Merge #8436

8436: Fix extract function's mutability of variables outliving the body r=matklad a=brandondong

**Reproduction:**
```rust
fn main() {
    let mut k = 1;
    let mut j = 2;
    j += 1;
    k += j;
}
```
1. Select the first to third lines of the main function. Use the "Extract into function" code assist.
2. The output is the following which does not compile because the outlived variable `k` is declared as immutable:
```rust
fn main() {
    let (k, j) = fun_name();
    k += j;
}

fn fun_name() -> (i32, i32) {
    let mut k = 1;
    let mut j = 2;
    j += 1;
    (k, j)
}
```
3. We would instead expect the output to be:
```rust
fn main() {
    let (mut k, j) = fun_name();
    k += j;
}
```

**Fix:**
- Instead of declaring outlived variables as immutable unconditionally, check for any mutable usages outside of the extracted function.

Co-authored-by: Brandon <[email protected]>

Author: bors[bot]

crates/ide_assists/src/handlers/extract_function.rs

@@ -75,7 +75,8 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext) -> Option
     let insert_after = scope_for_fn_insertion(&body, anchor)?;
     let module = ctx.sema.scope(&insert_after).module()?;
 
-    let vars_defined_in_body_and_outlive = vars_defined_in_body_and_outlive(ctx, &body);
+    let vars_defined_in_body_and_outlive =
+        vars_defined_in_body_and_outlive(ctx, &body, &node.parent().as_ref().unwrap_or(&node));
     let ret_ty = body_return_ty(ctx, &body)?;
 
     // FIXME: we compute variables that outlive here just to check `never!` condition
@@ -257,7 +258,7 @@ struct Function {
     control_flow: ControlFlow,
     ret_ty: RetType,
     body: FunctionBody,
-    vars_defined_in_body_and_outlive: Vec<Local>,
+    vars_defined_in_body_and_outlive: Vec<OutlivedLocal>,
 }
 
 #[derive(Debug)]
@@ -296,9 +297,9 @@ impl Function {
             RetType::Expr(ty) => FunType::Single(ty.clone()),
             RetType::Stmt => match self.vars_defined_in_body_and_outlive.as_slice() {
                 [] => FunType::Unit,
-                [var] => FunType::Single(var.ty(ctx.db())),
+                [var] => FunType::Single(var.local.ty(ctx.db())),
                 vars => {
-                    let types = vars.iter().map(|v| v.ty(ctx.db())).collect();
+                    let types = vars.iter().map(|v| v.local.ty(ctx.db())).collect();
                     FunType::Tuple(types)
                 }
             },
@@ -562,6 +563,12 @@ impl HasTokenAtOffset for FunctionBody {
     }
 }
 
+#[derive(Debug)]
+struct OutlivedLocal {
+    local: Local,
+    mut_usage_outside_body: bool,
+}
+
 /// Try to guess what user wants to extract
 ///
 /// We have basically have two cases:
@@ -707,10 +714,10 @@ fn has_exclusive_usages(ctx: &AssistContext, usages: &LocalUsages, body: &Functi
         .any(|reference| reference_is_exclusive(reference, body, ctx))
 }
 
-/// checks if this reference requires `&mut` access inside body
+/// checks if this reference requires `&mut` access inside node
 fn reference_is_exclusive(
     reference: &FileReference,
-    body: &FunctionBody,
+    node: &dyn HasTokenAtOffset,
     ctx: &AssistContext,
 ) -> bool {
     // we directly modify variable with set: `n = 0`, `n += 1`
@@ -719,7 +726,7 @@ fn reference_is_exclusive(
     }
 
     // we take `&mut` reference to variable: `&mut v`
-    let path = match path_element_of_reference(body, reference) {
+    let path = match path_element_of_reference(node, reference) {
         Some(path) => path,
         None => return false,
     };
@@ -820,10 +827,16 @@ fn vars_defined_in_body(body: &FunctionBody, ctx: &AssistContext) -> Vec<Local>
 }
 
 /// list local variables defined inside `body` that should be returned from extracted function
-fn vars_defined_in_body_and_outlive(ctx: &AssistContext, body: &FunctionBody) -> Vec<Local> {
-    let mut vars_defined_in_body = vars_defined_in_body(&body, ctx);
-    vars_defined_in_body.retain(|var| var_outlives_body(ctx, body, var));
+fn vars_defined_in_body_and_outlive(
+    ctx: &AssistContext,
+    body: &FunctionBody,
+    parent: &SyntaxNode,
+) -> Vec<OutlivedLocal> {
+    let vars_defined_in_body = vars_defined_in_body(&body, ctx);
     vars_defined_in_body
+        .into_iter()
+        .filter_map(|var| var_outlives_body(ctx, body, var, parent))
+        .collect()
 }
 
 /// checks if the relevant local was defined before(outside of) body
@@ -843,11 +856,23 @@ fn either_syntax(value: &Either<ast::IdentPat, ast::SelfParam>) -> &SyntaxNode {
     }
 }
 
-/// checks if local variable is used after(outside of) body
-fn var_outlives_body(ctx: &AssistContext, body: &FunctionBody, var: &Local) -> bool {
-    let usages = LocalUsages::find(ctx, *var);
+/// returns usage details if local variable is used after(outside of) body
+fn var_outlives_body(
+    ctx: &AssistContext,
+    body: &FunctionBody,
+    var: Local,
+    parent: &SyntaxNode,
+) -> Option<OutlivedLocal> {
+    let usages = LocalUsages::find(ctx, var);
     let has_usages = usages.iter().any(|reference| body.preceedes_range(reference.range));
-    has_usages
+    if !has_usages {
+        return None;
+    }
+    let has_mut_usages = usages
+        .iter()
+        .filter(|reference| body.preceedes_range(reference.range))
+        .any(|reference| reference_is_exclusive(reference, parent, ctx));
+    Some(OutlivedLocal { local: var, mut_usage_outside_body: has_mut_usages })
 }
 
 fn body_return_ty(ctx: &AssistContext, body: &FunctionBody) -> Option<RetType> {
@@ -927,16 +952,25 @@ fn format_replacement(ctx: &AssistContext, fun: &Function, indent: IndentLevel)
     let mut buf = String::new();
     match fun.vars_defined_in_body_and_outlive.as_slice() {
         [] => {}
-        [var] => format_to!(buf, "let {} = ", var.name(ctx.db()).unwrap()),
+        [var] => {
+            format_to!(buf, "let {}{} = ", mut_modifier(var), var.local.name(ctx.db()).unwrap())
+        }
         [v0, vs @ ..] => {
             buf.push_str("let (");
-            format_to!(buf, "{}", v0.name(ctx.db()).unwrap());
+            format_to!(buf, "{}{}", mut_modifier(v0), v0.local.name(ctx.db()).unwrap());
             for var in vs {
-                format_to!(buf, ", {}", var.name(ctx.db()).unwrap());
+                format_to!(buf, ", {}{}", mut_modifier(var), var.local.name(ctx.db()).unwrap());
             }
             buf.push_str(") = ");
         }
     }
+    fn mut_modifier(var: &OutlivedLocal) -> &'static str {
+        if var.mut_usage_outside_body {
+            "mut "
+        } else {
+            ""
+        }
+    }
     format_to!(buf, "{}", expr);
     if fun.ret_ty.is_unit()
         && (!fun.vars_defined_in_body_and_outlive.is_empty() || !expr.is_block_like())
@@ -1199,10 +1233,10 @@ fn make_body(
                 match fun.vars_defined_in_body_and_outlive.as_slice() {
                     [] => {}
                     [var] => {
-                        tail_expr = Some(path_expr_from_local(ctx, *var));
+                        tail_expr = Some(path_expr_from_local(ctx, var.local));
                     }
                     vars => {
-                        let exprs = vars.iter().map(|var| path_expr_from_local(ctx, *var));
+                        let exprs = vars.iter().map(|var| path_expr_from_local(ctx, var.local));
                         let expr = make::expr_tuple(exprs);
                         tail_expr = Some(expr);
                     }
@@ -2110,6 +2144,30 @@ fn $0fun_name(n: i32) -> i32 {
         );
     }
 
+    #[test]
+    fn variable_defined_inside_and_used_after_mutably_no_ret() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() {
+    let n = 1;
+    $0let mut k = n * n;$0
+    k += 1;
+}",
+            r"
+fn foo() {
+    let n = 1;
+    let mut k = fun_name(n);
+    k += 1;
+}
+
+fn $0fun_name(n: i32) -> i32 {
+    let mut k = n * n;
+    k
+}",
+        );
+    }
+
     #[test]
     fn two_variables_defined_inside_and_used_after_no_ret() {
         check_assist(
@@ -2136,6 +2194,38 @@ fn $0fun_name(n: i32) -> (i32, i32) {
         );
     }
 
+    #[test]
+    fn multi_variables_defined_inside_and_used_after_mutably_no_ret() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() {
+    let n = 1;
+    $0let mut k = n * n;
+    let mut m = k + 2;
+    let mut o = m + 3;
+    o += 1;$0
+    k += o;
+    m = 1;
+}",
+            r"
+fn foo() {
+    let n = 1;
+    let (mut k, mut m, o) = fun_name(n);
+    k += o;
+    m = 1;
+}
+
+fn $0fun_name(n: i32) -> (i32, i32, i32) {
+    let mut k = n * n;
+    let mut m = k + 2;
+    let mut o = m + 3;
+    o += 1;
+    (k, m, o)
+}",
+        );
+    }
+
     #[test]
     fn nontrivial_patterns_define_variables() {
         check_assist(