From 1979d3f9b50f0831dfc64e3f20adfed518583c04 Mon Sep 17 00:00:00 2001
From: Giga Bowser <45986823+Giga-Bowser@users.noreply.github.com>
Date: Mon, 9 Dec 2024 16:23:36 -0500
Subject: [PATCH] feat: Add an assist to extract an expression into a constant

---
 .../src/handlers/extract_variable.rs          | 896 ++++++++++++++----
 crates/ide-assists/src/tests.rs               | 122 ++-
 crates/ide-assists/src/tests/generated.rs     |  18 +
 3 files changed, 847 insertions(+), 189 deletions(-)

diff --git a/crates/ide-assists/src/handlers/extract_variable.rs b/crates/ide-assists/src/handlers/extract_variable.rs
index 61dc72e0b3..6670eac6a7 100644
--- a/crates/ide-assists/src/handlers/extract_variable.rs
+++ b/crates/ide-assists/src/handlers/extract_variable.rs
@@ -1,4 +1,4 @@
-use hir::TypeInfo;
+use hir::{HirDisplay, TypeInfo};
 use ide_db::syntax_helpers::suggest_name;
 use syntax::{
     ast::{
@@ -7,11 +7,11 @@ use syntax::{
     },
     syntax_editor::Position,
     NodeOrToken,
-    SyntaxKind::{BLOCK_EXPR, BREAK_EXPR, COMMENT, LOOP_EXPR, MATCH_GUARD, PATH_EXPR, RETURN_EXPR},
+    SyntaxKind::{self},
     SyntaxNode, T,
 };
 
-use crate::{AssistContext, AssistId, AssistKind, Assists};
+use crate::{utils::is_body_const, AssistContext, AssistId, AssistKind, Assists};
 
 // Assist: extract_variable
 //
@@ -29,6 +29,23 @@ use crate::{AssistContext, AssistId, AssistKind, Assists};
 //     var_name * 4;
 // }
 // ```
+
+// Assist: extract_constant
+//
+// Extracts subexpression into a constant.
+//
+// ```
+// fn main() {
+//     $0(1 + 2)$0 * 4;
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     const $0VAR_NAME: i32 = 1 + 2;
+//     VAR_NAME * 4;
+// }
+// ```
 pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
     let node = if ctx.has_empty_selection() {
         if let Some(t) = ctx.token_at_offset().find(|it| it.kind() == T![;]) {
@@ -41,7 +58,7 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
     } else {
         match ctx.covering_element() {
             NodeOrToken::Node(it) => it,
-            NodeOrToken::Token(it) if it.kind() == COMMENT => {
+            NodeOrToken::Token(it) if it.kind() == SyntaxKind::COMMENT => {
                 cov_mark::hit!(extract_var_in_comment_is_not_applicable);
                 return None;
             }
@@ -87,27 +104,39 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
             }
             _ => false,
         };
-
-    let anchor = Anchor::from(&to_extract)?;
+    let module = ctx.sema.scope(to_extract.syntax())?.module();
     let target = to_extract.syntax().text_range();
-    acc.add(
-        AssistId("extract_variable", AssistKind::RefactorExtract),
-        "Extract into variable",
-        target,
-        move |edit| {
-            let field_shorthand = to_extract
-                .syntax()
-                .parent()
-                .and_then(ast::RecordExprField::cast)
-                .filter(|field| field.name_ref().is_some());
+    let needs_mut = match &parent {
+        Some(ast::Expr::RefExpr(expr)) => expr.mut_token().is_some(),
+        _ => needs_adjust && !needs_ref && ty.as_ref().is_some_and(|ty| ty.is_mutable_reference()),
+    };
+    for kind in ExtractionKind::ALL {
+        let Some(anchor) = Anchor::from(&to_extract, kind) else {
+            continue;
+        };
+        let ty_string = match kind {
+            ExtractionKind::Constant => {
+                let Some(ty) = ty.clone() else {
+                    continue;
+                };
 
-            let (var_name, expr_replace) = match field_shorthand {
-                Some(field) => (field.to_string(), field.syntax().clone()),
-                None => (
-                    suggest_name::for_variable(&to_extract, &ctx.sema),
-                    to_extract.syntax().clone(),
-                ),
-            };
+                // We can't mutably reference a const, nor can we define
+                // one using a non-const expression or one of unknown type
+                if needs_mut || !is_body_const(&ctx.sema, &to_extract_no_ref) || ty.is_unknown() {
+                    continue;
+                }
+
+                let Ok(type_string) = ty.display_source_code(ctx.db(), module.into(), false) else {
+                    continue;
+                };
+
+                type_string
+            }
+            _ => "".to_owned(),
+        };
+
+        acc.add(kind.assist_id(), kind.label(), target, |edit| {
+            let (var_name, expr_replace) = kind.get_name_and_expr(ctx, &to_extract);
 
             let make = SyntaxFactory::new();
             let mut editor = edit.make_editor(&expr_replace);
@@ -120,35 +149,31 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
                 editor.add_annotation(pat_name.syntax().clone(), tabstop);
             }
 
-            let ident_pat = match parent {
-                Some(ast::Expr::RefExpr(expr)) if expr.mut_token().is_some() => {
-                    make.ident_pat(false, true, pat_name)
-                }
-                _ if needs_adjust
-                    && !needs_ref
-                    && ty.as_ref().is_some_and(|ty| ty.is_mutable_reference()) =>
-                {
-                    make.ident_pat(false, true, pat_name)
-                }
-                _ => make.ident_pat(false, false, pat_name),
-            };
-
-            let to_extract_no_ref = match ty.as_ref().filter(|_| needs_ref) {
+            let initializer = match ty.as_ref().filter(|_| needs_ref) {
                 Some(receiver_type) if receiver_type.is_mutable_reference() => {
-                    make.expr_ref(to_extract_no_ref, true)
+                    make.expr_ref(to_extract_no_ref.clone(), true)
                 }
                 Some(receiver_type) if receiver_type.is_reference() => {
-                    make.expr_ref(to_extract_no_ref, false)
+                    make.expr_ref(to_extract_no_ref.clone(), false)
                 }
-                _ => to_extract_no_ref,
+                _ => to_extract_no_ref.clone(),
             };
 
-            let let_stmt = make.let_stmt(ident_pat.into(), None, Some(to_extract_no_ref));
+            let new_stmt: ast::Stmt = match kind {
+                ExtractionKind::Variable => {
+                    let ident_pat = make.ident_pat(false, needs_mut, pat_name);
+                    make.let_stmt(ident_pat.into(), None, Some(initializer)).into()
+                }
+                ExtractionKind::Constant => {
+                    let ast_ty = make.ty(&ty_string);
+                    ast::Item::Const(make.item_const(None, pat_name, ast_ty, initializer)).into()
+                }
+            };
 
-            match anchor {
+            match &anchor {
                 Anchor::Before(place) => {
                     let prev_ws = place.prev_sibling_or_token().and_then(|it| it.into_token());
-                    let indent_to = IndentLevel::from_node(&place);
+                    let indent_to = IndentLevel::from_node(place);
 
                     // Adjust ws to insert depending on if this is all inline or on separate lines
                     let trailing_ws = if prev_ws.is_some_and(|it| it.text().starts_with('\n')) {
@@ -160,7 +185,7 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
                     editor.insert_all(
                         Position::before(place),
                         vec![
-                            let_stmt.syntax().clone().into(),
+                            new_stmt.syntax().clone().into(),
                             make::tokens::whitespace(&trailing_ws).into(),
                         ],
                     );
@@ -170,7 +195,7 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
                 Anchor::Replace(stmt) => {
                     cov_mark::hit!(test_extract_var_expr_stmt);
 
-                    editor.replace(stmt.syntax(), let_stmt.syntax());
+                    editor.replace(stmt.syntax(), new_stmt.syntax());
                 }
                 Anchor::WrapInBlock(to_wrap) => {
                     let indent_to = to_wrap.indent_level();
@@ -178,11 +203,11 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
                     let block = if to_wrap.syntax() == &expr_replace {
                         // Since `expr_replace` is the same that needs to be wrapped in a block,
                         // we can just directly replace it with a block
-                        make.block_expr([let_stmt.into()], Some(name_expr))
+                        make.block_expr([new_stmt], Some(name_expr))
                     } else {
                         // `expr_replace` is a descendant of `to_wrap`, so we just replace it with `name_expr`.
                         editor.replace(expr_replace, name_expr.syntax());
-                        make.block_expr([let_stmt.into()], Some(to_wrap.clone()))
+                        make.block_expr([new_stmt], Some(to_wrap.clone()))
                     };
 
                     editor.replace(to_wrap.syntax(), block.syntax());
@@ -195,8 +220,10 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
             editor.add_mappings(make.finish_with_mappings());
             edit.add_file_edits(ctx.file_id(), editor);
             edit.rename();
-        },
-    )
+        });
+    }
+
+    Some(())
 }
 
 fn peel_parens(mut expr: ast::Expr) -> ast::Expr {
@@ -211,17 +238,67 @@ fn peel_parens(mut expr: ast::Expr) -> ast::Expr {
 /// 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> {
     match node.kind() {
-        PATH_EXPR | LOOP_EXPR => None,
-        BREAK_EXPR => ast::BreakExpr::cast(node).and_then(|e| e.expr()),
-        RETURN_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()),
-        BLOCK_EXPR => {
+        SyntaxKind::PATH_EXPR | SyntaxKind::LOOP_EXPR => None,
+        SyntaxKind::BREAK_EXPR => ast::BreakExpr::cast(node).and_then(|e| e.expr()),
+        SyntaxKind::RETURN_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()),
+        SyntaxKind::BLOCK_EXPR => {
             ast::BlockExpr::cast(node).filter(|it| it.is_standalone()).map(ast::Expr::from)
         }
         _ => ast::Expr::cast(node),
     }
 }
 
-#[derive(Debug)]
+enum ExtractionKind {
+    Variable,
+    Constant,
+}
+
+impl ExtractionKind {
+    const ALL: &'static [ExtractionKind] = &[ExtractionKind::Variable, ExtractionKind::Constant];
+
+    fn assist_id(&self) -> AssistId {
+        let s = match self {
+            ExtractionKind::Variable => "extract_variable",
+            ExtractionKind::Constant => "extract_constant",
+        };
+
+        AssistId(s, AssistKind::RefactorExtract)
+    }
+
+    fn label(&self) -> &'static str {
+        match self {
+            ExtractionKind::Variable => "Extract into variable",
+            ExtractionKind::Constant => "Extract into constant",
+        }
+    }
+
+    fn get_name_and_expr(
+        &self,
+        ctx: &AssistContext<'_>,
+        to_extract: &ast::Expr,
+    ) -> (String, SyntaxNode) {
+        let field_shorthand = to_extract
+            .syntax()
+            .parent()
+            .and_then(ast::RecordExprField::cast)
+            .filter(|field| field.name_ref().is_some());
+        let (var_name, expr_replace) = match field_shorthand {
+            Some(field) => (field.to_string(), field.syntax().clone()),
+            None => {
+                (suggest_name::for_variable(to_extract, &ctx.sema), to_extract.syntax().clone())
+            }
+        };
+
+        let var_name = match self {
+            ExtractionKind::Variable => var_name,
+            ExtractionKind::Constant => var_name.to_uppercase(),
+        };
+
+        (var_name, expr_replace)
+    }
+}
+
+#[derive(Debug, Clone)]
 enum Anchor {
     Before(SyntaxNode),
     Replace(ast::ExprStmt),
@@ -229,8 +306,8 @@ enum Anchor {
 }
 
 impl Anchor {
-    fn from(to_extract: &ast::Expr) -> Option<Anchor> {
-        to_extract
+    fn from(to_extract: &ast::Expr, kind: &ExtractionKind) -> Option<Anchor> {
+        let result = to_extract
             .syntax()
             .ancestors()
             .take_while(|it| !ast::Item::can_cast(it.kind()) || ast::MacroCall::can_cast(it.kind()))
@@ -253,7 +330,7 @@ impl Anchor {
                         return parent.body().map(Anchor::WrapInBlock);
                     }
                     if let Some(parent) = ast::MatchArm::cast(parent) {
-                        if node.kind() == MATCH_GUARD {
+                        if node.kind() == SyntaxKind::MATCH_GUARD {
                             cov_mark::hit!(test_extract_var_in_match_guard);
                         } else {
                             cov_mark::hit!(test_extract_var_in_match_arm_no_block);
@@ -271,19 +348,57 @@ impl Anchor {
                     return Some(Anchor::Before(node));
                 }
                 None
-            })
+            });
+
+        match kind {
+            ExtractionKind::Constant if result.is_none() => {
+                to_extract.syntax().ancestors().find_map(|node| {
+                    let item = ast::Item::cast(node.clone())?;
+                    let parent = item.syntax().parent()?;
+                    match parent.kind() {
+                        SyntaxKind::ITEM_LIST
+                        | SyntaxKind::SOURCE_FILE
+                        | SyntaxKind::ASSOC_ITEM_LIST
+                        | SyntaxKind::STMT_LIST => Some(Anchor::Before(node)),
+                        _ => None,
+                    }
+                })
+            }
+            _ => result,
+        }
     }
 }
 
 #[cfg(test)]
 mod tests {
-    use crate::tests::{check_assist, check_assist_not_applicable, check_assist_target};
+    // NOTE: We use check_assist_by_label, but not check_assist_not_applicable_by_label
+    // because all of our not-applicable tests should behave that way for both assists
+    // extract_variable offers, and check_assist_not_applicable ensures neither is offered
+    use crate::tests::{
+        check_assist_by_label, check_assist_not_applicable, check_assist_not_applicable_by_label,
+        check_assist_target,
+    };
 
     use super::*;
 
     #[test]
-    fn test_extract_var_simple_without_select() {
-        check_assist(
+    fn now_bad() {
+        // unknown type
+        check_assist_not_applicable_by_label(
+            extract_variable,
+            r#"
+fn main() {
+    let a = Some(2);
+    a.is_some();$0
+}
+"#,
+            "Extract into constant",
+        );
+    }
+
+    #[test]
+    fn extract_var_simple_without_select() {
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() -> i32 {
@@ -304,9 +419,10 @@ fn main() -> i32 {
     var_name
 }
 "#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() -> i32 { 1 }
@@ -320,9 +436,10 @@ fn main() {
     let $0foo = foo();
 }
 "#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -336,9 +453,10 @@ fn main() {
     let $0is_some = a.is_some();
 }
 "#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -350,9 +468,10 @@ fn main() {
     let $0var_name = "hello";
 }
 "#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -364,9 +483,10 @@ fn main() {
     let $0var_name = 1  + 2;
 }
 "#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -384,11 +504,107 @@ fn main() {
     };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_unit_expr_without_select_not_applicable() {
+    fn extract_const_simple_without_select() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn main() -> i32 {
+    if true {
+        1
+    } else {
+        2
+    }$0
+}
+"#,
+            r#"
+fn main() -> i32 {
+    const $0VAR_NAME: i32 = if true {
+        1
+    } else {
+        2
+    };
+    VAR_NAME
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+const fn foo() -> i32 { 1 }
+fn main() {
+    foo();$0
+}
+"#,
+            r#"
+const fn foo() -> i32 { 1 }
+fn main() {
+    const $0FOO: i32 = foo();
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn main() {
+    "hello"$0;
+}
+"#,
+            r#"
+fn main() {
+    const $0VAR_NAME: &str = "hello";
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn main() {
+    1  + 2$0;
+}
+"#,
+            r#"
+fn main() {
+    const $0VAR_NAME: i32 = 1  + 2;
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn main() {
+    match () {
+        () if true => 1,
+        _ => 2,
+    };$0
+}
+"#,
+            r#"
+fn main() {
+    const $0VAR_NAME: i32 = match () {
+        () if true => 1,
+        _ => 2,
+    };
+}
+"#,
+            "Extract into constant",
+        );
+    }
+
+    #[test]
+    fn extract_var_unit_expr_without_select_not_applicable() {
         check_assist_not_applicable(
             extract_variable,
             r#"
@@ -414,8 +630,8 @@ fn foo() {
     }
 
     #[test]
-    fn test_extract_var_simple() {
-        check_assist(
+    fn extract_var_simple() {
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() {
@@ -426,19 +642,37 @@ fn foo() {
     let $0var_name = 1 + 1;
     foo(var_name);
 }"#,
+            "Extract into variable",
+        );
+    }
+
+    #[test]
+    fn extract_const_simple() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn foo() {
+    foo($01 + 1$0);
+}"#,
+            r#"
+fn foo() {
+    const $0VAR_NAME: i32 = 1 + 1;
+    foo(VAR_NAME);
+}"#,
+            "Extract into constant",
         );
     }
 
     #[test]
     fn extract_var_in_comment_is_not_applicable() {
         cov_mark::check!(extract_var_in_comment_is_not_applicable);
-        check_assist_not_applicable(extract_variable, "fn main() { 1 + /* $0comment$0 */ 1; }");
+        check_assist_not_applicable(extract_variable, r#"fn main() { 1 + /* $0comment$0 */ 1; }"#);
     }
 
     #[test]
-    fn test_extract_var_expr_stmt() {
+    fn extract_var_expr_stmt() {
         cov_mark::check!(test_extract_var_expr_stmt);
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() {
@@ -448,42 +682,94 @@ fn foo() {
 fn foo() {
     let $0var_name = 1 + 1;
 }"#,
+            "Extract into variable",
         );
-        check_assist(
+        check_assist_by_label(
             extract_variable,
-            r"
+            r#"
 fn foo() {
     $0{ let x = 0; x }$0;
     something_else();
-}",
-            r"
+}"#,
+            r#"
 fn foo() {
     let $0var_name = { let x = 0; x };
     something_else();
-}",
+}"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_part_of_expr_stmt() {
-        check_assist(
+    fn extract_const_expr_stmt() {
+        cov_mark::check!(test_extract_var_expr_stmt);
+        check_assist_by_label(
             extract_variable,
-            r"
+            r#"
+fn foo() {
+  $0  1 + 1$0;
+}"#,
+            r#"
+fn foo() {
+    const $0VAR_NAME: i32 = 1 + 1;
+}"#,
+            "Extract into constant",
+        );
+        // This is hilarious but as far as I know, it's valid
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn foo() {
+    $0{ let x = 0; x }$0;
+    something_else();
+}"#,
+            r#"
+fn foo() {
+    const $0VAR_NAME: i32 = { let x = 0; x };
+    something_else();
+}"#,
+            "Extract into constant",
+        );
+    }
+
+    #[test]
+    fn extract_var_part_of_expr_stmt() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
 fn foo() {
     $01$0 + 1;
-}",
-            r"
+}"#,
+            r#"
 fn foo() {
     let $0var_name = 1;
     var_name + 1;
-}",
+}"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_last_expr() {
+    fn extract_const_part_of_expr_stmt() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn foo() {
+    $01$0 + 1;
+}"#,
+            r#"
+fn foo() {
+    const $0VAR_NAME: i32 = 1;
+    VAR_NAME + 1;
+}"#,
+            "Extract into constant",
+        );
+    }
+
+    #[test]
+    fn extract_var_last_expr() {
         cov_mark::check!(test_extract_var_last_expr);
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() {
@@ -496,8 +782,9 @@ fn foo() {
     bar(var_name)
 }
 "#,
+            "Extract into variable",
         );
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() -> i32 {
@@ -518,13 +805,57 @@ fn bar(i: i32) -> i32 {
     i
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
-    fn test_extract_var_in_match_arm_no_block() {
+    fn extract_const_last_expr() {
+        cov_mark::check!(test_extract_var_last_expr);
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn foo() {
+    bar($01 + 1$0)
+}
+"#,
+            r#"
+fn foo() {
+    const $0VAR_NAME: i32 = 1 + 1;
+    bar(VAR_NAME)
+}
+"#,
+            "Extract into constant",
+        );
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn foo() -> i32 {
+    $0bar(1 + 1)$0
+}
+
+const fn bar(i: i32) -> i32 {
+    i
+}
+"#,
+            r#"
+fn foo() -> i32 {
+    const $0BAR: i32 = bar(1 + 1);
+    BAR
+}
+
+const fn bar(i: i32) -> i32 {
+    i
+}
+"#,
+            "Extract into constant",
+        )
+    }
+
+    #[test]
+    fn extract_var_in_match_arm_no_block() {
         cov_mark::check!(test_extract_var_in_match_arm_no_block);
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -547,12 +878,13 @@ fn main() {
     };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_in_match_arm_with_block() {
-        check_assist(
+    fn extract_var_in_match_arm_with_block() {
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -579,13 +911,14 @@ fn main() {
     };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_in_match_guard() {
+    fn extract_var_in_match_guard() {
         cov_mark::check!(test_extract_var_in_match_guard);
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -604,13 +937,14 @@ fn main() {
     };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_in_closure_no_block() {
+    fn extract_var_in_closure_no_block() {
         cov_mark::check!(test_extract_var_in_closure_no_block);
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -625,12 +959,13 @@ fn main() {
     };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_in_closure_with_block() {
-        check_assist(
+    fn extract_var_in_closure_with_block() {
+        check_assist_by_label(
             extract_variable,
             r#"
 fn main() {
@@ -642,104 +977,110 @@ fn main() {
     let lambda = |x: u32| { let $0var_name = x * 2; var_name };
 }
 "#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_path_simple() {
-        check_assist(
+    fn extract_var_path_simple() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn main() {
     let o = $0Some(true)$0;
 }
-",
-            "
+"#,
+            r#"
 fn main() {
     let $0var_name = Some(true);
     let o = var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_path_method() {
-        check_assist(
+    fn extract_var_path_method() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn main() {
     let v = $0bar.foo()$0;
 }
-",
-            "
+"#,
+            r#"
 fn main() {
     let $0foo = bar.foo();
     let v = foo;
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_return() {
-        check_assist(
+    fn extract_var_return() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn foo() -> u32 {
     $0return 2 + 2$0;
 }
-",
-            "
+"#,
+            r#"
 fn foo() -> u32 {
     let $0var_name = 2 + 2;
     return var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_does_not_add_extra_whitespace() {
-        check_assist(
+    fn extract_var_does_not_add_extra_whitespace() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn foo() -> u32 {
 
 
     $0return 2 + 2$0;
 }
-",
-            "
+"#,
+            r#"
 fn foo() -> u32 {
 
 
     let $0var_name = 2 + 2;
     return var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn foo() -> u32 {
 
         $0return 2 + 2$0;
 }
-",
-            "
+"#,
+            r#"
 fn foo() -> u32 {
 
         let $0var_name = 2 + 2;
         return var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
 
-        check_assist(
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn foo() -> u32 {
     let foo = 1;
 
@@ -748,8 +1089,8 @@ fn foo() -> u32 {
 
     $0return 2 + 2$0;
 }
-",
-            "
+"#,
+            r#"
 fn foo() -> u32 {
     let foo = 1;
 
@@ -759,53 +1100,56 @@ fn foo() -> u32 {
     let $0var_name = 2 + 2;
     return var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_break() {
-        check_assist(
+    fn extract_var_break() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn main() {
     let result = loop {
         $0break 2 + 2$0;
     };
 }
-",
-            "
+"#,
+            r#"
 fn main() {
     let result = loop {
         let $0var_name = 2 + 2;
         break var_name;
     };
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_for_cast() {
-        check_assist(
+    fn extract_var_for_cast() {
+        check_assist_by_label(
             extract_variable,
-            "
+            r#"
 fn main() {
     let v = $00f32 as u32$0;
 }
-",
-            "
+"#,
+            r#"
 fn main() {
     let $0var_name = 0f32 as u32;
     let v = var_name;
 }
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
     fn extract_var_field_shorthand() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S {
@@ -826,12 +1170,13 @@ fn main() {
     S { foo }
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_type() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct Test(i32);
@@ -848,12 +1193,13 @@ fn foo() -> Test {
     test
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_parameter() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn bar(test: u32, size: u32)
@@ -870,12 +1216,13 @@ fn foo() {
     bar(1, size);
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_parameter_name_has_precedence_over_type() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct TextSize(u32);
@@ -894,12 +1241,13 @@ fn foo() {
     bar(1, size);
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_function() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn is_required(test: u32, size: u32) -> bool
@@ -916,12 +1264,13 @@ fn foo() -> bool {
     is_required
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_method() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S;
@@ -944,12 +1293,13 @@ fn foo() -> u32 {
     bar
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_method_param() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S;
@@ -972,12 +1322,13 @@ fn foo() {
     S.bar(n, 2)
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_name_from_ufcs_method_param() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S;
@@ -1000,12 +1351,13 @@ fn foo() {
     S::bar(&S, n, 2)
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_var_parameter_name_has_precedence_over_function() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 fn bar(test: u32, size: u32)
@@ -1022,14 +1374,15 @@ fn foo() {
     bar(1, size);
 }
 "#,
+            "Extract into variable",
         )
     }
 
     #[test]
     fn extract_macro_call() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
-            r"
+            r#"
 struct Vec;
 macro_rules! vec {
     () => {Vec}
@@ -1037,8 +1390,8 @@ macro_rules! vec {
 fn main() {
     let _ = $0vec![]$0;
 }
-",
-            r"
+"#,
+            r#"
 struct Vec;
 macro_rules! vec {
     () => {Vec}
@@ -1047,22 +1400,47 @@ fn main() {
     let $0vec = vec![];
     let _ = vec;
 }
-",
+"#,
+            "Extract into variable",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+struct Vec;
+macro_rules! vec {
+    () => {Vec}
+}
+fn main() {
+    let _ = $0vec![]$0;
+}
+"#,
+            r#"
+struct Vec;
+macro_rules! vec {
+    () => {Vec}
+}
+fn main() {
+    const $0VEC: Vec = vec![];
+    let _ = VEC;
+}
+"#,
+            "Extract into constant",
         );
     }
 
     #[test]
-    fn test_extract_var_for_return_not_applicable() {
+    fn extract_var_for_return_not_applicable() {
         check_assist_not_applicable(extract_variable, "fn foo() { $0return$0; } ");
     }
 
     #[test]
-    fn test_extract_var_for_break_not_applicable() {
+    fn extract_var_for_break_not_applicable() {
         check_assist_not_applicable(extract_variable, "fn main() { loop { $0break$0; }; }");
     }
 
     #[test]
-    fn test_extract_var_unit_expr_not_applicable() {
+    fn extract_var_unit_expr_not_applicable() {
         check_assist_not_applicable(
             extract_variable,
             r#"
@@ -1080,11 +1458,11 @@ fn foo() {
     // FIXME: This is not quite correct, but good enough(tm) for the sorting heuristic
     #[test]
     fn extract_var_target() {
-        check_assist_target(extract_variable, "fn foo() -> u32 { $0return 2 + 2$0; }", "2 + 2");
+        check_assist_target(extract_variable, r#"fn foo() -> u32 { $0return 2 + 2$0; }"#, "2 + 2");
 
         check_assist_target(
             extract_variable,
-            "
+            r#"
 fn main() {
     let x = true;
     let tuple = match x {
@@ -1092,24 +1470,128 @@ fn main() {
         _ => (0, false)
     };
 }
-",
+"#,
             "2 + 2",
         );
     }
 
     #[test]
     fn extract_var_no_block_body() {
-        check_assist_not_applicable(
+        check_assist_not_applicable_by_label(
             extract_variable,
-            r"
+            r#"
 const X: usize = $0100$0;
-",
+"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_mutable_reference_parameter() {
-        check_assist(
+    fn extract_const_no_block_body() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+const FOO: i32 = foo($0100$0);
+"#,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+const $0X: i32 = 100;
+const FOO: i32 = foo(X);
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+mod foo {
+    enum Foo {
+        Bar,
+        Baz = $042$0,
+    }
+}
+"#,
+            r#"
+mod foo {
+    const $0VAR_NAME: isize = 42;
+    enum Foo {
+        Bar,
+        Baz = VAR_NAME,
+    }
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+trait Hello {
+    const World: i32;
+}
+
+struct Bar;
+impl Hello for Bar {
+    const World = foo($042$0);
+}
+"#,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+trait Hello {
+    const World: i32;
+}
+
+struct Bar;
+impl Hello for Bar {
+    const $0X: i32 = 42;
+    const World = foo(X);
+}
+"#,
+            "Extract into constant",
+        );
+
+        check_assist_by_label(
+            extract_variable,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+fn bar() {
+    const BAZ: i32 = foo($042$0);
+}
+"#,
+            r#"
+const fn foo(x: i32) -> i32 {
+    x
+}
+
+fn bar() {
+    const $0X: i32 = 42;
+    const BAZ: i32 = foo(X);
+}
+"#,
+            "Extract into constant",
+        );
+    }
+
+    #[test]
+    fn extract_var_mutable_reference_parameter() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S {
@@ -1138,12 +1620,34 @@ fn foo(s: &mut S) {
     let $0vec = &mut s.vec;
     vec.push(0);
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_mutable_reference_parameter_deep_nesting() {
-        check_assist(
+    fn dont_extract_const_mutable_reference_parameter() {
+        check_assist_not_applicable_by_label(
+            extract_variable,
+            r#"
+struct S {
+    vec: Vec<u8>
+}
+
+struct Vec<T>;
+impl<T> Vec<T> {
+    fn push(&mut self, _:usize) {}
+}
+
+fn foo(s: &mut S) {
+    $0s.vec$0.push(0);
+}"#,
+            "Extract into const",
+        );
+    }
+
+    #[test]
+    fn extract_var_mutable_reference_parameter_deep_nesting() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct Y {
@@ -1182,12 +1686,13 @@ fn foo(f: &mut Y) {
     let $0vec = &mut f.field.field.vec;
     vec.push(0);
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_reference_parameter() {
-        check_assist(
+    fn extract_var_reference_parameter() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct X;
@@ -1222,12 +1727,13 @@ fn foo(s: &S) {
     let $0x = &s.sub;
     x.do_thing();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_index_deref() {
-        check_assist(
+    fn extract_var_index_deref() {
+        check_assist_by_label(
             extract_variable,
             r#"
 //- minicore: index
@@ -1261,12 +1767,13 @@ fn foo(s: &S) {
     let $0sub = &s.sub;
     sub[0];
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_reference_parameter_deep_nesting() {
-        check_assist(
+    fn extract_var_reference_parameter_deep_nesting() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct Z;
@@ -1315,12 +1822,13 @@ fn foo(s: &S) {
     let $0z = &s.sub.field.field;
     z.do_thing();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_regular_parameter() {
-        check_assist(
+    fn extract_var_regular_parameter() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct X;
@@ -1355,12 +1863,13 @@ fn foo(s: S) {
     let $0x = &s.sub;
     x.do_thing();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_mutable_reference_local() {
-        check_assist(
+    fn extract_var_mutable_reference_local() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct X;
@@ -1421,12 +1930,13 @@ fn foo() {
     let $0x = &local.sub;
     x.do_thing();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_reference_local() {
-        check_assist(
+    fn extract_var_reference_local() {
+        check_assist_by_label(
             extract_variable,
             r#"
 struct X;
@@ -1487,12 +1997,13 @@ fn foo() {
     let $0x = &local.sub;
     x.do_thing();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
-    fn test_extract_var_for_mutable_borrow() {
-        check_assist(
+    fn extract_var_for_mutable_borrow() {
+        check_assist_by_label(
             extract_variable,
             r#"
 fn foo() {
@@ -1503,12 +2014,25 @@ fn foo() {
     let mut $0var_name = 0;
     let v = &mut var_name;
 }"#,
+            "Extract into variable",
+        );
+    }
+
+    #[test]
+    fn dont_extract_const_for_mutable_borrow() {
+        check_assist_not_applicable_by_label(
+            extract_variable,
+            r#"
+fn foo() {
+    let v = &mut $00$0;
+}"#,
+            "Extract into constant",
         );
     }
 
     #[test]
     fn generates_no_ref_on_calls() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S;
@@ -1529,12 +2053,13 @@ fn foo() {
     let mut $0bar = bar();
     bar.do_work();
 }"#,
+            "Extract into variable",
         );
     }
 
     #[test]
     fn generates_no_ref_for_deref() {
-        check_assist(
+        check_assist_by_label(
             extract_variable,
             r#"
 struct S;
@@ -1559,6 +2084,7 @@ fn foo() {
     s.do_work();
 }
 "#,
+            "Extract into variable",
         );
     }
 }
diff --git a/crates/ide-assists/src/tests.rs b/crates/ide-assists/src/tests.rs
index 6469957fe1..4f7f03764f 100644
--- a/crates/ide-assists/src/tests.rs
+++ b/crates/ide-assists/src/tests.rs
@@ -363,6 +363,7 @@ pub fn test_some_range(a: int) -> bool {
     expect![[r#"
         Convert integer base
         Extract into variable
+        Extract into constant
         Extract into function
         Replace if let with match
     "#]]
@@ -392,6 +393,7 @@ pub fn test_some_range(a: int) -> bool {
         expect![[r#"
             Convert integer base
             Extract into variable
+            Extract into constant
             Extract into function
             Replace if let with match
         "#]]
@@ -406,6 +408,7 @@ pub fn test_some_range(a: int) -> bool {
 
         expect![[r#"
             Extract into variable
+            Extract into constant
             Extract into function
         "#]]
         .assert_eq(&expected);
@@ -440,7 +443,7 @@ pub fn test_some_range(a: int) -> bool {
 
     {
         let assists = assists(&db, &cfg, AssistResolveStrategy::None, frange.into());
-        assert_eq!(2, assists.len());
+        assert_eq!(3, assists.len());
         let mut assists = assists.into_iter();
 
         let extract_into_variable_assist = assists.next().unwrap();
@@ -459,6 +462,22 @@ pub fn test_some_range(a: int) -> bool {
         "#]]
         .assert_debug_eq(&extract_into_variable_assist);
 
+        let extract_into_constant_assist = assists.next().unwrap();
+        expect![[r#"
+            Assist {
+                id: AssistId(
+                    "extract_constant",
+                    RefactorExtract,
+                ),
+                label: "Extract into constant",
+                group: None,
+                target: 59..60,
+                source_change: None,
+                command: None,
+            }
+        "#]]
+        .assert_debug_eq(&extract_into_constant_assist);
+
         let extract_into_function_assist = assists.next().unwrap();
         expect![[r#"
             Assist {
@@ -486,7 +505,7 @@ pub fn test_some_range(a: int) -> bool {
             }),
             frange.into(),
         );
-        assert_eq!(2, assists.len());
+        assert_eq!(3, assists.len());
         let mut assists = assists.into_iter();
 
         let extract_into_variable_assist = assists.next().unwrap();
@@ -505,6 +524,22 @@ pub fn test_some_range(a: int) -> bool {
         "#]]
         .assert_debug_eq(&extract_into_variable_assist);
 
+        let extract_into_constant_assist = assists.next().unwrap();
+        expect![[r#"
+            Assist {
+                id: AssistId(
+                    "extract_constant",
+                    RefactorExtract,
+                ),
+                label: "Extract into constant",
+                group: None,
+                target: 59..60,
+                source_change: None,
+                command: None,
+            }
+        "#]]
+        .assert_debug_eq(&extract_into_constant_assist);
+
         let extract_into_function_assist = assists.next().unwrap();
         expect![[r#"
             Assist {
@@ -532,7 +567,7 @@ pub fn test_some_range(a: int) -> bool {
             }),
             frange.into(),
         );
-        assert_eq!(2, assists.len());
+        assert_eq!(3, assists.len());
         let mut assists = assists.into_iter();
 
         let extract_into_variable_assist = assists.next().unwrap();
@@ -594,6 +629,22 @@ pub fn test_some_range(a: int) -> bool {
         "#]]
         .assert_debug_eq(&extract_into_variable_assist);
 
+        let extract_into_constant_assist = assists.next().unwrap();
+        expect![[r#"
+            Assist {
+                id: AssistId(
+                    "extract_constant",
+                    RefactorExtract,
+                ),
+                label: "Extract into constant",
+                group: None,
+                target: 59..60,
+                source_change: None,
+                command: None,
+            }
+        "#]]
+        .assert_debug_eq(&extract_into_constant_assist);
+
         let extract_into_function_assist = assists.next().unwrap();
         expect![[r#"
             Assist {
@@ -613,7 +664,7 @@ pub fn test_some_range(a: int) -> bool {
 
     {
         let assists = assists(&db, &cfg, AssistResolveStrategy::All, frange.into());
-        assert_eq!(2, assists.len());
+        assert_eq!(3, assists.len());
         let mut assists = assists.into_iter();
 
         let extract_into_variable_assist = assists.next().unwrap();
@@ -675,6 +726,69 @@ pub fn test_some_range(a: int) -> bool {
         "#]]
         .assert_debug_eq(&extract_into_variable_assist);
 
+        let extract_into_constant_assist = assists.next().unwrap();
+        expect![[r#"
+            Assist {
+                id: AssistId(
+                    "extract_constant",
+                    RefactorExtract,
+                ),
+                label: "Extract into constant",
+                group: None,
+                target: 59..60,
+                source_change: Some(
+                    SourceChange {
+                        source_file_edits: {
+                            FileId(
+                                0,
+                            ): (
+                                TextEdit {
+                                    indels: [
+                                        Indel {
+                                            insert: "const",
+                                            delete: 45..47,
+                                        },
+                                        Indel {
+                                            insert: "VAR_NAME:",
+                                            delete: 48..60,
+                                        },
+                                        Indel {
+                                            insert: "i32",
+                                            delete: 61..81,
+                                        },
+                                        Indel {
+                                            insert: "=",
+                                            delete: 82..86,
+                                        },
+                                        Indel {
+                                            insert: "5;\n    if let 2..6 = VAR_NAME {\n        true\n    } else {\n        false\n    }",
+                                            delete: 87..108,
+                                        },
+                                    ],
+                                },
+                                Some(
+                                    SnippetEdit(
+                                        [
+                                            (
+                                                0,
+                                                51..51,
+                                            ),
+                                        ],
+                                    ),
+                                ),
+                            ),
+                        },
+                        file_system_edits: [],
+                        is_snippet: true,
+                    },
+                ),
+                command: Some(
+                    Rename,
+                ),
+            }
+        "#]]
+        .assert_debug_eq(&extract_into_constant_assist);
+
         let extract_into_function_assist = assists.next().unwrap();
         expect![[r#"
             Assist {
diff --git a/crates/ide-assists/src/tests/generated.rs b/crates/ide-assists/src/tests/generated.rs
index 69ea200db1..c1799b48ed 100644
--- a/crates/ide-assists/src/tests/generated.rs
+++ b/crates/ide-assists/src/tests/generated.rs
@@ -932,6 +932,24 @@ enum TheEnum {
     )
 }
 
+#[test]
+fn doctest_extract_constant() {
+    check_doc_test(
+        "extract_constant",
+        r#####"
+fn main() {
+    $0(1 + 2)$0 * 4;
+}
+"#####,
+        r#####"
+fn main() {
+    const $0VAR_NAME: i32 = 1 + 2;
+    VAR_NAME * 4;
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_extract_expressions_from_format_string() {
     check_doc_test(