11 files changed, 1406 insertions, 0 deletions

diff --git a/crates/ra_assists/Cargo.toml b/crates/ra_assists/Cargo.toml
new file mode 100644
index 000000000..20bc253e3
--- /dev/null
+++ b/crates/ra_assists/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+edition = "2018"
+name = "ra_assists"
+version = "0.1.0"
+authors = ["Aleksey Kladov <[email protected]>"]
+
+[dependencies]
+join_to_string = "0.1.3"
+
+ra_ide_api_light = { path = "../ra_ide_api_light" }
+ra_syntax = { path = "../ra_syntax" }
+ra_text_edit = { path = "../ra_text_edit" }
+ra_db = { path = "../ra_db" }
+hir = { path = "../ra_hir", package = "ra_hir" }
+
+[dev-dependencies]
+test_utils = { path = "../test_utils" }
diff --git a/crates/ra_assists/src/add_derive.rs b/crates/ra_assists/src/add_derive.rs
new file mode 100644
index 000000000..01a4079f6
--- /dev/null
+++ b/crates/ra_assists/src/add_derive.rs
@@ -0,0 +1,85 @@
+use hir::db::HirDatabase;
+use ra_syntax::{
+    ast::{self, AstNode, AttrsOwner},
+    SyntaxKind::{WHITESPACE, COMMENT},
+    TextUnit,
+};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn add_derive(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let nominal = ctx.node_at_offset::<ast::NominalDef>()?;
+    let node_start = derive_insertion_offset(nominal)?;
+    ctx.build("add `#[derive]`", |edit| {
+        let derive_attr = nominal
+            .attrs()
+            .filter_map(|x| x.as_call())
+            .filter(|(name, _arg)| name == "derive")
+            .map(|(_name, arg)| arg)
+            .next();
+        let offset = match derive_attr {
+            None => {
+                edit.insert(node_start, "#[derive()]\n");
+                node_start + TextUnit::of_str("#[derive(")
+            }
+            Some(tt) => tt.syntax().range().end() - TextUnit::of_char(')'),
+        };
+        edit.set_cursor(offset)
+    })
+}
+
+// Insert `derive` after doc comments.
+fn derive_insertion_offset(nominal: &ast::NominalDef) -> Option<TextUnit> {
+    let non_ws_child = nominal
+        .syntax()
+        .children()
+        .find(|it| it.kind() != COMMENT && it.kind() != WHITESPACE)?;
+    Some(non_ws_child.range().start())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn add_derive_new() {
+        check_assist(
+            add_derive,
+            "struct Foo { a: i32, <|>}",
+            "#[derive(<|>)]\nstruct Foo { a: i32, }",
+        );
+        check_assist(
+            add_derive,
+            "struct Foo { <|> a: i32, }",
+            "#[derive(<|>)]\nstruct Foo {  a: i32, }",
+        );
+    }
+
+    #[test]
+    fn add_derive_existing() {
+        check_assist(
+            add_derive,
+            "#[derive(Clone)]\nstruct Foo { a: i32<|>, }",
+            "#[derive(Clone<|>)]\nstruct Foo { a: i32, }",
+        );
+    }
+
+    #[test]
+    fn add_derive_new_with_doc_comment() {
+        check_assist(
+            add_derive,
+            "
+/// `Foo` is a pretty important struct.
+/// It does stuff.
+struct Foo { a: i32<|>, }
+            ",
+            "
+/// `Foo` is a pretty important struct.
+/// It does stuff.
+#[derive(<|>)]
+struct Foo { a: i32, }
+            ",
+        );
+    }
+}
diff --git a/crates/ra_assists/src/add_impl.rs b/crates/ra_assists/src/add_impl.rs
new file mode 100644
index 000000000..699508f91
--- /dev/null
+++ b/crates/ra_assists/src/add_impl.rs
@@ -0,0 +1,67 @@
+use join_to_string::join;
+use hir::db::HirDatabase;
+use ra_syntax::{
+    ast::{self, AstNode, AstToken, NameOwner, TypeParamsOwner},
+    TextUnit,
+};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn add_impl(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let nominal = ctx.node_at_offset::<ast::NominalDef>()?;
+    let name = nominal.name()?;
+    ctx.build("add impl", |edit| {
+        let type_params = nominal.type_param_list();
+        let start_offset = nominal.syntax().range().end();
+        let mut buf = String::new();
+        buf.push_str("\n\nimpl");
+        if let Some(type_params) = type_params {
+            type_params.syntax().text().push_to(&mut buf);
+        }
+        buf.push_str(" ");
+        buf.push_str(name.text().as_str());
+        if let Some(type_params) = type_params {
+            let lifetime_params = type_params
+                .lifetime_params()
+                .filter_map(|it| it.lifetime())
+                .map(|it| it.text());
+            let type_params = type_params
+                .type_params()
+                .filter_map(|it| it.name())
+                .map(|it| it.text());
+            join(lifetime_params.chain(type_params))
+                .surround_with("<", ">")
+                .to_buf(&mut buf);
+        }
+        buf.push_str(" {\n");
+        edit.set_cursor(start_offset + TextUnit::of_str(&buf));
+        buf.push_str("\n}");
+        edit.insert(start_offset, buf);
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn test_add_impl() {
+        check_assist(
+            add_impl,
+            "struct Foo {<|>}\n",
+            "struct Foo {}\n\nimpl Foo {\n<|>\n}\n",
+        );
+        check_assist(
+            add_impl,
+            "struct Foo<T: Clone> {<|>}",
+            "struct Foo<T: Clone> {}\n\nimpl<T: Clone> Foo<T> {\n<|>\n}",
+        );
+        check_assist(
+            add_impl,
+            "struct Foo<'a, T: Foo<'a>> {<|>}",
+            "struct Foo<'a, T: Foo<'a>> {}\n\nimpl<'a, T: Foo<'a>> Foo<'a, T> {\n<|>\n}",
+        );
+    }
+
+}
diff --git a/crates/ra_assists/src/assist_ctx.rs b/crates/ra_assists/src/assist_ctx.rs
new file mode 100644
index 000000000..6d09bde52
--- /dev/null
+++ b/crates/ra_assists/src/assist_ctx.rs
@@ -0,0 +1,154 @@
+use hir::db::HirDatabase;
+use ra_text_edit::TextEditBuilder;
+use ra_db::FileRange;
+use ra_syntax::{
+    SourceFile, TextRange, AstNode, TextUnit, SyntaxNode,
+    algo::{find_leaf_at_offset, find_node_at_offset, find_covering_node, LeafAtOffset},
+};
+use ra_ide_api_light::formatting::{leading_indent, reindent};
+
+use crate::{AssistLabel, AssistAction};
+
+pub(crate) enum Assist {
+    Unresolved(AssistLabel),
+    Resolved(AssistLabel, AssistAction),
+}
+
+/// `AssistCtx` allows to apply an assist or check if it could be applied.
+///
+/// Assists use a somewhat overengineered approach, given the current needs. The
+/// assists workflow consists of two phases. In the first phase, a user asks for
+/// the list of available assists. In the second phase, the user picks a
+/// particular assist and it gets applied.
+///
+/// There are two peculiarities here:
+///
+/// * first, we ideally avoid computing more things then necessary to answer
+///   "is assist applicable" in the first phase.
+/// * second, when we are applying assist, we don't have a guarantee that there
+///   weren't any changes between the point when user asked for assists and when
+///   they applied a particular assist. So, when applying assist, we need to do
+///   all the checks from scratch.
+///
+/// To avoid repeating the same code twice for both "check" and "apply"
+/// functions, we use an approach reminiscent of that of Django's function based
+/// views dealing with forms. Each assist receives a runtime parameter,
+/// `should_compute_edit`. It first check if an edit is applicable (potentially
+/// computing info required to compute the actual edit). If it is applicable,
+/// and `should_compute_edit` is `true`, it then computes the actual edit.
+///
+/// So, to implement the original assists workflow, we can first apply each edit
+/// with `should_compute_edit = false`, and then applying the selected edit
+/// again, with `should_compute_edit = true` this time.
+///
+/// Note, however, that we don't actually use such two-phase logic at the
+/// moment, because the LSP API is pretty awkward in this place, and it's much
+/// easier to just compute the edit eagerly :-)#[derive(Debug, Clone)]
+#[derive(Debug)]
+pub(crate) struct AssistCtx<'a, DB> {
+    pub(crate) db: &'a DB,
+    pub(crate) frange: FileRange,
+    source_file: &'a SourceFile,
+    should_compute_edit: bool,
+}
+
+impl<'a, DB> Clone for AssistCtx<'a, DB> {
+    fn clone(&self) -> Self {
+        AssistCtx {
+            db: self.db,
+            frange: self.frange,
+            source_file: self.source_file,
+            should_compute_edit: self.should_compute_edit,
+        }
+    }
+}
+
+impl<'a, DB: HirDatabase> AssistCtx<'a, DB> {
+    pub(crate) fn with_ctx<F, T>(db: &DB, frange: FileRange, should_compute_edit: bool, f: F) -> T
+    where
+        F: FnOnce(AssistCtx<DB>) -> T,
+    {
+        let source_file = &db.parse(frange.file_id);
+        let ctx = AssistCtx {
+            db,
+            frange,
+            source_file,
+            should_compute_edit,
+        };
+        f(ctx)
+    }
+
+    pub(crate) fn build(
+        self,
+        label: impl Into<String>,
+        f: impl FnOnce(&mut AssistBuilder),
+    ) -> Option<Assist> {
+        let label = AssistLabel {
+            label: label.into(),
+        };
+        if !self.should_compute_edit {
+            return Some(Assist::Unresolved(label));
+        }
+        let action = {
+            let mut edit = AssistBuilder::default();
+            f(&mut edit);
+            edit.build()
+        };
+        Some(Assist::Resolved(label, action))
+    }
+
+    pub(crate) fn leaf_at_offset(&self) -> LeafAtOffset<&'a SyntaxNode> {
+        find_leaf_at_offset(self.source_file.syntax(), self.frange.range.start())
+    }
+
+    pub(crate) fn node_at_offset<N: AstNode>(&self) -> Option<&'a N> {
+        find_node_at_offset(self.source_file.syntax(), self.frange.range.start())
+    }
+    pub(crate) fn covering_node(&self) -> &'a SyntaxNode {
+        find_covering_node(self.source_file.syntax(), self.frange.range)
+    }
+}
+
+#[derive(Default)]
+pub(crate) struct AssistBuilder {
+    edit: TextEditBuilder,
+    cursor_position: Option<TextUnit>,
+}
+
+impl AssistBuilder {
+    pub(crate) fn replace(&mut self, range: TextRange, replace_with: impl Into<String>) {
+        self.edit.replace(range, replace_with.into())
+    }
+
+    pub(crate) fn replace_node_and_indent(
+        &mut self,
+        node: &SyntaxNode,
+        replace_with: impl Into<String>,
+    ) {
+        let mut replace_with = replace_with.into();
+        if let Some(indent) = leading_indent(node) {
+            replace_with = reindent(&replace_with, indent)
+        }
+        self.replace(node.range(), replace_with)
+    }
+
+    #[allow(unused)]
+    pub(crate) fn delete(&mut self, range: TextRange) {
+        self.edit.delete(range)
+    }
+
+    pub(crate) fn insert(&mut self, offset: TextUnit, text: impl Into<String>) {
+        self.edit.insert(offset, text.into())
+    }
+
+    pub(crate) fn set_cursor(&mut self, offset: TextUnit) {
+        self.cursor_position = Some(offset)
+    }
+
+    fn build(self) -> AssistAction {
+        AssistAction {
+            edit: self.edit.finish(),
+            cursor_position: self.cursor_position,
+        }
+    }
+}
diff --git a/crates/ra_assists/src/change_visibility.rs b/crates/ra_assists/src/change_visibility.rs
new file mode 100644
index 000000000..4cd32985e
--- /dev/null
+++ b/crates/ra_assists/src/change_visibility.rs
@@ -0,0 +1,166 @@
+use hir::db::HirDatabase;
+use ra_syntax::{
+    AstNode, SyntaxNode, TextUnit,
+    ast::{self, VisibilityOwner, NameOwner},
+    SyntaxKind::{VISIBILITY, FN_KW, MOD_KW, STRUCT_KW, ENUM_KW, TRAIT_KW, FN_DEF, MODULE, STRUCT_DEF, ENUM_DEF, TRAIT_DEF, IDENT, WHITESPACE, COMMENT, ATTR},
+};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn change_visibility(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    if let Some(vis) = ctx.node_at_offset::<ast::Visibility>() {
+        return change_vis(ctx, vis);
+    }
+    add_vis(ctx)
+}
+
+fn add_vis(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let item_keyword = ctx.leaf_at_offset().find(|leaf| match leaf.kind() {
+        FN_KW | MOD_KW | STRUCT_KW | ENUM_KW | TRAIT_KW => true,
+        _ => false,
+    });
+
+    let offset = if let Some(keyword) = item_keyword {
+        let parent = keyword.parent()?;
+        let def_kws = vec![FN_DEF, MODULE, STRUCT_DEF, ENUM_DEF, TRAIT_DEF];
+        // Parent is not a definition, can't add visibility
+        if !def_kws.iter().any(|&def_kw| def_kw == parent.kind()) {
+            return None;
+        }
+        // Already have visibility, do nothing
+        if parent.children().any(|child| child.kind() == VISIBILITY) {
+            return None;
+        }
+        vis_offset(parent)
+    } else {
+        let ident = ctx.leaf_at_offset().find(|leaf| leaf.kind() == IDENT)?;
+        let field = ident.ancestors().find_map(ast::NamedFieldDef::cast)?;
+        if field.name()?.syntax().range() != ident.range() && field.visibility().is_some() {
+            return None;
+        }
+        vis_offset(field.syntax())
+    };
+
+    ctx.build("make pub(crate)", |edit| {
+        edit.insert(offset, "pub(crate) ");
+        edit.set_cursor(offset);
+    })
+}
+
+fn vis_offset(node: &SyntaxNode) -> TextUnit {
+    node.children()
+        .skip_while(|it| match it.kind() {
+            WHITESPACE | COMMENT | ATTR => true,
+            _ => false,
+        })
+        .next()
+        .map(|it| it.range().start())
+        .unwrap_or(node.range().start())
+}
+
+fn change_vis(ctx: AssistCtx<impl HirDatabase>, vis: &ast::Visibility) -> Option<Assist> {
+    if vis.syntax().text() == "pub" {
+        return ctx.build("chage to pub(crate)", |edit| {
+            edit.replace(vis.syntax().range(), "pub(crate)");
+            edit.set_cursor(vis.syntax().range().start());
+        });
+    }
+    if vis.syntax().text() == "pub(crate)" {
+        return ctx.build("chage to pub", |edit| {
+            edit.replace(vis.syntax().range(), "pub");
+            edit.set_cursor(vis.syntax().range().start());
+        });
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn change_visibility_adds_pub_crate_to_items() {
+        check_assist(
+            change_visibility,
+            "<|>fn foo() {}",
+            "<|>pub(crate) fn foo() {}",
+        );
+        check_assist(
+            change_visibility,
+            "f<|>n foo() {}",
+            "<|>pub(crate) fn foo() {}",
+        );
+        check_assist(
+            change_visibility,
+            "<|>struct Foo {}",
+            "<|>pub(crate) struct Foo {}",
+        );
+        check_assist(
+            change_visibility,
+            "<|>mod foo {}",
+            "<|>pub(crate) mod foo {}",
+        );
+        check_assist(
+            change_visibility,
+            "<|>trait Foo {}",
+            "<|>pub(crate) trait Foo {}",
+        );
+        check_assist(change_visibility, "m<|>od {}", "<|>pub(crate) mod {}");
+        check_assist(
+            change_visibility,
+            "unsafe f<|>n foo() {}",
+            "<|>pub(crate) unsafe fn foo() {}",
+        );
+    }
+
+    #[test]
+    fn change_visibility_works_with_struct_fields() {
+        check_assist(
+            change_visibility,
+            "struct S { <|>field: u32 }",
+            "struct S { <|>pub(crate) field: u32 }",
+        )
+    }
+
+    #[test]
+    fn change_visibility_pub_to_pub_crate() {
+        check_assist(
+            change_visibility,
+            "<|>pub fn foo() {}",
+            "<|>pub(crate) fn foo() {}",
+        )
+    }
+
+    #[test]
+    fn change_visibility_pub_crate_to_pub() {
+        check_assist(
+            change_visibility,
+            "<|>pub(crate) fn foo() {}",
+            "<|>pub fn foo() {}",
+        )
+    }
+
+    #[test]
+    fn change_visibility_handles_comment_attrs() {
+        check_assist(
+            change_visibility,
+            "
+            /// docs
+
+            // comments
+
+            #[derive(Debug)]
+            <|>struct Foo;
+            ",
+            "
+            /// docs
+
+            // comments
+
+            #[derive(Debug)]
+            <|>pub(crate) struct Foo;
+            ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/fill_match_arms.rs b/crates/ra_assists/src/fill_match_arms.rs
new file mode 100644
index 000000000..9aa37d94c
--- /dev/null
+++ b/crates/ra_assists/src/fill_match_arms.rs
@@ -0,0 +1,145 @@
+use std::fmt::Write;
+
+use hir::{
+    AdtDef, Ty, FieldSource, source_binder,
+    db::HirDatabase,
+};
+use ra_syntax::ast::{self, AstNode};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn fill_match_arms(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let match_expr = ctx.node_at_offset::<ast::MatchExpr>()?;
+
+    // We already have some match arms, so we don't provide any assists.
+    match match_expr.match_arm_list() {
+        Some(arm_list) if arm_list.arms().count() > 0 => {
+            return None;
+        }
+        _ => {}
+    }
+
+    let expr = match_expr.expr()?;
+    let function =
+        source_binder::function_from_child_node(ctx.db, ctx.frange.file_id, expr.syntax())?;
+    let infer_result = function.infer(ctx.db);
+    let syntax_mapping = function.body_syntax_mapping(ctx.db);
+    let node_expr = syntax_mapping.node_expr(expr)?;
+    let match_expr_ty = infer_result[node_expr].clone();
+    let enum_def = match match_expr_ty {
+        Ty::Adt {
+            def_id: AdtDef::Enum(e),
+            ..
+        } => e,
+        _ => return None,
+    };
+    let enum_name = enum_def.name(ctx.db)?;
+    let db = ctx.db;
+
+    ctx.build("fill match arms", |edit| {
+        let mut buf = format!("match {} {{\n", expr.syntax().text().to_string());
+        let variants = enum_def.variants(db);
+        for variant in variants {
+            let name = match variant.name(db) {
+                Some(it) => it,
+                None => continue,
+            };
+            write!(&mut buf, "    {}::{}", enum_name, name.to_string()).unwrap();
+
+            let pat = variant
+                .fields(db)
+                .into_iter()
+                .map(|field| {
+                    let name = field.name(db).to_string();
+                    let (_, source) = field.source(db);
+                    match source {
+                        FieldSource::Named(_) => name,
+                        FieldSource::Pos(_) => "_".to_string(),
+                    }
+                })
+                .collect::<Vec<_>>();
+
+            match pat.first().map(|s| s.as_str()) {
+                Some("_") => write!(&mut buf, "({})", pat.join(", ")).unwrap(),
+                Some(_) => write!(&mut buf, "{{{}}}", pat.join(", ")).unwrap(),
+                None => (),
+            };
+
+            buf.push_str(" => (),\n");
+        }
+        buf.push_str("}");
+        edit.set_cursor(expr.syntax().range().start());
+        edit.replace_node_and_indent(match_expr.syntax(), buf);
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::helpers::check_assist;
+
+    use super::fill_match_arms;
+
+    #[test]
+    fn fill_match_arms_empty_body() {
+        check_assist(
+            fill_match_arms,
+            r#"
+            enum A {
+                As,
+                Bs,
+                Cs(String),
+                Ds(String, String),
+                Es{x: usize, y: usize}
+            }
+
+            fn main() {
+                let a = A::As;
+                match a<|> {}
+            }
+            "#,
+            r#"
+            enum A {
+                As,
+                Bs,
+                Cs(String),
+                Ds(String, String),
+                Es{x: usize, y: usize}
+            }
+
+            fn main() {
+                let a = A::As;
+                match <|>a {
+                    A::As => (),
+                    A::Bs => (),
+                    A::Cs(_) => (),
+                    A::Ds(_, _) => (),
+                    A::Es{x, y} => (),
+                }
+            }
+            "#,
+        );
+    }
+    #[test]
+    fn fill_match_arms_no_body() {
+        check_assist(
+            fill_match_arms,
+            r#"
+            enum E { X, Y}
+
+            fn main() {
+                match E::X<|>
+            }
+            "#,
+            r#"
+            enum E { X, Y}
+
+            fn main() {
+                match <|>E::X {
+                    E::X => (),
+                    E::Y => (),
+                }
+            }
+            "#,
+        );
+    }
+}
diff --git a/crates/ra_assists/src/flip_comma.rs b/crates/ra_assists/src/flip_comma.rs
new file mode 100644
index 000000000..a49820c29
--- /dev/null
+++ b/crates/ra_assists/src/flip_comma.rs
@@ -0,0 +1,33 @@
+use hir::db::HirDatabase;
+use ra_syntax::{
+    Direction,
+    SyntaxKind::COMMA,
+};
+
+use crate::{AssistCtx, Assist, non_trivia_sibling};
+
+pub(crate) fn flip_comma(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let comma = ctx.leaf_at_offset().find(|leaf| leaf.kind() == COMMA)?;
+    let prev = non_trivia_sibling(comma, Direction::Prev)?;
+    let next = non_trivia_sibling(comma, Direction::Next)?;
+    ctx.build("flip comma", |edit| {
+        edit.replace(prev.range(), next.text());
+        edit.replace(next.range(), prev.text());
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn flip_comma_works_for_function_parameters() {
+        check_assist(
+            flip_comma,
+            "fn foo(x: i32,<|> y: Result<(), ()>) {}",
+            "fn foo(y: Result<(), ()>,<|> x: i32) {}",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/introduce_variable.rs b/crates/ra_assists/src/introduce_variable.rs
new file mode 100644
index 000000000..c937a816c
--- /dev/null
+++ b/crates/ra_assists/src/introduce_variable.rs
@@ -0,0 +1,432 @@
+use hir::db::HirDatabase;
+use ra_syntax::{
+    ast::{self, AstNode},
+    SyntaxKind::{
+        WHITESPACE, MATCH_ARM, LAMBDA_EXPR, PATH_EXPR, BREAK_EXPR, LOOP_EXPR, RETURN_EXPR, COMMENT
+    }, SyntaxNode, TextUnit,
+};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn introduce_variable<'a>(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let node = ctx.covering_node();
+    if !valid_covering_node(node) {
+        return None;
+    }
+    let expr = node.ancestors().filter_map(valid_target_expr).next()?;
+    let (anchor_stmt, wrap_in_block) = anchor_stmt(expr)?;
+    let indent = anchor_stmt.prev_sibling()?;
+    if indent.kind() != WHITESPACE {
+        return None;
+    }
+    ctx.build("introduce variable", move |edit| {
+        let mut buf = String::new();
+
+        let cursor_offset = if wrap_in_block {
+            buf.push_str("{ let var_name = ");
+            TextUnit::of_str("{ let ")
+        } else {
+            buf.push_str("let var_name = ");
+            TextUnit::of_str("let ")
+        };
+
+        expr.syntax().text().push_to(&mut buf);
+        let full_stmt = ast::ExprStmt::cast(anchor_stmt);
+        let is_full_stmt = if let Some(expr_stmt) = full_stmt {
+            Some(expr.syntax()) == expr_stmt.expr().map(|e| e.syntax())
+        } else {
+            false
+        };
+        if is_full_stmt {
+            if !full_stmt.unwrap().has_semi() {
+                buf.push_str(";");
+            }
+            edit.replace(expr.syntax().range(), buf);
+        } else {
+            buf.push_str(";");
+            indent.text().push_to(&mut buf);
+            edit.replace(expr.syntax().range(), "var_name".to_string());
+            edit.insert(anchor_stmt.range().start(), buf);
+            if wrap_in_block {
+                edit.insert(anchor_stmt.range().end(), " }");
+            }
+        }
+        edit.set_cursor(anchor_stmt.range().start() + cursor_offset);
+    })
+}
+
+fn valid_covering_node(node: &SyntaxNode) -> bool {
+    node.kind() != COMMENT
+}
+/// Check wether the node is a valid expression which can be extracted to a variable.
+/// In general that's true for any expression, but in some cases that would produce invalid code.
+fn valid_target_expr(node: &SyntaxNode) -> Option<&ast::Expr> {
+    return match node.kind() {
+        PATH_EXPR => None,
+        BREAK_EXPR => ast::BreakExpr::cast(node).and_then(|e| e.expr()),
+        RETURN_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()),
+        LOOP_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()),
+        _ => ast::Expr::cast(node),
+    };
+}
+
+/// Returns the syntax node which will follow the freshly introduced var
+/// and a boolean indicating whether we have to wrap it within a { } block
+/// to produce correct code.
+/// It can be a statement, the last in a block expression or a wanna be block
+/// expression like a lamba or match arm.
+fn anchor_stmt(expr: &ast::Expr) -> Option<(&SyntaxNode, bool)> {
+    expr.syntax().ancestors().find_map(|node| {
+        if ast::Stmt::cast(node).is_some() {
+            return Some((node, false));
+        }
+
+        if let Some(expr) = node
+            .parent()
+            .and_then(ast::Block::cast)
+            .and_then(|it| it.expr())
+        {
+            if expr.syntax() == node {
+                return Some((node, false));
+            }
+        }
+
+        if let Some(parent) = node.parent() {
+            if parent.kind() == MATCH_ARM || parent.kind() == LAMBDA_EXPR {
+                return Some((node, true));
+            }
+        }
+
+        None
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::{check_assist, check_assist_not_applicable, check_assist_range};
+
+    #[test]
+    fn test_introduce_var_simple() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    foo(<|>1 + 1<|>);
+}",
+            "
+fn foo() {
+    let <|>var_name = 1 + 1;
+    foo(var_name);
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_expr_stmt() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    <|>1 + 1<|>;
+}",
+            "
+fn foo() {
+    let <|>var_name = 1 + 1;
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_part_of_expr_stmt() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    <|>1<|> + 1;
+}",
+            "
+fn foo() {
+    let <|>var_name = 1;
+    var_name + 1;
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_last_expr() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    bar(<|>1 + 1<|>)
+}",
+            "
+fn foo() {
+    let <|>var_name = 1 + 1;
+    bar(var_name)
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_last_full_expr() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    <|>bar(1 + 1)<|>
+}",
+            "
+fn foo() {
+    let <|>var_name = bar(1 + 1);
+    var_name
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_block_expr_second_to_last() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn foo() {
+    <|>{ let x = 0; x }<|>
+    something_else();
+}",
+            "
+fn foo() {
+    let <|>var_name = { let x = 0; x };
+    something_else();
+}",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_in_match_arm_no_block() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => (<|>2 + 2<|>, true)
+        _ => (0, false)
+    };
+}
+",
+            "
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => { let <|>var_name = 2 + 2; (var_name, true) }
+        _ => (0, false)
+    };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_in_match_arm_with_block() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => {
+            let y = 1;
+            (<|>2 + y<|>, true)
+        }
+        _ => (0, false)
+    };
+}
+",
+            "
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => {
+            let y = 1;
+            let <|>var_name = 2 + y;
+            (var_name, true)
+        }
+        _ => (0, false)
+    };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_in_closure_no_block() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn main() {
+    let lambda = |x: u32| <|>x * 2<|>;
+}
+",
+            "
+fn main() {
+    let lambda = |x: u32| { let <|>var_name = x * 2; var_name };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_in_closure_with_block() {
+        check_assist_range(
+            introduce_variable,
+            "
+fn main() {
+    let lambda = |x: u32| { <|>x * 2<|> };
+}
+",
+            "
+fn main() {
+    let lambda = |x: u32| { let <|>var_name = x * 2; var_name };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_path_simple() {
+        check_assist(
+            introduce_variable,
+            "
+fn main() {
+    let o = S<|>ome(true);
+}
+",
+            "
+fn main() {
+    let <|>var_name = Some(true);
+    let o = var_name;
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_path_method() {
+        check_assist(
+            introduce_variable,
+            "
+fn main() {
+    let v = b<|>ar.foo();
+}
+",
+            "
+fn main() {
+    let <|>var_name = bar.foo();
+    let v = var_name;
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_return() {
+        check_assist(
+            introduce_variable,
+            "
+fn foo() -> u32 {
+    r<|>eturn 2 + 2;
+}
+",
+            "
+fn foo() -> u32 {
+    let <|>var_name = 2 + 2;
+    return var_name;
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_break() {
+        check_assist(
+            introduce_variable,
+            "
+fn main() {
+    let result = loop {
+        b<|>reak 2 + 2;
+    };
+}
+",
+            "
+fn main() {
+    let result = loop {
+        let <|>var_name = 2 + 2;
+        break var_name;
+    };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_for_cast() {
+        check_assist(
+            introduce_variable,
+            "
+fn main() {
+    let v = 0f32 a<|>s u32;
+}
+",
+            "
+fn main() {
+    let <|>var_name = 0f32 as u32;
+    let v = var_name;
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_for_return_not_applicable() {
+        check_assist_not_applicable(
+            introduce_variable,
+            "
+fn foo() {
+    r<|>eturn;
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_for_break_not_applicable() {
+        check_assist_not_applicable(
+            introduce_variable,
+            "
+fn main() {
+    loop {
+        b<|>reak;
+    };
+}
+",
+        );
+    }
+
+    #[test]
+    fn test_introduce_var_in_comment_not_applicable() {
+        check_assist_not_applicable(
+            introduce_variable,
+            "
+fn main() {
+    let x = true;
+    let tuple = match x {
+        // c<|>omment
+        true => (2 + 2, true)
+        _ => (0, false)
+    };
+}
+",
+        );
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
new file mode 100644
index 000000000..4e97a84c2
--- /dev/null
+++ b/crates/ra_assists/src/lib.rs
@@ -0,0 +1,170 @@
+//! `ra_assits` crate provides a bunch of code assists, aslo known as code
+//! actions (in LSP) or intentions (in IntelliJ).
+//!
+//! An assist is a micro-refactoring, which is automatically activated in
+//! certain context. For example, if the cursor is over `,`, a "swap `,`" assist
+//! becomes available.
+
+mod assist_ctx;
+
+use ra_text_edit::TextEdit;
+use ra_syntax::{TextUnit, SyntaxNode, Direction};
+use ra_db::FileRange;
+use hir::db::HirDatabase;
+
+pub(crate) use crate::assist_ctx::{AssistCtx, Assist};
+
+#[derive(Debug)]
+pub struct AssistLabel {
+    /// Short description of the assist, as shown in the UI.
+    pub label: String,
+}
+
+pub struct AssistAction {
+    pub edit: TextEdit,
+    pub cursor_position: Option<TextUnit>,
+}
+
+/// Return all the assists applicable at the given position.
+///
+/// Assists are returned in the "unresolved" state, that is only labels are
+/// returned, without actual edits.
+pub fn applicable_assists<H>(db: &H, range: FileRange) -> Vec<AssistLabel>
+where
+    H: HirDatabase + 'static,
+{
+    AssistCtx::with_ctx(db, range, false, |ctx| {
+        all_assists()
+            .iter()
+            .filter_map(|f| f(ctx.clone()))
+            .map(|a| match a {
+                Assist::Unresolved(label) => label,
+                Assist::Resolved(..) => unreachable!(),
+            })
+            .collect()
+    })
+}
+
+/// Return all the assists applicable at the given position.
+///
+/// Assists are returned in the "resolved" state, that is with edit fully
+/// computed.
+pub fn assists<H>(db: &H, range: FileRange) -> Vec<(AssistLabel, AssistAction)>
+where
+    H: HirDatabase + 'static,
+{
+    AssistCtx::with_ctx(db, range, false, |ctx| {
+        all_assists()
+            .iter()
+            .filter_map(|f| f(ctx.clone()))
+            .map(|a| match a {
+                Assist::Resolved(label, action) => (label, action),
+                Assist::Unresolved(..) => unreachable!(),
+            })
+            .collect()
+    })
+}
+
+mod add_derive;
+mod add_impl;
+mod flip_comma;
+mod change_visibility;
+mod fill_match_arms;
+mod introduce_variable;
+mod replace_if_let_with_match;
+mod split_import;
+fn all_assists<DB: HirDatabase>() -> &'static [fn(AssistCtx<DB>) -> Option<Assist>] {
+    &[
+        add_derive::add_derive,
+        add_impl::add_impl,
+        change_visibility::change_visibility,
+        fill_match_arms::fill_match_arms,
+        flip_comma::flip_comma,
+        introduce_variable::introduce_variable,
+        replace_if_let_with_match::replace_if_let_with_match,
+        split_import::split_import,
+    ]
+}
+
+fn non_trivia_sibling(node: &SyntaxNode, direction: Direction) -> Option<&SyntaxNode> {
+    node.siblings(direction)
+        .skip(1)
+        .find(|node| !node.kind().is_trivia())
+}
+
+#[cfg(test)]
+mod helpers {
+    use hir::mock::MockDatabase;
+    use ra_syntax::TextRange;
+    use ra_db::FileRange;
+    use test_utils::{extract_offset, assert_eq_text, add_cursor, extract_range};
+
+    use crate::{AssistCtx, Assist};
+
+    pub(crate) fn check_assist(
+        assist: fn(AssistCtx<MockDatabase>) -> Option<Assist>,
+        before: &str,
+        after: &str,
+    ) {
+        let (before_cursor_pos, before) = extract_offset(before);
+        let (db, _source_root, file_id) = MockDatabase::with_single_file(&before);
+        let frange = FileRange {
+            file_id,
+            range: TextRange::offset_len(before_cursor_pos, 0.into()),
+        };
+        let assist =
+            AssistCtx::with_ctx(&db, frange, true, assist).expect("code action is not applicable");
+        let action = match assist {
+            Assist::Unresolved(_) => unreachable!(),
+            Assist::Resolved(_, it) => it,
+        };
+
+        let actual = action.edit.apply(&before);
+        let actual_cursor_pos = match action.cursor_position {
+            None => action
+                .edit
+                .apply_to_offset(before_cursor_pos)
+                .expect("cursor position is affected by the edit"),
+            Some(off) => off,
+        };
+        let actual = add_cursor(&actual, actual_cursor_pos);
+        assert_eq_text!(after, &actual);
+    }
+
+    pub(crate) fn check_assist_range(
+        assist: fn(AssistCtx<MockDatabase>) -> Option<Assist>,
+        before: &str,
+        after: &str,
+    ) {
+        let (range, before) = extract_range(before);
+        let (db, _source_root, file_id) = MockDatabase::with_single_file(&before);
+        let frange = FileRange { file_id, range };
+        let assist =
+            AssistCtx::with_ctx(&db, frange, true, assist).expect("code action is not applicable");
+        let action = match assist {
+            Assist::Unresolved(_) => unreachable!(),
+            Assist::Resolved(_, it) => it,
+        };
+
+        let mut actual = action.edit.apply(&before);
+        if let Some(pos) = action.cursor_position {
+            actual = add_cursor(&actual, pos);
+        }
+        assert_eq_text!(after, &actual);
+    }
+
+    pub(crate) fn check_assist_not_applicable(
+        assist: fn(AssistCtx<MockDatabase>) -> Option<Assist>,
+        before: &str,
+    ) {
+        let (before_cursor_pos, before) = extract_offset(before);
+        let (db, _source_root, file_id) = MockDatabase::with_single_file(&before);
+        let frange = FileRange {
+            file_id,
+            range: TextRange::offset_len(before_cursor_pos, 0.into()),
+        };
+        let assist = AssistCtx::with_ctx(&db, frange, true, assist);
+        assert!(assist.is_none());
+    }
+
+}
diff --git a/crates/ra_assists/src/replace_if_let_with_match.rs b/crates/ra_assists/src/replace_if_let_with_match.rs
new file mode 100644
index 000000000..f6af47ec9
--- /dev/null
+++ b/crates/ra_assists/src/replace_if_let_with_match.rs
@@ -0,0 +1,80 @@
+use ra_syntax::{AstNode, ast};
+use ra_ide_api_light::formatting::extract_trivial_expression;
+use hir::db::HirDatabase;
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn replace_if_let_with_match(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let if_expr: &ast::IfExpr = ctx.node_at_offset()?;
+    let cond = if_expr.condition()?;
+    let pat = cond.pat()?;
+    let expr = cond.expr()?;
+    let then_block = if_expr.then_branch()?;
+    let else_block = match if_expr.else_branch()? {
+        ast::ElseBranchFlavor::Block(it) => it,
+        ast::ElseBranchFlavor::IfExpr(_) => return None,
+    };
+
+    ctx.build("replace with match", |edit| {
+        let match_expr = build_match_expr(expr, pat, then_block, else_block);
+        edit.replace_node_and_indent(if_expr.syntax(), match_expr);
+        edit.set_cursor(if_expr.syntax().range().start())
+    })
+}
+
+fn build_match_expr(
+    expr: &ast::Expr,
+    pat1: &ast::Pat,
+    arm1: &ast::Block,
+    arm2: &ast::Block,
+) -> String {
+    let mut buf = String::new();
+    buf.push_str(&format!("match {} {{\n", expr.syntax().text()));
+    buf.push_str(&format!(
+        "    {} => {}\n",
+        pat1.syntax().text(),
+        format_arm(arm1)
+    ));
+    buf.push_str(&format!("    _ => {}\n", format_arm(arm2)));
+    buf.push_str("}");
+    buf
+}
+
+fn format_arm(block: &ast::Block) -> String {
+    match extract_trivial_expression(block) {
+        None => block.syntax().text().to_string(),
+        Some(e) => format!("{},", e.syntax().text()),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn test_replace_if_let_with_match_unwraps_simple_expressions() {
+        check_assist(
+            replace_if_let_with_match,
+            "
+impl VariantData {
+    pub fn is_struct(&self) -> bool {
+        if <|>let VariantData::Struct(..) = *self {
+            true
+        } else {
+            false
+        }
+    }
+}           ",
+            "
+impl VariantData {
+    pub fn is_struct(&self) -> bool {
+        <|>match *self {
+            VariantData::Struct(..) => true,
+            _ => false,
+        }
+    }
+}           ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/split_import.rs b/crates/ra_assists/src/split_import.rs
new file mode 100644
index 000000000..7e34be087
--- /dev/null
+++ b/crates/ra_assists/src/split_import.rs
@@ -0,0 +1,57 @@
+use hir::db::HirDatabase;
+use ra_syntax::{
+    TextUnit, AstNode, SyntaxKind::COLONCOLON,
+    ast,
+    algo::generate,
+};
+
+use crate::{AssistCtx, Assist};
+
+pub(crate) fn split_import(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let colon_colon = ctx
+        .leaf_at_offset()
+        .find(|leaf| leaf.kind() == COLONCOLON)?;
+    let path = colon_colon.parent().and_then(ast::Path::cast)?;
+    let top_path = generate(Some(path), |it| it.parent_path()).last()?;
+
+    let use_tree = top_path.syntax().ancestors().find_map(ast::UseTree::cast);
+    if use_tree.is_none() {
+        return None;
+    }
+
+    let l_curly = colon_colon.range().end();
+    let r_curly = match top_path.syntax().parent().and_then(ast::UseTree::cast) {
+        Some(tree) => tree.syntax().range().end(),
+        None => top_path.syntax().range().end(),
+    };
+
+    ctx.build("split import", |edit| {
+        edit.insert(l_curly, "{");
+        edit.insert(r_curly, "}");
+        edit.set_cursor(l_curly + TextUnit::of_str("{"));
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::check_assist;
+
+    #[test]
+    fn test_split_import() {
+        check_assist(
+            split_import,
+            "use crate::<|>db::RootDatabase;",
+            "use crate::{<|>db::RootDatabase};",
+        )
+    }
+
+    #[test]
+    fn split_import_works_with_trees() {
+        check_assist(
+            split_import,
+            "use algo:<|>:visitor::{Visitor, visit}",
+            "use algo::{<|>visitor::{Visitor, visit}}",
+        )
+    }
+}