7 files changed, 1504 insertions, 9 deletions
diff --git a/crates/ide_assists/src/handlers/add_turbo_fish.rs b/crates/ide_assists/src/handlers/add_turbo_fish.rs
index 8e9ea4fad..a08b55ebb 100644
--- a/crates/ide_assists/src/handlers/add_turbo_fish.rs
+++ b/crates/ide_assists/src/handlers/add_turbo_fish.rs
@@ -31,6 +31,7 @@ pub(crate) fn add_turbo_fish(acc: &mut Assists, ctx: &AssistContext) -> Option<(
            return None;
        }
        mark::hit!(add_turbo_fish_after_call);
+        mark::hit!(add_type_ascription_after_call);
        arg_list.l_paren_token()?.prev_token().filter(|it| it.kind() == SyntaxKind::IDENT)
    })?;
    let next_token = ident.next_token()?;
@@ -52,6 +53,24 @@ pub(crate) fn add_turbo_fish(acc: &mut Assists, ctx: &AssistContext) -> Option<(
        mark::hit!(add_turbo_fish_non_generic);
        return None;
    }
+    if let Some(let_stmt) = ctx.find_node_at_offset::<ast::LetStmt>() {
+        if let_stmt.colon_token().is_none() {
+            let type_pos = let_stmt.pat()?.syntax().last_token()?.text_range().end();
+            acc.add(
+                AssistId("add_type_ascription", AssistKind::RefactorRewrite),
+                "Add `: _` before assignment operator",
+                ident.text_range(),
+                |builder| match ctx.config.snippet_cap {
+                    Some(cap) => builder.insert_snippet(cap, type_pos, ": ${0:_}"),
+                    None => builder.insert(type_pos, ": _"),
+                },
+            )?
+        } else {
+            mark::hit!(add_type_ascription_already_typed);
+        }
+    }
    acc.add(
        AssistId("add_turbo_fish", AssistKind::RefactorRewrite),
        "Add `::<>`",
@@ -65,7 +84,7 @@ pub(crate) fn add_turbo_fish(acc: &mut Assists, ctx: &AssistContext) -> Option<(
 #[cfg(test)]
 mod tests {
-    use crate::tests::{check_assist, check_assist_not_applicable};
+    use crate::tests::{check_assist, check_assist_by_label, check_assist_not_applicable};
    use super::*;
    use test_utils::mark;
@@ -161,4 +180,91 @@ fn main() {
 "#,
        );
    }
+    #[test]
+    fn add_type_ascription_function() {
+        check_assist_by_label(
+            add_turbo_fish,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x = make$0();
+}
+"#,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: ${0:_} = make();
+}
+"#,
+            "Add `: _` before assignment operator",
+        );
+    }
+    #[test]
+    fn add_type_ascription_after_call() {
+        mark::check!(add_type_ascription_after_call);
+        check_assist_by_label(
+            add_turbo_fish,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x = make()$0;
+}
+"#,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: ${0:_} = make();
+}
+"#,
+            "Add `: _` before assignment operator",
+        );
+    }
+    #[test]
+    fn add_type_ascription_method() {
+        check_assist_by_label(
+            add_turbo_fish,
+            r#"
+struct S;
+impl S {
+    fn make<T>(&self) -> T {}
+}
+fn main() {
+    let x = S.make$0();
+}
+"#,
+            r#"
+struct S;
+impl S {
+    fn make<T>(&self) -> T {}
+}
+fn main() {
+    let x: ${0:_} = S.make();
+}
+"#,
+            "Add `: _` before assignment operator",
+        );
+    }
+    #[test]
+    fn add_type_ascription_already_typed() {
+        mark::check!(add_type_ascription_already_typed);
+        check_assist(
+            add_turbo_fish,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: () = make$0();
+}
+"#,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: () = make::<${0:_}>();
+}
+"#,
+        );
+    }
 }
diff --git a/crates/ide_assists/src/handlers/apply_demorgan.rs b/crates/ide_assists/src/handlers/apply_demorgan.rs
index 6997ea048..128b1eb56 100644
--- a/crates/ide_assists/src/handlers/apply_demorgan.rs
+++ b/crates/ide_assists/src/handlers/apply_demorgan.rs
@@ -1,4 +1,5 @@
 use syntax::ast::{self, AstNode};
+use test_utils::mark;
 use crate::{utils::invert_boolean_expression, AssistContext, AssistId, AssistKind, Assists};
@@ -43,9 +44,36 @@ pub(crate) fn apply_demorgan(acc: &mut Assists, ctx: &AssistContext) -> Option<(
        "Apply De Morgan's law",
        op_range,
        |edit| {
+            let paren_expr = expr.syntax().parent().and_then(|parent| ast::ParenExpr::cast(parent));
+            let neg_expr = paren_expr
+                .clone()
+                .and_then(|paren_expr| paren_expr.syntax().parent())
+                .and_then(|parent| ast::PrefixExpr::cast(parent))
+                .and_then(|prefix_expr| {
+                    if prefix_expr.op_kind().unwrap() == ast::PrefixOp::Not {
+                        Some(prefix_expr)
+                    } else {
+                        None
+                    }
+                });
            edit.replace(op_range, opposite_op);
-            edit.replace(lhs_range, format!("!({}", not_lhs.syntax().text()));
-            edit.replace(rhs_range, format!("{})", not_rhs.syntax().text()));
+            if let Some(paren_expr) = paren_expr {
+                edit.replace(lhs_range, not_lhs.syntax().text());
+                edit.replace(rhs_range, not_rhs.syntax().text());
+                if let Some(neg_expr) = neg_expr {
+                    mark::hit!(demorgan_double_negation);
+                    edit.replace(neg_expr.op_token().unwrap().text_range(), "");
+                } else {
+                    mark::hit!(demorgan_double_parens);
+                    edit.replace(paren_expr.l_paren_token().unwrap().text_range(), "!(");
+                }
+            } else {
+                edit.replace(lhs_range, format!("!({}", not_lhs.syntax().text()));
+                edit.replace(rhs_range, format!("{})", not_rhs.syntax().text()));
+            }
        },
    )
 }
@@ -62,6 +90,7 @@ fn opposite_logic_op(kind: ast::BinOp) -> Option<&'static str> {
 #[cfg(test)]
 mod tests {
    use ide_db::helpers::FamousDefs;
+    use test_utils::mark;
    use super::*;
@@ -156,4 +185,16 @@ fn f() {
    fn demorgan_doesnt_apply_with_cursor_not_on_op() {
        check_assist_not_applicable(apply_demorgan, "fn f() { $0 !x || !x }")
    }
+    #[test]
+    fn demorgan_doesnt_double_negation() {
+        mark::check!(demorgan_double_negation);
+        check_assist(apply_demorgan, "fn f() { !(x ||$0 x) }", "fn f() { (!x && !x) }")
+    }
+    #[test]
+    fn demorgan_doesnt_double_parens() {
+        mark::check!(demorgan_double_parens);
+        check_assist(apply_demorgan, "fn f() { (x ||$0 x) }", "fn f() { !(!x && !x) }")
+    }
 }
diff --git a/crates/ide_assists/src/handlers/convert_comment_block.rs b/crates/ide_assists/src/handlers/convert_comment_block.rs
new file mode 100644
index 000000000..cdc45fc42
--- /dev/null
+++ b/crates/ide_assists/src/handlers/convert_comment_block.rs
@@ -0,0 +1,419 @@
+use itertools::Itertools;
+use std::convert::identity;
+use syntax::{
+    ast::{
+        self,
+        edit::IndentLevel,
+        Comment, CommentKind,
+        CommentPlacement::{Inner, Outer},
+        CommentShape::{self, Block, Line},
+        Whitespace,
+    },
+    AstToken, Direction, SyntaxElement, TextRange,
+};
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+/// Assist: line_to_block
+///
+/// Converts comments between block and single-line form
+///
+/// ```
+///    // Multi-line
+///    // comment
+/// ```
+/// ->
+/// ```
+///   /**
+///   Multi-line
+///   comment
+///   */
+/// ```
+pub(crate) fn convert_comment_block(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    if let Some(comment) = ctx.find_token_at_offset::<ast::Comment>() {
+        // Only allow comments which are alone on their line
+        if let Some(prev) = comment.syntax().prev_token() {
+            if Whitespace::cast(prev).filter(|w| w.text().contains('\n')).is_none() {
+                return None;
+            }
+        }
+        return match comment.kind().shape {
+            ast::CommentShape::Block => block_to_line(acc, comment),
+            ast::CommentShape::Line => line_to_block(acc, comment),
+        };
+    }
+    return None;
+}
+fn block_to_line(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
+    let target = comment.syntax().text_range();
+    acc.add(
+        AssistId("block_to_line", AssistKind::RefactorRewrite),
+        "Replace block comment with line comments",
+        target,
+        |edit| {
+            let indentation = IndentLevel::from_token(comment.syntax());
+            let line_prefix =
+                comment_kind_prefix(CommentKind { shape: CommentShape::Line, ..comment.kind() });
+            let text = comment.text();
+            let text = &text[comment.prefix().len()..(text.len() - "*/".len())].trim();
+            let lines = text.lines().peekable();
+            let indent_spaces = indentation.to_string();
+            let output = lines
+                .map(|l| l.trim_start_matches(&indent_spaces))
+                .map(|l| {
+                    // Don't introduce trailing whitespace
+                    if l.is_empty() {
+                        line_prefix.to_string()
+                    } else {
+                        format!("{} {}", line_prefix, l.trim_start_matches(&indent_spaces))
+                    }
+                })
+                .join(&format!("\n{}", indent_spaces));
+            edit.replace(target, output)
+        },
+    )
+}
+fn line_to_block(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
+    // Find all the comments we'll be collapsing into a block
+    let comments = relevant_line_comments(&comment);
+    // Establish the target of our edit based on the comments we found
+    let target = TextRange::new(
+        comments[0].syntax().text_range().start(),
+        comments.last().unwrap().syntax().text_range().end(),
+    );
+    acc.add(
+        AssistId("line_to_block", AssistKind::RefactorRewrite),
+        "Replace line comments with a single block comment",
+        target,
+        |edit| {
+            // We pick a single indentation level for the whole block comment based on the
+            // comment where the assist was invoked. This will be prepended to the
+            // contents of each line comment when they're put into the block comment.
+            let indentation = IndentLevel::from_token(&comment.syntax());
+            let block_comment_body =
+                comments.into_iter().map(|c| line_comment_text(indentation, c)).join("\n");
+            let block_prefix =
+                comment_kind_prefix(CommentKind { shape: CommentShape::Block, ..comment.kind() });
+            let output =
+                format!("{}\n{}\n{}*/", block_prefix, block_comment_body, indentation.to_string());
+            edit.replace(target, output)
+        },
+    )
+}
+/// The line -> block assist can  be invoked from anywhere within a sequence of line comments.
+/// relevant_line_comments crawls backwards and forwards finding the complete sequence of comments that will
+/// be joined.
+fn relevant_line_comments(comment: &ast::Comment) -> Vec<Comment> {
+    // The prefix identifies the kind of comment we're dealing with
+    let prefix = comment.prefix();
+    let same_prefix = |c: &ast::Comment| c.prefix() == prefix;
+    // These tokens are allowed to exist between comments
+    let skippable = |not: &SyntaxElement| {
+        not.clone()
+            .into_token()
+            .and_then(Whitespace::cast)
+            .map(|w| !w.spans_multiple_lines())
+            .unwrap_or(false)
+    };
+    // Find all preceding comments (in reverse order) that have the same prefix
+    let prev_comments = comment
+        .syntax()
+        .siblings_with_tokens(Direction::Prev)
+        .filter(|s| !skippable(s))
+        .map(|not| not.into_token().and_then(Comment::cast).filter(same_prefix))
+        .take_while(|opt_com| opt_com.is_some())
+        .filter_map(identity)
+        .skip(1); // skip the first element so we don't duplicate it in next_comments
+    let next_comments = comment
+        .syntax()
+        .siblings_with_tokens(Direction::Next)
+        .filter(|s| !skippable(s))
+        .map(|not| not.into_token().and_then(Comment::cast).filter(same_prefix))
+        .take_while(|opt_com| opt_com.is_some())
+        .filter_map(identity);
+    let mut comments: Vec<_> = prev_comments.collect();
+    comments.reverse();
+    comments.extend(next_comments);
+    comments
+}
+// Line comments usually begin with a single space character following the prefix as seen here:
+//^
+// But comments can also include indented text:
+//    > Hello there
+//
+// We handle this by stripping *AT MOST* one space character from the start of the line
+// This has its own problems because it can cause alignment issues:
+//
+//              /*
+// a      ----> a
+//b       ----> b
+//              */
+//
+// But since such comments aren't idiomatic we're okay with this.
+fn line_comment_text(indentation: IndentLevel, comm: ast::Comment) -> String {
+    let contents_without_prefix = comm.text().strip_prefix(comm.prefix()).unwrap();
+    let contents = contents_without_prefix.strip_prefix(' ').unwrap_or(contents_without_prefix);
+    // Don't add the indentation if the line is empty
+    if contents.is_empty() {
+        contents.to_owned()
+    } else {
+        indentation.to_string() + &contents
+    }
+}
+fn comment_kind_prefix(ck: ast::CommentKind) -> &'static str {
+    match (ck.shape, ck.doc) {
+        (Line, Some(Inner)) => "//!",
+        (Line, Some(Outer)) => "///",
+        (Line, None) => "//",
+        (Block, Some(Inner)) => "/*!",
+        (Block, Some(Outer)) => "/**",
+        (Block, None) => "/*",
+    }
+}
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+    use super::*;
+    #[test]
+    fn single_line_to_block() {
+        check_assist(
+            convert_comment_block,
+            r#"
+// line$0 comment
+fn main() {
+    foo();
+}
+"#,
+            r#"
+/*
+line comment
+*/
+fn main() {
+    foo();
+}
+"#,
+        );
+    }
+    #[test]
+    fn single_line_to_block_indented() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // line$0 comment
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /*
+    line comment
+    */
+    foo();
+}
+"#,
+        );
+    }
+    #[test]
+    fn multiline_to_block() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // above
+    // line$0 comment
+    //
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /*
+    above
+    line comment
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+    #[test]
+    fn end_of_line_to_block() {
+        check_assist_not_applicable(
+            convert_comment_block,
+            r#"
+fn main() {
+    foo(); // end-of-line$0 comment
+}
+"#,
+        );
+    }
+    #[test]
+    fn single_line_different_kinds() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    /// different prefix
+    // line$0 comment
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /// different prefix
+    /*
+    line comment
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+    #[test]
+    fn single_line_separate_chunks() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // different chunk
+    // line$0 comment
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    // different chunk
+    /*
+    line comment
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+    #[test]
+    fn doc_block_comment_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/**
+ hi$0 there
+*/
+"#,
+            r#"
+/// hi there
+"#,
+        );
+    }
+    #[test]
+    fn block_comment_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/*
+ hi$0 there
+*/
+"#,
+            r#"
+// hi there
+"#,
+        );
+    }
+    #[test]
+    fn inner_doc_block_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/*!
+ hi$0 there
+*/
+"#,
+            r#"
+//! hi there
+"#,
+        );
+    }
+    #[test]
+    fn block_to_lines_indent() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    /*!
+    hi$0 there
+    ```
+      code_sample
+    ```
+    */
+}
+"#,
+            r#"
+fn main() {
+    //! hi there
+    //!
+    //! ```
+    //!   code_sample
+    //! ```
+}
+"#,
+        );
+    }
+    #[test]
+    fn end_of_line_block_to_line() {
+        check_assist_not_applicable(
+            convert_comment_block,
+            r#"
+fn main() {
+    foo(); /* end-of-line$0 comment */
+}
+"#,
+        );
+    }
+}
diff --git a/crates/ide_assists/src/handlers/extract_variable.rs b/crates/ide_assists/src/handlers/extract_variable.rs
index 98f3dc6ca..312ac7ac4 100644
--- a/crates/ide_assists/src/handlers/extract_variable.rs
+++ b/crates/ide_assists/src/handlers/extract_variable.rs
@@ -8,7 +8,7 @@ use syntax::{
 };
 use test_utils::mark;
-use crate::{AssistContext, AssistId, AssistKind, Assists};
+use crate::{utils::suggest_name, AssistContext, AssistId, AssistKind, Assists};
 // Assist: extract_variable
 //
@@ -54,7 +54,7 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext) -> Option
            let var_name = match &field_shorthand {
                Some(it) => it.to_string(),
-                None => "var_name".to_string(),
+                None => suggest_name::variable(&to_extract, &ctx.sema),
            };
            let expr_range = match &field_shorthand {
                Some(it) => it.syntax().text_range().cover(to_extract.syntax().text_range()),
@@ -274,8 +274,8 @@ fn foo() {
 "#,
            r#"
 fn foo() {
-    let $0var_name = bar(1 + 1);
+    let $0bar = bar(1 + 1);
-    var_name
+    bar
 }
 "#,
        )
@@ -401,8 +401,8 @@ fn main() {
 ",
            "
 fn main() {
-    let $0var_name = bar.foo();
+    let $0foo = bar.foo();
-    let v = var_name;
+    let v = foo;
 }
 ",
        );
@@ -557,6 +557,202 @@ fn main() {
    }
    #[test]
+    fn extract_var_name_from_type() {
+        check_assist(
+            extract_variable,
+            r#"
+struct Test(i32);
+fn foo() -> Test {
+    $0{ Test(10) }$0
+}
+"#,
+            r#"
+struct Test(i32);
+fn foo() -> Test {
+    let $0test = { Test(10) };
+    test
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_name_from_parameter() {
+        check_assist(
+            extract_variable,
+            r#"
+fn bar(test: u32, size: u32)
+fn foo() {
+    bar(1, $01+1$0);
+}
+"#,
+            r#"
+fn bar(test: u32, size: u32)
+fn foo() {
+    let $0size = 1+1;
+    bar(1, size);
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_parameter_name_has_precedence_over_type() {
+        check_assist(
+            extract_variable,
+            r#"
+struct TextSize(u32);
+fn bar(test: u32, size: TextSize)
+fn foo() {
+    bar(1, $0{ TextSize(1+1) }$0);
+}
+"#,
+            r#"
+struct TextSize(u32);
+fn bar(test: u32, size: TextSize)
+fn foo() {
+    let $0size = { TextSize(1+1) };
+    bar(1, size);
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_name_from_function() {
+        check_assist(
+            extract_variable,
+            r#"
+fn is_required(test: u32, size: u32) -> bool
+fn foo() -> bool {
+    $0is_required(1, 2)$0
+}
+"#,
+            r#"
+fn is_required(test: u32, size: u32) -> bool
+fn foo() -> bool {
+    let $0is_required = is_required(1, 2);
+    is_required
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_name_from_method() {
+        check_assist(
+            extract_variable,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32) -> u32 { n }
+}
+fn foo() -> u32 {
+    $0S.bar(1)$0
+}
+"#,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32) -> u32 { n }
+}
+fn foo() -> u32 {
+    let $0bar = S.bar(1);
+    bar
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_name_from_method_param() {
+        check_assist(
+            extract_variable,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32, size: u32) { n }
+}
+fn foo() {
+    S.bar($01 + 1$0, 2)
+}
+"#,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32, size: u32) { n }
+}
+fn foo() {
+    let $0n = 1 + 1;
+    S.bar(n, 2)
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_name_from_ufcs_method_param() {
+        check_assist(
+            extract_variable,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32, size: u32) { n }
+}
+fn foo() {
+    S::bar(&S, $01 + 1$0, 2)
+}
+"#,
+            r#"
+struct S;
+impl S {
+    fn bar(&self, n: u32, size: u32) { n }
+}
+fn foo() {
+    let $0n = 1 + 1;
+    S::bar(&S, n, 2)
+}
+"#,
+        )
+    }
+    #[test]
+    fn extract_var_parameter_name_has_precedence_over_function() {
+        check_assist(
+            extract_variable,
+            r#"
+fn bar(test: u32, size: u32)
+fn foo() {
+    bar(1, $0symbol_size(1, 2)$0);
+}
+"#,
+            r#"
+fn bar(test: u32, size: u32)
+fn foo() {
+    let $0size = symbol_size(1, 2);
+    bar(1, size);
+}
+"#,
+        )
+    }
+    #[test]
    fn test_extract_var_for_return_not_applicable() {
        check_assist_not_applicable(extract_variable, "fn foo() { $0return$0; } ");
    }
diff --git a/crates/ide_assists/src/lib.rs b/crates/ide_assists/src/lib.rs
index 53542d433..9c8148462 100644
--- a/crates/ide_assists/src/lib.rs
+++ b/crates/ide_assists/src/lib.rs
@@ -115,6 +115,7 @@ mod handlers {
    mod auto_import;
    mod change_visibility;
    mod convert_integer_literal;
+    mod convert_comment_block;
    mod early_return;
    mod expand_glob_import;
    mod extract_function;
@@ -178,6 +179,7 @@ mod handlers {
            auto_import::auto_import,
            change_visibility::change_visibility,
            convert_integer_literal::convert_integer_literal,
+            convert_comment_block::convert_comment_block,
            early_return::convert_to_guarded_return,
            expand_glob_import::expand_glob_import,
            extract_struct_from_enum_variant::extract_struct_from_enum_variant,
diff --git a/crates/ide_assists/src/utils.rs b/crates/ide_assists/src/utils.rs
index 880ab6fe3..62f959082 100644
--- a/crates/ide_assists/src/utils.rs
+++ b/crates/ide_assists/src/utils.rs
@@ -1,5 +1,7 @@
 //! Assorted functions shared by several assists.
+pub(crate) mod suggest_name;
 use std::ops;
 use ast::TypeBoundsOwner;
diff --git a/crates/ide_assists/src/utils/suggest_name.rs b/crates/ide_assists/src/utils/suggest_name.rs
new file mode 100644
index 000000000..533624c1f
--- /dev/null
+++ b/crates/ide_assists/src/utils/suggest_name.rs
@@ -0,0 +1,729 @@
+//! This module contains functions to suggest names for expressions, functions and other items
+use hir::Semantics;
+use ide_db::RootDatabase;
+use itertools::Itertools;
+use stdx::to_lower_snake_case;
+use syntax::{
+    ast::{self, NameOwner},
+    match_ast, AstNode,
+};
+/// Trait names, that will be ignored when in `impl Trait` and `dyn Trait`
+const USELESS_TRAITS: &[&str] = &["Send", "Sync", "Copy", "Clone", "Eq", "PartialEq"];
+/// Identifier names that won't be suggested, ever
+///
+/// **NOTE**: they all must be snake lower case
+const USELESS_NAMES: &[&str] =
+    &["new", "default", "option", "some", "none", "ok", "err", "str", "string"];
+/// Generic types replaced by their first argument
+///
+/// # Examples
+/// `Option<Name>` -> `Name`
+/// `Result<User, Error>` -> `User`
+const WRAPPER_TYPES: &[&str] = &["Box", "Option", "Result"];
+/// Prefixes to strip from methods names
+///
+/// # Examples
+/// `vec.as_slice()` -> `slice`
+/// `args.into_config()` -> `config`
+/// `bytes.to_vec()` -> `vec`
+const USELESS_METHOD_PREFIXES: &[&str] = &["into_", "as_", "to_"];
+/// Useless methods that are stripped from expression
+///
+/// # Examples
+/// `var.name().to_string()` -> `var.name()`
+const USELESS_METHODS: &[&str] = &[
+    "to_string",
+    "as_str",
+    "to_owned",
+    "as_ref",
+    "clone",
+    "cloned",
+    "expect",
+    "expect_none",
+    "unwrap",
+    "unwrap_none",
+    "unwrap_or",
+    "unwrap_or_default",
+    "unwrap_or_else",
+    "unwrap_unchecked",
+    "iter",
+    "into_iter",
+    "iter_mut",
+];
+/// Suggest name of variable for given expression
+///
+/// **NOTE**: it is caller's responsibility to guarantee uniqueness of the name.
+/// I.e. it doesn't look for names in scope.
+///
+/// # Current implementation
+///
+/// In current implementation, the function tries to get the name from
+/// the following sources:
+///
+/// * if expr is an argument to function/method, use paramter name
+/// * if expr is a function/method call, use function name
+/// * expression type name if it exists (E.g. `()`, `fn() -> ()` or `!` do not have names)
+/// * fallback: `var_name`
+///
+/// It also applies heuristics to filter out less informative names
+///
+/// Currently it sticks to the first name found.
+pub(crate) fn variable(expr: &ast::Expr, sema: &Semantics<'_, RootDatabase>) -> String {
+    // `from_param` does not benifit from stripping
+    // it need the largest context possible
+    // so we check firstmost
+    if let Some(name) = from_param(expr, sema) {
+        return name;
+    }
+    let mut next_expr = Some(expr.clone());
+    while let Some(expr) = next_expr {
+        let name = from_call(&expr).or_else(|| from_type(&expr, sema));
+        if let Some(name) = name {
+            return name;
+        }
+        match expr {
+            ast::Expr::RefExpr(inner) => next_expr = inner.expr(),
+            ast::Expr::BoxExpr(inner) => next_expr = inner.expr(),
+            ast::Expr::AwaitExpr(inner) => next_expr = inner.expr(),
+            // ast::Expr::BlockExpr(block) => expr = block.tail_expr(),
+            ast::Expr::CastExpr(inner) => next_expr = inner.expr(),
+            ast::Expr::MethodCallExpr(method) if is_useless_method(&method) => {
+                next_expr = method.receiver();
+            }
+            ast::Expr::ParenExpr(inner) => next_expr = inner.expr(),
+            ast::Expr::TryExpr(inner) => next_expr = inner.expr(),
+            ast::Expr::PrefixExpr(prefix) if prefix.op_kind() == Some(ast::PrefixOp::Deref) => {
+                next_expr = prefix.expr()
+            }
+            _ => break,
+        }
+    }
+    "var_name".to_string()
+}
+fn normalize(name: &str) -> Option<String> {
+    let name = to_lower_snake_case(name);
+    if USELESS_NAMES.contains(&name.as_str()) {
+        return None;
+    }
+    if !is_valid_name(&name) {
+        return None;
+    }
+    Some(name)
+}
+fn is_valid_name(name: &str) -> bool {
+    match syntax::lex_single_syntax_kind(name) {
+        Some((syntax::SyntaxKind::IDENT, _error)) => true,
+        _ => false,
+    }
+}
+fn is_useless_method(method: &ast::MethodCallExpr) -> bool {
+    let ident = method.name_ref().and_then(|it| it.ident_token());
+    if let Some(ident) = ident {
+        USELESS_METHODS.contains(&ident.text())
+    } else {
+        false
+    }
+}
+fn from_call(expr: &ast::Expr) -> Option<String> {
+    from_func_call(expr).or_else(|| from_method_call(expr))
+}
+fn from_func_call(expr: &ast::Expr) -> Option<String> {
+    let call = match expr {
+        ast::Expr::CallExpr(call) => call,
+        _ => return None,
+    };
+    let func = match call.expr()? {
+        ast::Expr::PathExpr(path) => path,
+        _ => return None,
+    };
+    let ident = func.path()?.segment()?.name_ref()?.ident_token()?;
+    normalize(ident.text())
+}
+fn from_method_call(expr: &ast::Expr) -> Option<String> {
+    let method = match expr {
+        ast::Expr::MethodCallExpr(call) => call,
+        _ => return None,
+    };
+    let ident = method.name_ref()?.ident_token()?;
+    let mut name = ident.text();
+    if USELESS_METHODS.contains(&name) {
+        return None;
+    }
+    for prefix in USELESS_METHOD_PREFIXES {
+        if let Some(suffix) = name.strip_prefix(prefix) {
+            name = suffix;
+            break;
+        }
+    }
+    normalize(&name)
+}
+fn from_param(expr: &ast::Expr, sema: &Semantics<'_, RootDatabase>) -> Option<String> {
+    let arg_list = expr.syntax().parent().and_then(ast::ArgList::cast)?;
+    let args_parent = arg_list.syntax().parent()?;
+    let func = match_ast! {
+        match args_parent {
+            ast::CallExpr(call) => {
+                let func = call.expr()?;
+                let func_ty = sema.type_of_expr(&func)?;
+                func_ty.as_callable(sema.db)?
+            },
+            ast::MethodCallExpr(method) => sema.resolve_method_call_as_callable(&method)?,
+            _ => return None,
+        }
+    };
+    let (idx, _) = arg_list.args().find_position(|it| it == expr).unwrap();
+    let (pat, _) = func.params(sema.db).into_iter().nth(idx)?;
+    let pat = match pat? {
+        either::Either::Right(pat) => pat,
+        _ => return None,
+    };
+    let name = var_name_from_pat(&pat)?;
+    normalize(&name.to_string())
+}
+fn var_name_from_pat(pat: &ast::Pat) -> Option<ast::Name> {
+    match pat {
+        ast::Pat::IdentPat(var) => var.name(),
+        ast::Pat::RefPat(ref_pat) => var_name_from_pat(&ref_pat.pat()?),
+        ast::Pat::BoxPat(box_pat) => var_name_from_pat(&box_pat.pat()?),
+        _ => None,
+    }
+}
+fn from_type(expr: &ast::Expr, sema: &Semantics<'_, RootDatabase>) -> Option<String> {
+    let ty = sema.type_of_expr(expr)?;
+    let ty = ty.remove_ref().unwrap_or(ty);
+    name_of_type(&ty, sema.db)
+}
+fn name_of_type(ty: &hir::Type, db: &RootDatabase) -> Option<String> {
+    let name = if let Some(adt) = ty.as_adt() {
+        let name = adt.name(db).to_string();
+        if WRAPPER_TYPES.contains(&name.as_str()) {
+            let inner_ty = ty.type_parameters().next()?;
+            return name_of_type(&inner_ty, db);
+        }
+        name
+    } else if let Some(trait_) = ty.as_dyn_trait() {
+        trait_name(&trait_, db)?
+    } else if let Some(traits) = ty.as_impl_traits(db) {
+        let mut iter = traits.into_iter().filter_map(|t| trait_name(&t, db));
+        let name = iter.next()?;
+        if iter.next().is_some() {
+            return None;
+        }
+        name
+    } else {
+        return None;
+    };
+    normalize(&name)
+}
+fn trait_name(trait_: &hir::Trait, db: &RootDatabase) -> Option<String> {
+    let name = trait_.name(db).to_string();
+    if USELESS_TRAITS.contains(&name.as_str()) {
+        return None;
+    }
+    Some(name)
+}
+#[cfg(test)]
+mod tests {
+    use ide_db::base_db::{fixture::WithFixture, FileRange};
+    use super::*;
+    #[track_caller]
+    fn check(ra_fixture: &str, expected: &str) {
+        let (db, file_id, range_or_offset) = RootDatabase::with_range_or_offset(ra_fixture);
+        let frange = FileRange { file_id, range: range_or_offset.into() };
+        let sema = Semantics::new(&db);
+        let source_file = sema.parse(frange.file_id);
+        let element = source_file.syntax().covering_element(frange.range);
+        let expr =
+            element.ancestors().find_map(ast::Expr::cast).expect("selection is not an expression");
+        assert_eq!(
+            expr.syntax().text_range(),
+            frange.range,
+            "selection is not an expression(yet contained in one)"
+        );
+        let name = variable(&expr, &sema);
+        assert_eq!(&name, expected);
+    }
+    #[test]
+    fn no_args() {
+        check(r#"fn foo() { $0bar()$0 }"#, "bar");
+        check(r#"fn foo() { $0bar.frobnicate()$0 }"#, "frobnicate");
+    }
+    #[test]
+    fn single_arg() {
+        check(r#"fn foo() { $0bar(1)$0 }"#, "bar");
+    }
+    #[test]
+    fn many_args() {
+        check(r#"fn foo() { $0bar(1, 2, 3)$0 }"#, "bar");
+    }
+    #[test]
+    fn path() {
+        check(r#"fn foo() { $0i32::bar(1, 2, 3)$0 }"#, "bar");
+    }
+    #[test]
+    fn generic_params() {
+        check(r#"fn foo() { $0bar::<i32>(1, 2, 3)$0 }"#, "bar");
+        check(r#"fn foo() { $0bar.frobnicate::<i32, u32>()$0 }"#, "frobnicate");
+    }
+    #[test]
+    fn to_name() {
+        check(
+            r#"
+struct Args;
+struct Config;
+impl Args {
+    fn to_config(&self) -> Config {}
+}
+fn foo() {
+    $0Args.to_config()$0;
+}
+"#,
+            "config",
+        );
+    }
+    #[test]
+    fn plain_func() {
+        check(
+            r#"
+fn bar(n: i32, m: u32);
+fn foo() { bar($01$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn mut_param() {
+        check(
+            r#"
+fn bar(mut n: i32, m: u32);
+fn foo() { bar($01$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn func_does_not_exist() {
+        check(r#"fn foo() { bar($01$0, 2) }"#, "var_name");
+    }
+    #[test]
+    fn unnamed_param() {
+        check(
+            r#"
+fn bar(_: i32, m: u32);
+fn foo() { bar($01$0, 2) }
+"#,
+            "var_name",
+        );
+    }
+    #[test]
+    fn tuple_pat() {
+        check(
+            r#"
+fn bar((n, k): (i32, i32), m: u32);
+fn foo() {
+    bar($0(1, 2)$0, 3)
+}
+"#,
+            "var_name",
+        );
+    }
+    #[test]
+    fn ref_pat() {
+        check(
+            r#"
+fn bar(&n: &i32, m: u32);
+fn foo() { bar($0&1$0, 3) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn box_pat() {
+        check(
+            r#"
+fn bar(box n: &i32, m: u32);
+fn foo() { bar($01$0, 3) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn param_out_of_index() {
+        check(
+            r#"
+fn bar(n: i32, m: u32);
+fn foo() { bar(1, 2, $03$0) }
+"#,
+            "var_name",
+        );
+    }
+    #[test]
+    fn generic_param_resolved() {
+        check(
+            r#"
+fn bar<T>(n: T, m: u32);
+fn foo() { bar($01$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn generic_param_unresolved() {
+        check(
+            r#"
+fn bar<T>(n: T, m: u32);
+fn foo<T>(x: T) { bar($0x$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn method() {
+        check(
+            r#"
+struct S;
+impl S { fn bar(&self, n: i32, m: u32); }
+fn foo() { S.bar($01$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn method_ufcs() {
+        check(
+            r#"
+struct S;
+impl S { fn bar(&self, n: i32, m: u32); }
+fn foo() { S::bar(&S, $01$0, 2) }
+"#,
+            "n",
+        );
+    }
+    #[test]
+    fn method_self() {
+        check(
+            r#"
+struct S;
+impl S { fn bar(&self, n: i32, m: u32); }
+fn foo() { S::bar($0&S$0, 1, 2) }
+"#,
+            "s",
+        );
+    }
+    #[test]
+    fn method_self_named() {
+        check(
+            r#"
+struct S;
+impl S { fn bar(strukt: &Self, n: i32, m: u32); }
+fn foo() { S::bar($0&S$0, 1, 2) }
+"#,
+            "strukt",
+        );
+    }
+    #[test]
+    fn i32() {
+        check(r#"fn foo() { let _: i32 = $01$0; }"#, "var_name");
+    }
+    #[test]
+    fn u64() {
+        check(r#"fn foo() { let _: u64 = $01$0; }"#, "var_name");
+    }
+    #[test]
+    fn bool() {
+        check(r#"fn foo() { let _: bool = $0true$0; }"#, "var_name");
+    }
+    #[test]
+    fn struct_unit() {
+        check(
+            r#"
+struct Seed;
+fn foo() { let _ = $0Seed$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn struct_unit_to_snake() {
+        check(
+            r#"
+struct SeedState;
+fn foo() { let _ = $0SeedState$0; }
+"#,
+            "seed_state",
+        );
+    }
+    #[test]
+    fn struct_single_arg() {
+        check(
+            r#"
+struct Seed(u32);
+fn foo() { let _ = $0Seed(0)$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn struct_with_fields() {
+        check(
+            r#"
+struct Seed { value: u32 }
+fn foo() { let _ = $0Seed { value: 0 }$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn enum_() {
+        check(
+            r#"
+enum Kind { A, B }
+fn foo() { let _ = $0Kind::A$0; }
+"#,
+            "kind",
+        );
+    }
+    #[test]
+    fn enum_generic_resolved() {
+        check(
+            r#"
+enum Kind<T> { A { x: T }, B }
+fn foo() { let _ = $0Kind::A { x:1 }$0; }
+"#,
+            "kind",
+        );
+    }
+    #[test]
+    fn enum_generic_unresolved() {
+        check(
+            r#"
+enum Kind<T> { A { x: T }, B }
+fn foo<T>(x: T) { let _ = $0Kind::A { x }$0; }
+"#,
+            "kind",
+        );
+    }
+    #[test]
+    fn dyn_trait() {
+        check(
+            r#"
+trait DynHandler {}
+fn bar() -> dyn DynHandler {}
+fn foo() { $0(bar())$0; }
+"#,
+            "dyn_handler",
+        );
+    }
+    #[test]
+    fn impl_trait() {
+        check(
+            r#"
+trait StaticHandler {}
+fn bar() -> impl StaticHandler {}
+fn foo() { $0(bar())$0; }
+"#,
+            "static_handler",
+        );
+    }
+    #[test]
+    fn impl_trait_plus_clone() {
+        check(
+            r#"
+trait StaticHandler {}
+trait Clone {}
+fn bar() -> impl StaticHandler + Clone {}
+fn foo() { $0(bar())$0; }
+"#,
+            "static_handler",
+        );
+    }
+    #[test]
+    fn impl_trait_plus_lifetime() {
+        check(
+            r#"
+trait StaticHandler {}
+trait Clone {}
+fn bar<'a>(&'a i32) -> impl StaticHandler + 'a {}
+fn foo() { $0(bar(&1))$0; }
+"#,
+            "static_handler",
+        );
+    }
+    #[test]
+    fn impl_trait_plus_trait() {
+        check(
+            r#"
+trait Handler {}
+trait StaticHandler {}
+fn bar() -> impl StaticHandler + Handler {}
+fn foo() { $0(bar())$0; }
+"#,
+            "bar",
+        );
+    }
+    #[test]
+    fn ref_value() {
+        check(
+            r#"
+struct Seed;
+fn bar() -> &Seed {}
+fn foo() { $0(bar())$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn box_value() {
+        check(
+            r#"
+struct Box<T>(*const T);
+struct Seed;
+fn bar() -> Box<Seed> {}
+fn foo() { $0(bar())$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn box_generic() {
+        check(
+            r#"
+struct Box<T>(*const T);
+fn bar<T>() -> Box<T> {}
+fn foo<T>() { $0(bar::<T>())$0; }
+"#,
+            "bar",
+        );
+    }
+    #[test]
+    fn option_value() {
+        check(
+            r#"
+enum Option<T> { Some(T) }
+struct Seed;
+fn bar() -> Option<Seed> {}
+fn foo() { $0(bar())$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn result_value() {
+        check(
+            r#"
+enum Result<T, E> { Ok(T), Err(E) }
+struct Seed;
+struct Error;
+fn bar() -> Result<Seed, Error> {}
+fn foo() { $0(bar())$0; }
+"#,
+            "seed",
+        );
+    }
+    #[test]
+    fn ref_call() {
+        check(
+            r#"
+fn foo() { $0&bar(1, 3)$0 }
+"#,
+            "bar",
+        );
+    }
+    #[test]
+    fn name_to_string() {
+        check(
+            r#"
+fn foo() { $0function.name().to_string()$0 }
+"#,
+            "name",
+        );
+    }
+    #[test]
+    fn nested_useless_method() {
+        check(
+            r#"
+fn foo() { $0function.name().as_ref().unwrap().to_string()$0 }
+"#,
+            "name",
+        );
+    }
+}