Merge #4521 #4522

4521: Use snippets in add_function r=matklad a=matklad

bors r+
🤖

4522: Explain the purpose of `ast::make` module more clearly r=matklad a=matklad



bors r+
🤖

Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>

crates/ra_assists/src/handlers/add_explicit_type.rs

@@ -25,9 +25,8 @@ pub(crate) fn add_explicit_type(acc: &mut Assists, ctx: &AssistContext) -> Optio
     let stmt = ctx.find_node_at_offset::<LetStmt>()?;
     let module = ctx.sema.scope(stmt.syntax()).module()?;
     let expr = stmt.initializer()?;
-    let pat = stmt.pat()?;
     // Must be a binding
-    let pat = match pat {
+    let pat = match stmt.pat()? {
         ast::Pat::BindPat(bind_pat) => bind_pat,
         _ => return None,
     };
@@ -46,7 +45,7 @@ pub(crate) fn add_explicit_type(acc: &mut Assists, ctx: &AssistContext) -> Optio
     // Assist not applicable if the type has already been specified
     // and it has no placeholders
     let ascribed_ty = stmt.ascribed_type();
-    if let Some(ref ty) = ascribed_ty {
+    if let Some(ty) = &ascribed_ty {
         if ty.syntax().descendants().find_map(ast::PlaceholderType::cast).is_none() {
             return None;
         }

crates/ra_assists/src/handlers/add_from_impl_for_enum.rs

@@ -1,6 +1,5 @@
 use ra_ide_db::RootDatabase;
 use ra_syntax::ast::{self, AstNode, NameOwner};
-use stdx::format_to;
 use test_utils::tested_by;
 
 use crate::{utils::FamousDefs, AssistContext, AssistId, Assists};
@@ -35,7 +34,7 @@ pub(crate) fn add_from_impl_for_enum(acc: &mut Assists, ctx: &AssistContext) ->
     }
     let field_type = field_list.fields().next()?.type_ref()?;
     let path = match field_type {
-        ast::TypeRef::PathType(p) => p,
+        ast::TypeRef::PathType(it) => it,
         _ => return None,
     };
 
@@ -51,9 +50,7 @@ pub(crate) fn add_from_impl_for_enum(acc: &mut Assists, ctx: &AssistContext) ->
         target,
         |edit| {
             let start_offset = variant.parent_enum().syntax().text_range().end();
-            let mut buf = String::new();
+            let buf = format!(
-            format_to!(
-                buf,
                 r#"
 
 impl From<{0}> for {1} {{

crates/ra_assists/src/handlers/add_function.rs

@@ -4,13 +4,13 @@ use ra_syntax::{
     ast::{
         self,
         edit::{AstNodeEdit, IndentLevel},
-        ArgListOwner, AstNode, ModuleItemOwner,
+        make, ArgListOwner, AstNode, ModuleItemOwner,
     },
     SyntaxKind, SyntaxNode, TextSize,
 };
 use rustc_hash::{FxHashMap, FxHashSet};
 
-use crate::{AssistContext, AssistId, Assists};
+use crate::{assist_config::SnippetCap, utils::render_snippet, AssistContext, AssistId, Assists};
 
 // Assist: add_function
 //
@@ -33,7 +33,7 @@ use crate::{AssistContext, AssistId, Assists};
 // }
 //
 // fn bar(arg: &str, baz: Baz) {
-//     todo!()
+//     ${0:todo!()}
 // }
 //
 // ```
@@ -58,21 +58,36 @@ pub(crate) fn add_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()>
     let function_builder = FunctionBuilder::from_call(&ctx, &call, &path, target_module)?;
 
     let target = call.syntax().text_range();
-    acc.add(AssistId("add_function"), "Add function", target, |edit| {
+    acc.add(AssistId("add_function"), "Add function", target, |builder| {
         let function_template = function_builder.render();
-        edit.set_file(function_template.file);
+        builder.set_file(function_template.file);
-        edit.set_cursor(function_template.cursor_offset);
+        let new_fn = function_template.to_string(ctx.config.snippet_cap);
-        edit.insert(function_template.insert_offset, function_template.fn_def.to_string());
+        match ctx.config.snippet_cap {
+            Some(cap) => builder.insert_snippet(cap, function_template.insert_offset, new_fn),
+            None => builder.insert(function_template.insert_offset, new_fn),
+        }
     })
 }
 
 struct FunctionTemplate {
     insert_offset: TextSize,
-    cursor_offset: TextSize,
+    placeholder_expr: ast::MacroCall,
-    fn_def: ast::SourceFile,
+    leading_ws: String,
+    fn_def: ast::FnDef,
+    trailing_ws: String,
     file: FileId,
 }
 
+impl FunctionTemplate {
+    fn to_string(&self, cap: Option<SnippetCap>) -> String {
+        let f = match cap {
+            Some(cap) => render_snippet(cap, self.fn_def.syntax(), self.placeholder_expr.syntax()),
+            None => self.fn_def.to_string(),
+        };
+        format!("{}{}{}", self.leading_ws, f, self.trailing_ws)
+    }
+}
+
 struct FunctionBuilder {
     target: GeneratedFunctionTarget,
     fn_name: ast::Name,
@@ -110,35 +125,41 @@ impl FunctionBuilder {
     }
 
     fn render(self) -> FunctionTemplate {
-        let placeholder_expr = ast::make::expr_todo();
+        let placeholder_expr = make::expr_todo();
-        let fn_body = ast::make::block_expr(vec![], Some(placeholder_expr));
+        let fn_body = make::block_expr(vec![], Some(placeholder_expr));
-        let mut fn_def = ast::make::fn_def(self.fn_name, self.type_params, self.params, fn_body);
+        let visibility = if self.needs_pub { Some(make::visibility_pub_crate()) } else { None };
-        if self.needs_pub {
+        let mut fn_def =
-            fn_def = ast::make::add_pub_crate_modifier(fn_def);
+            make::fn_def(visibility, self.fn_name, self.type_params, self.params, fn_body);
-        }
+        let leading_ws;
+        let trailing_ws;
 
-        let (fn_def, insert_offset) = match self.target {
+        let insert_offset = match self.target {
             GeneratedFunctionTarget::BehindItem(it) => {
-                let with_leading_blank_line = ast::make::add_leading_newlines(2, fn_def);
+                let indent = IndentLevel::from_node(&it);
-                let indented = with_leading_blank_line.indent(IndentLevel::from_node(&it));
+                leading_ws = format!("\n\n{}", indent);
-                (indented, it.text_range().end())
+                fn_def = fn_def.indent(indent);
+                trailing_ws = String::new();
+                it.text_range().end()
             }
             GeneratedFunctionTarget::InEmptyItemList(it) => {
-                let indent_once = IndentLevel(1);
                 let indent = IndentLevel::from_node(it.syntax());
-                let fn_def = ast::make::add_leading_newlines(1, fn_def);
+                leading_ws = format!("\n{}", indent + 1);
-                let fn_def = fn_def.indent(indent_once);
+                fn_def = fn_def.indent(indent + 1);
-                let fn_def = ast::make::add_trailing_newlines(1, fn_def);
+                trailing_ws = format!("\n{}", indent);
-                let fn_def = fn_def.indent(indent);
+                it.syntax().text_range().start() + TextSize::of('{')
-                (fn_def, it.syntax().text_range().start() + TextSize::of('{'))
             }
         };
 
         let placeholder_expr =
             fn_def.syntax().descendants().find_map(ast::MacroCall::cast).unwrap();
-        let cursor_offset_from_fn_start = placeholder_expr.syntax().text_range().start();
+        FunctionTemplate {
-        let cursor_offset = insert_offset + cursor_offset_from_fn_start;
+            insert_offset,
-        FunctionTemplate { insert_offset, cursor_offset, fn_def, file: self.file }
+            placeholder_expr,
+            leading_ws,
+            fn_def,
+            trailing_ws,
+            file: self.file,
+        }
     }
 }
 
@@ -158,7 +179,7 @@ impl GeneratedFunctionTarget {
 
 fn fn_name(call: &ast::Path) -> Option<ast::Name> {
     let name = call.segment()?.syntax().to_string();
-    Some(ast::make::name(&name))
+    Some(make::name(&name))
 }
 
 /// Computes the type variables and arguments required for the generated function
@@ -180,8 +201,8 @@ fn fn_args(
         });
     }
     deduplicate_arg_names(&mut arg_names);
-    let params = arg_names.into_iter().zip(arg_types).map(|(name, ty)| ast::make::param(name, ty));
+    let params = arg_names.into_iter().zip(arg_types).map(|(name, ty)| make::param(name, ty));
-    Some((None, ast::make::param_list(params)))
+    Some((None, make::param_list(params)))
 }
 
 /// Makes duplicate argument names unique by appending incrementing numbers.
@@ -316,7 +337,7 @@ fn foo() {
 }
 
 fn bar() {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -343,7 +364,7 @@ impl Foo {
 }
 
 fn bar() {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -367,7 +388,7 @@ fn foo1() {
 }
 
 fn bar() {
-    <|>todo!()
+    ${0:todo!()}
 }
 
 fn foo2() {}
@@ -393,7 +414,7 @@ mod baz {
     }
 
     fn bar() {
-        <|>todo!()
+        ${0:todo!()}
     }
 }
 ",
@@ -419,7 +440,7 @@ fn foo() {
 }
 
 fn bar(baz: Baz) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         );
@@ -452,7 +473,7 @@ impl Baz {
 }
 
 fn bar(baz: Baz) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -473,7 +494,7 @@ fn foo() {
 }
 
 fn bar(arg: &str) {
-    <|>todo!()
+    ${0:todo!()}
 }
 "#,
         )
@@ -494,7 +515,7 @@ fn foo() {
 }
 
 fn bar(arg: char) {
-    <|>todo!()
+    ${0:todo!()}
 }
 "#,
         )
@@ -515,7 +536,7 @@ fn foo() {
 }
 
 fn bar(arg: i32) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -536,7 +557,7 @@ fn foo() {
 }
 
 fn bar(arg: u8) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -561,7 +582,7 @@ fn foo() {
 }
 
 fn bar(x: u8) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -584,7 +605,7 @@ fn foo() {
 }
 
 fn bar(worble: ()) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -613,7 +634,7 @@ fn baz() {
 }
 
 fn bar(foo: impl Foo) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -640,7 +661,7 @@ fn foo() {
 }
 
 fn bar(baz: &Baz) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -669,7 +690,7 @@ fn foo() {
 }
 
 fn bar(baz: Baz::Bof) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -692,7 +713,7 @@ fn foo<T>(t: T) {
 }
 
 fn bar<T>(t: T) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -723,7 +744,7 @@ fn foo() {
 }
 
 fn bar(arg: fn() -> Baz) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -748,7 +769,7 @@ fn foo() {
 }
 
 fn bar(closure: impl Fn(i64) -> i64) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -769,7 +790,7 @@ fn foo() {
 }
 
 fn bar(baz: ()) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -794,7 +815,7 @@ fn foo() {
 }
 
 fn bar(baz_1: Baz, baz_2: Baz) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -819,7 +840,7 @@ fn foo() {
 }
 
 fn bar(baz_1: Baz, baz_2: Baz, arg_1: &str, arg_2: &str) {
-    <|>todo!()
+    ${0:todo!()}
 }
 "#,
         )
@@ -839,7 +860,7 @@ fn foo() {
             r"
 mod bar {
     pub(crate) fn my_fn() {
-        <|>todo!()
+        ${0:todo!()}
     }
 }
 
@@ -878,7 +899,7 @@ fn bar() {
 }
 
 fn baz(foo: foo::Foo) {
-    <|>todo!()
+    ${0:todo!()}
 }
 ",
         )
@@ -902,7 +923,7 @@ mod bar {
     fn something_else() {}
 
     pub(crate) fn my_fn() {
-        <|>todo!()
+        ${0:todo!()}
     }
 }
 
@@ -930,7 +951,7 @@ fn foo() {
 mod bar {
     mod baz {
         pub(crate) fn my_fn() {
-            <|>todo!()
+            ${0:todo!()}
         }
     }
 }
@@ -959,7 +980,7 @@ fn main() {
 
 
 pub(crate) fn bar() {
-    <|>todo!()
+    ${0:todo!()}
 }",
         )
     }

crates/ra_assists/src/handlers/replace_unwrap_with_match.rs

@@ -51,7 +51,7 @@ pub(crate) fn replace_unwrap_with_match(acc: &mut Assists, ctx: &AssistContext)
         let bind_path = make::path_unqualified(make::path_segment(make::name_ref("a")));
         let ok_arm = make::match_arm(iter::once(ok_tuple), make::expr_path(bind_path));
 
-        let unreachable_call = make::unreachable_macro_call().into();
+        let unreachable_call = make::expr_unreachable();
         let err_arm = make::match_arm(iter::once(make::placeholder_pat().into()), unreachable_call);
 
         let match_arm_list = make::match_arm_list(vec![ok_arm, err_arm]);

crates/ra_assists/src/tests/generated.rs

@@ -78,7 +78,7 @@ fn foo() {
 }
 
 fn bar(arg: &str, baz: Baz) {
-    todo!()
+    ${0:todo!()}
 }
 
 "#####,

crates/ra_assists/src/utils.rs

@@ -1,18 +1,44 @@
 //! Assorted functions shared by several assists.
 pub(crate) mod insert_use;
 
-use std::iter;
+use std::{iter, ops};
 
 use hir::{Adt, Crate, Semantics, Trait, Type};
 use ra_ide_db::RootDatabase;
 use ra_syntax::{
     ast::{self, make, NameOwner},
-    AstNode, T,
+    AstNode, SyntaxNode, T,
 };
 use rustc_hash::FxHashSet;
 
+use crate::assist_config::SnippetCap;
+
 pub(crate) use insert_use::insert_use_statement;
 
+pub(crate) fn render_snippet(
+    _cap: SnippetCap,
+    node: &SyntaxNode,
+    placeholder: &SyntaxNode,
+) -> String {
+    assert!(placeholder.ancestors().any(|it| it == *node));
+    let range = placeholder.text_range() - node.text_range().start();
+    let range: ops::Range<usize> = range.into();
+
+    let mut placeholder = placeholder.to_string();
+    escape(&mut placeholder);
+    let tab_stop = format!("${{0:{}}}", placeholder);
+
+    let mut buf = node.to_string();
+    buf.replace_range(range, &tab_stop);
+    return buf;
+
+    fn escape(buf: &mut String) {
+        stdx::replace(buf, '{', r"\{");
+        stdx::replace(buf, '}', r"\}");
+        stdx::replace(buf, '$', r"\$");
+    }
+}
+
 pub fn get_missing_assoc_items(
     sema: &Semantics<RootDatabase>,
     impl_def: &ast::ImplDef,

crates/ra_syntax/src/algo.rs

@@ -266,6 +266,15 @@ impl<'a> SyntaxRewriter<'a> {
         let replacement = Replacement::Single(with.clone().into());
         self.replacements.insert(what, replacement);
     }
+    pub fn replace_with_many<T: Clone + Into<SyntaxElement>>(
+        &mut self,
+        what: &T,
+        with: Vec<SyntaxElement>,
+    ) {
+        let what = what.clone().into();
+        let replacement = Replacement::Many(with);
+        self.replacements.insert(what, replacement);
+    }
     pub fn replace_ast<T: AstNode>(&mut self, what: &T, with: &T) {
         self.replace(what.syntax(), with.syntax())
     }
@@ -302,31 +311,41 @@ impl<'a> SyntaxRewriter<'a> {
 
     fn rewrite_children(&self, node: &SyntaxNode) -> SyntaxNode {
         //  FIXME: this could be made much faster.
-        let new_children =
+        let mut new_children = Vec::new();
-            node.children_with_tokens().flat_map(|it| self.rewrite_self(&it)).collect::<Vec<_>>();
+        for child in node.children_with_tokens() {
+            self.rewrite_self(&mut new_children, &child);
+        }
         with_children(node, new_children)
     }
 
     fn rewrite_self(
         &self,
+        acc: &mut Vec<NodeOrToken<rowan::GreenNode, rowan::GreenToken>>,
         element: &SyntaxElement,
-    ) -> Option<NodeOrToken<rowan::GreenNode, rowan::GreenToken>> {
+    ) {
         if let Some(replacement) = self.replacement(&element) {
-            return match replacement {
+            match replacement {
                 Replacement::Single(NodeOrToken::Node(it)) => {
-                    Some(NodeOrToken::Node(it.green().clone()))
+                    acc.push(NodeOrToken::Node(it.green().clone()))
                 }
                 Replacement::Single(NodeOrToken::Token(it)) => {
-                    Some(NodeOrToken::Token(it.green().clone()))
+                    acc.push(NodeOrToken::Token(it.green().clone()))
                 }
-                Replacement::Delete => None,
+                Replacement::Many(replacements) => {
+                    acc.extend(replacements.iter().map(|it| match it {
+                        NodeOrToken::Node(it) => NodeOrToken::Node(it.green().clone()),
+                        NodeOrToken::Token(it) => NodeOrToken::Token(it.green().clone()),
+                    }))
+                }
+                Replacement::Delete => (),
             };
+            return;
         }
         let res = match element {
             NodeOrToken::Token(it) => NodeOrToken::Token(it.green().clone()),
             NodeOrToken::Node(it) => NodeOrToken::Node(self.rewrite_children(it).green().clone()),
         };
-        Some(res)
+        acc.push(res)
     }
 }
 
@@ -341,6 +360,7 @@ impl ops::AddAssign for SyntaxRewriter<'_> {
 enum Replacement {
     Delete,
     Single(SyntaxElement),
+    Many(Vec<SyntaxElement>),
 }
 
 fn with_children(

crates/ra_syntax/src/ast/edit.rs

@@ -1,7 +1,10 @@
 //! This module contains functions for editing syntax trees. As the trees are
 //! immutable, all function here return a fresh copy of the tree, instead of
 //! doing an in-place modification.
-use std::{iter, ops::RangeInclusive};
+use std::{
+    fmt, iter,
+    ops::{self, RangeInclusive},
+};
 
 use arrayvec::ArrayVec;
 
@@ -437,6 +440,28 @@ impl From<u8> for IndentLevel {
     }
 }
 
+impl fmt::Display for IndentLevel {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let spaces = "                                        ";
+        let buf;
+        let len = self.0 as usize * 4;
+        let indent = if len <= spaces.len() {
+            &spaces[..len]
+        } else {
+            buf = iter::repeat(' ').take(len).collect::<String>();
+            &buf
+        };
+        fmt::Display::fmt(indent, f)
+    }
+}
+
+impl ops::Add<u8> for IndentLevel {
+    type Output = IndentLevel;
+    fn add(self, rhs: u8) -> IndentLevel {
+        IndentLevel(self.0 + rhs)
+    }
+}
+
 impl IndentLevel {
     pub fn from_node(node: &SyntaxNode) -> IndentLevel {
         let first_token = match node.first_token() {
@@ -453,6 +478,14 @@ impl IndentLevel {
         IndentLevel(0)
     }
 
+    /// XXX: this intentionally doesn't change the indent of the very first token.
+    /// Ie, in something like
+    /// ```
+    /// fn foo() {
+    ///    92
+    /// }
+    /// ```
+    /// if you indent the block, the `{` token would stay put.
     fn increase_indent(self, node: SyntaxNode) -> SyntaxNode {
         let mut rewriter = SyntaxRewriter::default();
         node.descendants_with_tokens()
@@ -463,12 +496,7 @@ impl IndentLevel {
                 text.contains('\n')
             })
             .for_each(|ws| {
-                let new_ws = make::tokens::whitespace(&format!(
+                let new_ws = make::tokens::whitespace(&format!("{}{}", ws.syntax(), self,));
-                    "{}{:width$}",
-                    ws.syntax().text(),
-                    "",
-                    width = self.0 as usize * 4
-                ));
                 rewriter.replace(ws.syntax(), &new_ws)
             });
         rewriter.rewrite(&node)
@@ -485,7 +513,7 @@ impl IndentLevel {
             })
             .for_each(|ws| {
                 let new_ws = make::tokens::whitespace(
-                    &ws.syntax().text().replace(&format!("\n{:1$}", "", self.0 as usize * 4), "\n"),
+                    &ws.syntax().text().replace(&format!("\n{}", self), "\n"),
                 );
                 rewriter.replace(ws.syntax(), &new_ws)
             });

crates/ra_syntax/src/ast/make.rs

@@ -1,5 +1,9 @@
 //! This module contains free-standing functions for creating AST fragments out
 //! of smaller pieces.
+//!
+//! Note that all functions here intended to be stupid constructors, which just
+//! assemble a finish node from immediate children. If you want to do something
+//! smarter than that, it probably doesn't belong in this module.
 use itertools::Itertools;
 use stdx::format_to;
 
@@ -95,6 +99,9 @@ pub fn expr_empty_block() -> ast::Expr {
 pub fn expr_unimplemented() -> ast::Expr {
     expr_from_text("unimplemented!()")
 }
+pub fn expr_unreachable() -> ast::Expr {
+    expr_from_text("unreachable!()")
+}
 pub fn expr_todo() -> ast::Expr {
     expr_from_text("todo!()")
 }
@@ -264,10 +271,6 @@ pub fn token(kind: SyntaxKind) -> SyntaxToken {
         .unwrap_or_else(|| panic!("unhandled token: {:?}", kind))
 }
 
-pub fn unreachable_macro_call() -> ast::MacroCall {
-    ast_from_text(&format!("unreachable!()"))
-}
-
 pub fn param(name: String, ty: String) -> ast::Param {
     ast_from_text(&format!("fn f({}: {}) {{ }}", name, ty))
 }
@@ -277,7 +280,12 @@ pub fn param_list(pats: impl IntoIterator<Item = ast::Param>) -> ast::ParamList
     ast_from_text(&format!("fn f({}) {{ }}", args))
 }
 
+pub fn visibility_pub_crate() -> ast::Visibility {
+    ast_from_text("pub(crate) struct S")
+}
+
 pub fn fn_def(
+    visibility: Option<ast::Visibility>,
     fn_name: ast::Name,
     type_params: Option<ast::TypeParamList>,
     params: ast::ParamList,
@@ -285,21 +293,11 @@ pub fn fn_def(
 ) -> ast::FnDef {
     let type_params =
         if let Some(type_params) = type_params { format!("<{}>", type_params) } else { "".into() };
-    ast_from_text(&format!("fn {}{}{} {}", fn_name, type_params, params, body))
+    let visibility = match visibility {
-}
+        None => String::new(),
-
+        Some(it) => format!("{} ", it),
-pub fn add_leading_newlines(amount_of_newlines: usize, t: impl AstNode) -> ast::SourceFile {
+    };
-    let newlines = "\n".repeat(amount_of_newlines);
+    ast_from_text(&format!("{}fn {}{}{} {}", visibility, fn_name, type_params, params, body))
-    ast_from_text(&format!("{}{}", newlines, t.syntax()))
-}
-
-pub fn add_trailing_newlines(amount_of_newlines: usize, t: impl AstNode) -> ast::SourceFile {
-    let newlines = "\n".repeat(amount_of_newlines);
-    ast_from_text(&format!("{}{}", t.syntax(), newlines))
-}
-
-pub fn add_pub_crate_modifier(fn_def: ast::FnDef) -> ast::FnDef {
-    ast_from_text(&format!("pub(crate) {}", fn_def))
 }
 
 fn ast_from_text<N: AstNode>(text: &str) -> N {

crates/rust-analyzer/src/to_proto.rs

@@ -639,7 +639,7 @@ fn main() <fold>{
 }
 
 pub(crate) fn code_action(world: &WorldSnapshot, assist: Assist) -> Result<lsp_ext::CodeAction> {
-    let res = if assist.source_change.is_snippet {
+    let res = if assist.source_change.cursor_position.is_none() {
         lsp_ext::CodeAction {
             title: assist.label,
             kind: Some(String::new()),
@@ -647,6 +647,7 @@ pub(crate) fn code_action(world: &WorldSnapshot, assist: Assist) -> Result<lsp_e
             command: None,
         }
     } else {
+        assert!(!assist.source_change.is_snippet);
         let source_change = source_change(&world, assist.source_change)?;
         let arg = serde_json::to_value(source_change)?;
         let title = assist.label;

crates/stdx/src/lib.rs

@@ -116,3 +116,11 @@ pub fn to_lower_snake_case(s: &str) -> String {
     }
     buf
 }
+
+pub fn replace(buf: &mut String, from: char, to: &str) {
+    if !buf.contains(from) {
+        return;
+    }
+    // FIXME: do this in place.
+    *buf = buf.replace(from, to)
+}

docs/user/assists.md

@@ -77,7 +77,7 @@ fn foo() {
 }
 
 fn bar(arg: &str, baz: Baz) {
-    todo!()
+    ${0:todo!()}
 }
 
 ```