+    }
+
+    #[must_use]
+    fn replace_children<'a>(
+        &self,
+        to_delete: RangeInclusive<SyntaxElement<'_>>,
+        to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+    ) -> TreeArc<N> {
+        let new_syntax = self.ast().syntax().replace_children(to_delete, to_insert);
+        N::cast(&new_syntax).unwrap().to_owned()
+    }
+
+    fn do_make_multiline(&mut self) {
+        let l_curly =
+            match self.ast().syntax().children_with_tokens().find(|it| it.kind() == L_CURLY) {
+                Some(it) => it,
+                None => return,
+            };
+        let sibling = match l_curly.next_sibling_or_token() {
+            Some(it) => it,
+            None => return,
+        };
+        let existing_ws = match sibling.as_token() {
+            None => None,
+            Some(tok) if tok.kind() != WHITESPACE => None,
+            Some(ws) => {
+                if ws.text().contains('\n') {
+                    return;
+                }
+                Some(ws)
+            }
+        };
+
+        let indent = leading_indent(self.ast().syntax()).unwrap_or("");
+        let ws = tokens::WsBuilder::new(&format!("\n{}", indent));
+        let to_insert = iter::once(ws.ws().into());
+        self.ast = match existing_ws {
+            None => self.insert_children(InsertPosition::After(l_curly), to_insert),
+            Some(ws) => self.replace_children(RangeInclusive::new(ws.into(), ws.into()), to_insert),
+        };
+    }
+}
+
+impl AstEditor<ast::NamedFieldList> {
+    pub fn append_field(&mut self, field: &ast::NamedField) {
+        self.insert_field(InsertPosition::Last, field)
+    }
+
+    pub fn make_multiline(&mut self) {
+        self.do_make_multiline()
+    }
+
+    pub fn insert_field(
+        &mut self,
+        position: InsertPosition<&'_ ast::NamedField>,
+        field: &ast::NamedField,
+    ) {
+        let is_multiline = self.ast().syntax().text().contains('\n');
+        let ws;
+        let space = if is_multiline {
+            ws = tokens::WsBuilder::new(&format!(
+                "\n{}    ",
+                leading_indent(self.ast().syntax()).unwrap_or("")
+            ));
+            ws.ws()
+        } else {
+            tokens::single_space()
+        };
+
+        let mut to_insert: ArrayVec<[SyntaxElement; 4]> = ArrayVec::new();
+        to_insert.push(space.into());
+        to_insert.push(field.syntax().into());
+        to_insert.push(tokens::comma().into());
+
+        macro_rules! after_l_curly {
+            () => {{
+                let anchor = match self.l_curly() {
+                    Some(it) => it,
+                    None => return,
+                };
+                InsertPosition::After(anchor)
+            }};
+        }
+
+        macro_rules! after_field {
+            ($anchor:expr) => {
+                if let Some(comma) = $anchor
+                    .syntax()
+                    .siblings_with_tokens(Direction::Next)
+                    .find(|it| it.kind() == COMMA)
+                {
+                    InsertPosition::After(comma)
+                } else {
+                    to_insert.insert(0, tokens::comma().into());
+                    InsertPosition::After($anchor.syntax().into())
+                }
+            };
+        };
+
+        let position = match position {
+            InsertPosition::First => after_l_curly!(),
+            InsertPosition::Last => {
+                if !is_multiline {
+                    // don't insert comma before curly
+                    to_insert.pop();
+                }
+                match self.ast().fields().last() {
+                    Some(it) => after_field!(it),
+                    None => after_l_curly!(),
+                }
+            }
+            InsertPosition::Before(anchor) => InsertPosition::Before(anchor.syntax().into()),
+            InsertPosition::After(anchor) => after_field!(anchor),
+        };
+
+        self.ast = self.insert_children(position, to_insert.iter().cloned());
+    }
+
+    fn l_curly(&self) -> Option<SyntaxElement> {
+        self.ast().syntax().children_with_tokens().find(|it| it.kind() == L_CURLY)
+    }
+}
+
+impl AstEditor<ast::ItemList> {
+    pub fn make_multiline(&mut self) {
+        self.do_make_multiline()
+    }
+
+    pub fn append_functions<'a>(&mut self, fns: impl Iterator<Item = &'a ast::FnDef>) {
+        fns.for_each(|it| self.append_function(it))
+    }
+
+    pub fn append_function(&mut self, fn_def: &ast::FnDef) {
+        let (indent, position) = match self.ast().impl_items().last() {
+            Some(it) => (
+                leading_indent(it.syntax()).unwrap_or("").to_string(),
+                InsertPosition::After(it.syntax().into()),
+            ),
+            None => match self.l_curly() {
+                Some(it) => (
+                    "    ".to_string() + leading_indent(self.ast().syntax()).unwrap_or(""),
+                    InsertPosition::After(it),
+                ),
+                None => return,
+            },
+        };
+        let ws = tokens::WsBuilder::new(&format!("\n{}", indent));
+        let to_insert: ArrayVec<[SyntaxElement; 2]> =
+            [ws.ws().into(), fn_def.syntax().into()].into();
+        self.ast = self.insert_children(position, to_insert.into_iter());
+    }
+
+    fn l_curly(&self) -> Option<SyntaxElement> {
+        self.ast().syntax().children_with_tokens().find(|it| it.kind() == L_CURLY)
+    }
+}
+
+impl AstEditor<ast::FnDef> {
+    pub fn set_body(&mut self, body: &ast::Block) {
+        let mut to_insert: ArrayVec<[SyntaxElement; 2]> = ArrayVec::new();
+        let old_body_or_semi: SyntaxElement = if let Some(old_body) = self.ast().body() {
+            old_body.syntax().into()
+        } else if let Some(semi) = self.ast().semicolon_token() {
+            to_insert.push(tokens::single_space().into());
+            semi.into()
+        } else {
+            to_insert.push(tokens::single_space().into());
+            to_insert.push(body.syntax().into());
+            self.ast = self.insert_children(InsertPosition::Last, to_insert.into_iter());
+            return;
+        };
+        to_insert.push(body.syntax().into());
+        let replace_range = RangeInclusive::new(old_body_or_semi, old_body_or_semi);
+        self.ast = self.replace_children(replace_range, to_insert.into_iter())
+    }
+
+    pub fn strip_attrs_and_docs(&mut self) {
+        loop {
+            if let Some(start) = self
+                .ast()
+                .syntax()
+                .children_with_tokens()
+                .find(|it| it.kind() == ATTR || it.kind() == COMMENT)
+            {
+                let end = match start.next_sibling_or_token() {
+                    Some(el) if el.kind() == WHITESPACE => el,
+                    Some(_) | None => start,
+                };
+                self.ast = self.replace_children(RangeInclusive::new(start, end), iter::empty());
+            } else {
+                break;
+            }
+        }
+    }
+}
+
+pub struct AstBuilder<N: AstNode> {
+    _phantom: std::marker::PhantomData<N>,
+}
+
+impl AstBuilder<ast::NamedField> {
+    fn from_text(text: &str) -> TreeArc<ast::NamedField> {
+        ast_node_from_file_text(&format!("fn f() {{ S {{ {}, }} }}", text))
+    }
+
+    pub fn from_pieces(name: &ast::NameRef, expr: Option<&ast::Expr>) -> TreeArc<ast::NamedField> {
+        match expr {
+            Some(expr) => Self::from_text(&format!("{}: {}", name.syntax(), expr.syntax())),
+            None => Self::from_text(&name.syntax().to_string()),
+        }
+    }
+}
+
+impl AstBuilder<ast::Block> {
+    fn from_text(text: &str) -> TreeArc<ast::Block> {
+        ast_node_from_file_text(&format!("fn f() {}", text))
+    }
+
+    pub fn single_expr(e: &ast::Expr) -> TreeArc<ast::Block> {
+        Self::from_text(&format!("{{ {} }}", e.syntax()))
+    }
+}
+
+impl AstBuilder<ast::Expr> {
+    fn from_text(text: &str) -> TreeArc<ast::Expr> {
+        ast_node_from_file_text(&format!("fn f() {{ {}; }}", text))
+    }
+
+    pub fn unit() -> TreeArc<ast::Expr> {
+        Self::from_text("()")
+    }
+
+    pub fn unimplemented() -> TreeArc<ast::Expr> {
+        Self::from_text("unimplemented!()")
+    }
+}
+
+impl AstBuilder<ast::NameRef> {
+    pub fn new(text: &str) -> TreeArc<ast::NameRef> {
+        ast_node_from_file_text(&format!("fn f() {{ {}; }}", text))
+    }
+}
+
+fn ast_node_from_file_text<N: AstNode>(text: &str) -> TreeArc<N> {
+    let file = SourceFile::parse(text);
+    let res = file.syntax().descendants().find_map(N::cast).unwrap().to_owned();
+    res
+}
+
+mod tokens {
+    use lazy_static::lazy_static;
+    use ra_syntax::{AstNode, SourceFile, TreeArc, SyntaxToken, SyntaxKind::*};
+
+    lazy_static! {
+        static ref SOURCE_FILE: TreeArc<SourceFile> = SourceFile::parse(",\n; ;");
+    }
+
+    pub(crate) fn comma() -> SyntaxToken<'static> {
+        SOURCE_FILE
+            .syntax()
+            .descendants_with_tokens()
+            .filter_map(|it| it.as_token())
+            .find(|it| it.kind() == COMMA)
+            .unwrap()
+    }
+
+    pub(crate) fn single_space() -> SyntaxToken<'static> {
+        SOURCE_FILE
+            .syntax()
+            .descendants_with_tokens()
+            .filter_map(|it| it.as_token())
+            .find(|it| it.kind() == WHITESPACE && it.text().as_str() == " ")
+            .unwrap()
+    }
+
+    #[allow(unused)]
+    pub(crate) fn single_newline() -> SyntaxToken<'static> {
+        SOURCE_FILE
+            .syntax()
+            .descendants_with_tokens()
+            .filter_map(|it| it.as_token())
+            .find(|it| it.kind() == WHITESPACE && it.text().as_str() == "\n")
+            .unwrap()
+    }
+
+    pub(crate) struct WsBuilder(TreeArc<SourceFile>);
+
+    impl WsBuilder {
+        pub(crate) fn new(text: &str) -> WsBuilder {
+            WsBuilder(SourceFile::parse(text))
+        }
+        pub(crate) fn ws(&self) -> SyntaxToken<'_> {
+            self.0.syntax().first_child_or_token().unwrap().as_token().unwrap()
+        }
+    }
+
+}
diff --git a/crates/ra_assists/src/fill_struct_fields.rs b/crates/ra_assists/src/fill_struct_fields.rs
index 663b4f669..54b70e17d 100644
--- a/crates/ra_assists/src/fill_struct_fields.rs
+++ b/crates/ra_assists/src/fill_struct_fields.rs
@@ -1,94 +1,55 @@
-use std::fmt::Write;
-
 use hir::{AdtDef, db::HirDatabase};
 
 use ra_syntax::ast::{self, AstNode};
 
-use crate::{AssistCtx, Assist, AssistId};
+use crate::{AssistCtx, Assist, AssistId, ast_editor::{AstEditor, AstBuilder}};
 
 pub(crate) fn fill_struct_fields(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
     let struct_lit = ctx.node_at_offset::<ast::StructLit>()?;
-    let mut fsf = FillStructFields {
-        ctx: &mut ctx,
-        named_field_list: struct_lit.named_field_list()?,
-        struct_fields: vec![],
-        struct_lit,
-    };
-    fsf.evaluate_struct_def_fields()?;
-    if fsf.struct_lit_and_def_have_the_same_number_of_fields() {
-        return None;
-    }
-    fsf.remove_already_included_fields()?;
-    fsf.add_action()?;
-    ctx.build()
-}
+    let named_field_list = struct_lit.named_field_list()?;
 
-struct FillStructFields<'a, 'b: 'a, DB> {
-    ctx: &'a mut AssistCtx<'b, DB>,
-    named_field_list: &'a ast::NamedFieldList,
-    struct_fields: Vec<(String, String)>,
-    struct_lit: &'a ast::StructLit,
-}
-
-impl<DB> FillStructFields<'_, '_, DB>
-where
-    DB: HirDatabase,
-{
-    fn add_action(&mut self) -> Option<()> {
-        let named_field_list = self.named_field_list;
-        let struct_fields_string = self.struct_fields_string()?;
-        let struct_lit = self.struct_lit;
-        self.ctx.add_action(AssistId("fill_struct_fields"), "fill struct fields", |edit| {
-            edit.target(struct_lit.syntax().range());
-            edit.set_cursor(struct_lit.syntax().range().start());
-            edit.replace_node_and_indent(named_field_list.syntax(), struct_fields_string);
-        });
-        Some(())
-    }
-
-    fn struct_lit_and_def_have_the_same_number_of_fields(&self) -> bool {
-        self.named_field_list.fields().count() == self.struct_fields.len()
-    }
-
-    fn evaluate_struct_def_fields(&mut self) -> Option<()> {
-        let analyzer = hir::SourceAnalyzer::new(
-            self.ctx.db,
-            self.ctx.frange.file_id,
-            self.struct_lit.syntax(),
-            None,
-        );
-        let struct_lit_ty = analyzer.type_of(self.ctx.db, self.struct_lit.into())?;
+    // Collect all fields from struct definition
+    let mut fields = {
+        let analyzer =
+            hir::SourceAnalyzer::new(ctx.db, ctx.frange.file_id, struct_lit.syntax(), None);
+        let struct_lit_ty = analyzer.type_of(ctx.db, struct_lit.into())?;
         let struct_def = match struct_lit_ty.as_adt() {
             Some((AdtDef::Struct(s), _)) => s,
             _ => return None,
         };
-        self.struct_fields = struct_def
-            .fields(self.ctx.db)
-            .into_iter()
-            .map(|f| (f.name(self.ctx.db).to_string(), "()".into()))
-            .collect();
-        Some(())
-    }
+        struct_def.fields(ctx.db)
+    };
 
-    fn remove_already_included_fields(&mut self) -> Option<()> {
-        for ast_field in self.named_field_list.fields() {
-            let expr = ast_field.expr()?.syntax().text().to_string();
-            let name_from_ast = ast_field.name_ref()?.text().to_string();
-            if let Some(idx) = self.struct_fields.iter().position(|(n, _)| n == &name_from_ast) {
-                self.struct_fields[idx] = (name_from_ast, expr);
-            }
-        }
-        Some(())
+    // Filter out existing fields
+    for ast_field in named_field_list.fields() {
+        let name_from_ast = ast_field.name_ref()?.text().to_string();
+        fields.retain(|field| field.name(ctx.db).to_string() != name_from_ast);
+    }
+    if fields.is_empty() {
+        return None;
     }
 
-    fn struct_fields_string(&mut self) -> Option<String> {
-        let mut buf = String::from("{\n");
-        for (name, expr) in &self.struct_fields {
-            write!(&mut buf, "    {}: {},\n", name, expr).unwrap();
+    let db = ctx.db;
+    ctx.add_action(AssistId("fill_struct_fields"), "fill struct fields", |edit| {
+        let mut ast_editor = AstEditor::new(named_field_list);
+        if named_field_list.fields().count() == 0 && fields.len() > 2 {
+            ast_editor.make_multiline();
+        };
+
+        for field in fields {
+            let field = AstBuilder::<ast::NamedField>::from_pieces(
+                &AstBuilder::<ast::NameRef>::new(&field.name(db).to_string()),
+                Some(&AstBuilder::<ast::Expr>::unit()),
+            );
+            ast_editor.append_field(&field);
         }
-        buf.push_str("}");
-        Some(buf)
-    }
+
+        edit.target(struct_lit.syntax().range());
+        edit.set_cursor(struct_lit.syntax().range().start());
+
+        ast_editor.into_text_edit(edit.text_edit_builder());
+    });
+    ctx.build()
 }
 
 #[cfg(test)]
@@ -225,14 +186,41 @@ mod tests {
 
             fn main() {
                 let s = <|>S {
+                    c: (1, 2),
+                    e: "foo",
                     a: (),
                     b: (),
-                    c: (1, 2),
                     d: (),
-                    e: "foo",
                 }
             }
             "#,
         );
     }
+
+    #[test]
+    fn fill_struct_short() {
+        check_assist(
+            fill_struct_fields,
+            r#"
+            struct S {
+                foo: u32,
+                bar: String,
+            }
+
+            fn main() {
+                let s = S {<|> };
+            }
+            "#,
+            r#"
+            struct S {
+                foo: u32,
+                bar: String,
+            }
+
+            fn main() {
+                let s = <|>S { foo: (), bar: () };
+            }
+            "#,
+        );
+    }
 }
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index ded401b63..60b4d5c63 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -7,6 +7,7 @@
 
 mod assist_ctx;
 mod marks;
+pub mod ast_editor;
 
 use itertools::Itertools;
 
@@ -36,7 +37,7 @@ pub struct AssistAction {
     pub target: Option<TextRange>,
 }
 
-/// Return all the assists applicable at the given position.
+/// Return all the assists eapplicable at the given position.
 ///
 /// Assists are returned in the "unresolved" state, that is only labels are
 /// returned, without actual edits.
diff --git a/crates/ra_ide_api/src/completion/complete_postfix.rs b/crates/ra_ide_api/src/completion/complete_postfix.rs
index 4dfa5f176..278b1b797 100644
--- a/crates/ra_ide_api/src/completion/complete_postfix.rs
+++ b/crates/ra_ide_api/src/completion/complete_postfix.rs
@@ -40,7 +40,7 @@ pub(super) fn complete_postfix(acc: &mut Completions, ctx: &CompletionContext) {
             ctx,
             "match",
             "match expr {}",
-            &format!("match {} {{\n${{1:_}} => {{$0\\}},\n}}", receiver_text),
+            &format!("match {} {{\n    ${{1:_}} => {{$0\\}},\n}}", receiver_text),
         )
         .add_to(acc);
         postfix_snippet(
diff --git a/crates/ra_ide_api/src/completion/snapshots/completion_item__postfix_completion_works_for_trivial_path_expression.snap b/crates/ra_ide_api/src/completion/snapshots/completion_item__postfix_completion_works_for_trivial_path_expression.snap
index 2b5435f0c..3bbc9e3c4 100644
--- a/crates/ra_ide_api/src/completion/snapshots/completion_item__postfix_completion_works_for_trivial_path_expression.snap
+++ b/crates/ra_ide_api/src/completion/snapshots/completion_item__postfix_completion_works_for_trivial_path_expression.snap
@@ -1,6 +1,6 @@
 ---
-created: "2019-02-18T09:22:24.127119709Z"
-creator: insta@0.6.2
+created: "2019-04-22T07:37:13.981826301Z"
+creator: insta@0.7.4
 source: crates/ra_ide_api/src/completion/completion_item.rs
 expression: kind_completions
 ---
@@ -23,7 +23,7 @@ expression: kind_completions
         label: "match",
         source_range: [76; 76),
         delete: [72; 76),
-        insert: "match bar {\n${1:_} => {$0\\},\n}",
+        insert: "match bar {\n    ${1:_} => {$0\\},\n}",
         detail: "match expr {}"
     },
     CompletionItem {
diff --git a/crates/ra_syntax/src/ast/extensions.rs b/crates/ra_syntax/src/ast/extensions.rs
index 5c4c0ffc1..9cbd2c6b8 100644
--- a/crates/ra_syntax/src/ast/extensions.rs
+++ b/crates/ra_syntax/src/ast/extensions.rs
@@ -210,6 +210,15 @@ impl ast::EnumVariant {
     }
 }
 
+impl ast::FnDef {
+    pub fn semicolon_token(&self) -> Option<SyntaxToken<'_>> {
+        self.syntax()
+            .last_child_or_token()
+            .and_then(|it| it.as_token())
+            .filter(|it| it.kind() == SEMI)
+    }
+}
+
 impl ast::LetStmt {
     pub fn has_semi(&self) -> bool {
         match self.syntax().last_child_or_token() {
diff --git a/crates/ra_syntax/src/lib.rs b/crates/ra_syntax/src/lib.rs
index a6ce14f06..9cb66b76b 100644
--- a/crates/ra_syntax/src/lib.rs
+++ b/crates/ra_syntax/src/lib.rs
@@ -38,7 +38,7 @@ pub use crate::{
     ast::AstNode,
     syntax_error::{SyntaxError, SyntaxErrorKind, Location},
     syntax_text::SyntaxText,
-    syntax_node::{Direction,  SyntaxNode, WalkEvent, TreeArc, SyntaxTreeBuilder, SyntaxElement, SyntaxToken},
+    syntax_node::{Direction,  SyntaxNode, WalkEvent, TreeArc, SyntaxTreeBuilder, SyntaxElement, SyntaxToken, InsertPosition},
     ptr::{SyntaxNodePtr, AstPtr},
     parsing::{tokenize, classify_literal, Token},
 };
diff --git a/crates/ra_syntax/src/ptr.rs b/crates/ra_syntax/src/ptr.rs
index 15a8b94cd..b0816b135 100644
--- a/crates/ra_syntax/src/ptr.rs
+++ b/crates/ra_syntax/src/ptr.rs
@@ -10,7 +10,7 @@ use crate::{
 /// specific node across reparses of the same file.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SyntaxNodePtr {
-    range: TextRange,
+    pub(crate) range: TextRange,
     kind: SyntaxKind,
 }
 
diff --git a/crates/ra_syntax/src/syntax_node.rs b/crates/ra_syntax/src/syntax_node.rs
index dc2352c76..92c15234e 100644
--- a/crates/ra_syntax/src/syntax_node.rs
+++ b/crates/ra_syntax/src/syntax_node.rs
@@ -7,6 +7,7 @@
 //! modules just wraps its API.
 
 use std::{
+    ops::RangeInclusive,
     fmt::{self, Write},
     any::Any,
     borrow::Borrow,
@@ -17,13 +18,21 @@ use ra_parser::ParseError;
 use rowan::{TransparentNewType, GreenNodeBuilder};
 
 use crate::{
-    SmolStr, SyntaxKind, TextUnit, TextRange, SyntaxText, SourceFile, AstNode,
+    SmolStr, SyntaxKind, TextUnit, TextRange, SyntaxText, SourceFile, AstNode, SyntaxNodePtr,
     syntax_error::{SyntaxError, SyntaxErrorKind},
 };
 
 pub use rowan::WalkEvent;
 pub(crate) use rowan::{GreenNode, GreenToken};
 
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum InsertPosition<T> {
+    First,
+    Last,
+    Before(T),
+    After(T),
+}
+
 /// Marker trait for CST and AST nodes
 pub trait SyntaxNodeWrapper: TransparentNewType<Repr = rowan::SyntaxNode> {}
 impl<T: TransparentNewType<Repr = rowan::SyntaxNode>> SyntaxNodeWrapper for T {}
@@ -309,6 +318,97 @@ impl SyntaxNode {
     pub(crate) fn replace_with(&self, replacement: GreenNode) -> GreenNode {
         self.0.replace_with(replacement)
     }
+
+    /// Adds specified children (tokens or nodes) to the current node at the
+    /// specific position.
+    ///
+    /// 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 insert_children<'a>(
+        &self,
+        position: InsertPosition<SyntaxElement<'_>>,
+        to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+    ) -> TreeArc<SyntaxNode> {
+        let mut delta = TextUnit::default();
+        let to_insert = to_insert.map(|element| {
+            delta += element.text_len();
+            to_green_element(element)
+        });
+
+        let old_children = self.0.green().children();
+
+        let new_children = match position {
+            InsertPosition::First => {
+                to_insert.chain(old_children.iter().cloned()).collect::<Box<[_]>>()
+            }
+            InsertPosition::Last => {
+                old_children.iter().cloned().chain(to_insert).collect::<Box<[_]>>()
+            }
+            InsertPosition::Before(anchor) | InsertPosition::After(anchor) => {
+                let take_anchor = if let InsertPosition::After(_) = position { 1 } else { 0 };
+                let split_at = self.position_of_child(anchor) + take_anchor;
+                let (before, after) = old_children.split_at(split_at);
+                before
+                    .iter()
+                    .cloned()
+                    .chain(to_insert)
+                    .chain(after.iter().cloned())
+                    .collect::<Box<[_]>>()
+            }
+        };
+
+        self.with_children(new_children)
+    }
+
+    /// Replaces all nodes in `to_delete` with nodes from `to_insert`
+    ///
+    /// 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_children<'a>(
+        &self,
+        to_delete: RangeInclusive<SyntaxElement<'_>>,
+        to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+    ) -> TreeArc<SyntaxNode> {
+        let start = self.position_of_child(*to_delete.start());
+        let end = self.position_of_child(*to_delete.end());
+        let old_children = self.0.green().children();
+
+        let new_children = old_children[..start]
+            .iter()
+            .cloned()
+            .chain(to_insert.map(to_green_element))
+            .chain(old_children[end + 1..].iter().cloned())
+            .collect::<Box<[_]>>();
+        self.with_children(new_children)
+    }
+
+    fn with_children(&self, new_children: Box<[rowan::GreenElement]>) -> TreeArc<SyntaxNode> {
+        let len = new_children.iter().map(|it| it.text_len()).sum::<TextUnit>();
+        let new_node = GreenNode::new(rowan::SyntaxKind(self.kind() as u16), new_children);
+        let new_file_node = self.replace_with(new_node);
+        let file = SourceFile::new(new_file_node, Vec::new());
+
+        // FIXME: use a more elegant way to re-fetch the node (#1185), make
+        // `range` private afterwards
+        let mut ptr = SyntaxNodePtr::new(self);
+        ptr.range = TextRange::offset_len(ptr.range().start(), len);
+        return ptr.to_node(&file).to_owned();
+    }
+
+    fn position_of_child(&self, child: SyntaxElement) -> usize {
+        self.children_with_tokens()
+            .position(|it| it == child)
+            .expect("elemetn is not a child of current element")
+    }
+}
+
+fn to_green_element(element: SyntaxElement) -> rowan::GreenElement {
+    match element {
+        SyntaxElement::Node(node) => node.0.green().clone().into(),
+        SyntaxElement::Token(tok) => {
+            GreenToken::new(rowan::SyntaxKind(tok.kind() as u16), tok.text().clone()).into()
+        }
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
@@ -451,6 +551,13 @@ impl<'a> SyntaxElement<'a> {
         }
         .ancestors()
     }
+
+    fn text_len(&self) -> TextUnit {
+        match self {
+            SyntaxElement::Node(node) => node.0.green().text_len(),
+            SyntaxElement::Token(token) => TextUnit::of_str(token.0.text()),
+        }
+    }
 }
 
 impl<'a> From<rowan::SyntaxElement<'a>> for SyntaxElement<'a> {