rust-analyzer/crates/ide-diagnostics/src/handlers/mutability_errors.rs

use ide_db::source_change::SourceChange;
use syntax::{AstNode, SyntaxKind, SyntaxNode, SyntaxToken, T};
use text_edit::TextEdit;

use crate::{fix, Diagnostic, DiagnosticsContext, Severity};

// Diagnostic: need-mut
//
// This diagnostic is triggered on mutating an immutable variable.
pub(crate) fn need_mut(ctx: &DiagnosticsContext<'_>, d: &hir::NeedMut) -> Diagnostic {
    let fixes = (|| {
        if d.local.is_ref(ctx.sema.db) {
            // There is no simple way to add `mut` to `ref x` and `ref mut x`
            return None;
        }
        let file_id = d.span.file_id.file_id()?;
        let mut edit_builder = TextEdit::builder();
        let use_range = d.span.value.text_range();
        for source in d.local.sources(ctx.sema.db) {
            let Some(ast) = source.name() else { continue };
            edit_builder.insert(ast.syntax().text_range().start(), "mut ".to_string());
        }
        let edit = edit_builder.finish();
        Some(vec![fix(
            "remove_mut",
            "Remove unnecessary `mut`",
            SourceChange::from_text_edit(file_id, edit),
            use_range,
        )])
    })();
    Diagnostic::new(
        "need-mut",
        format!("cannot mutate immutable variable `{}`", d.local.name(ctx.sema.db)),
        ctx.sema.diagnostics_display_range(d.span.clone()).range,
    )
    .with_fixes(fixes)
}

// Diagnostic: unused-mut
//
// This diagnostic is triggered when a mutable variable isn't actually mutated.
pub(crate) fn unused_mut(ctx: &DiagnosticsContext<'_>, d: &hir::UnusedMut) -> Diagnostic {
    let ast = d.local.primary_source(ctx.sema.db).syntax_ptr();
    let fixes = (|| {
        let file_id = ast.file_id.file_id()?;
        let mut edit_builder = TextEdit::builder();
        let use_range = ast.value.text_range();
        for source in d.local.sources(ctx.sema.db) {
            let ast = source.syntax();
            let Some(mut_token) = token(ast, T![mut]) else { continue };
            edit_builder.delete(mut_token.text_range());
            if let Some(token) = mut_token.next_token() {
                if token.kind() == SyntaxKind::WHITESPACE {
                    edit_builder.delete(token.text_range());
                }
            }
        }
        let edit = edit_builder.finish();
        Some(vec![fix(
            "remove_mut",
            "Remove unnecessary `mut`",
            SourceChange::from_text_edit(file_id, edit),
            use_range,
        )])
    })();
    let ast = d.local.primary_source(ctx.sema.db).syntax_ptr();
    Diagnostic::new(
        "unused-mut",
        "remove this `mut`",
        ctx.sema.diagnostics_display_range(ast).range,
    )
    .severity(Severity::WeakWarning)
    .with_fixes(fixes)
}

pub(super) fn token(parent: &SyntaxNode, kind: SyntaxKind) -> Option<SyntaxToken> {
    parent.children_with_tokens().filter_map(|it| it.into_token()).find(|it| it.kind() == kind)
}

#[cfg(test)]
mod tests {
    use crate::tests::{check_diagnostics, check_fix};

    #[test]
    fn unused_mut_simple() {
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let mut x = 2;
      //^^^^^ 💡 weak: remove this `mut`
    f(x);
}
"#,
        );
    }

    #[test]
    fn no_false_positive_simple() {
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let x = 2;
    f(x);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let mut x = 2;
    x = 5;
    f(x);
}
"#,
        );
    }

    #[test]
    fn multiple_errors_for_single_variable() {
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let x = 2;
    x = 10;
  //^^^^^^ 💡 error: cannot mutate immutable variable `x`
    x = 5;
  //^^^^^ 💡 error: cannot mutate immutable variable `x`
    &mut x;
  //^^^^^^ 💡 error: cannot mutate immutable variable `x`
    f(x);
}
"#,
        );
    }

    #[test]
    fn unused_mut_fix() {
        check_fix(
            r#"
fn f(_: i32) {}
fn main() {
    let mu$0t x = 2;
    f(x);
}
"#,
            r#"
fn f(_: i32) {}
fn main() {
    let x = 2;
    f(x);
}
"#,
        );
        check_fix(
            r#"
fn f(_: i32) {}
fn main() {
    let ((mu$0t x, _) | (_, mut x)) = (2, 3);
    f(x);
}
"#,
            r#"
fn f(_: i32) {}
fn main() {
    let ((x, _) | (_, x)) = (2, 3);
    f(x);
}
"#,
        );
    }

    #[test]
    fn need_mut_fix() {
        check_fix(
            r#"
fn f(_: i32) {}
fn main() {
    let x = 2;
    x$0 = 5;
    f(x);
}
"#,
            r#"
fn f(_: i32) {}
fn main() {
    let mut x = 2;
    x = 5;
    f(x);
}
"#,
        );
        check_fix(
            r#"
fn f(_: i32) {}
fn main() {
    let ((x, _) | (_, x)) = (2, 3);
    x =$0 4;
    f(x);
}
"#,
            r#"
fn f(_: i32) {}
fn main() {
    let ((mut x, _) | (_, mut x)) = (2, 3);
    x = 4;
    f(x);
}
"#,
        );

        check_fix(
            r#"
struct Foo(i32);

impl Foo {
    fn foo(self) {
        self = Fo$0o(5);
    }
}
"#,
            r#"
struct Foo(i32);

impl Foo {
    fn foo(mut self) {
        self = Foo(5);
    }
}
"#,
        );
    }

    #[test]
    fn need_mut_fix_not_applicable_on_ref() {
        check_diagnostics(
            r#"
fn main() {
    let ref x = 2;
    x = &5;
  //^^^^^^ error: cannot mutate immutable variable `x`
}
"#,
        );
        check_diagnostics(
            r#"
fn main() {
    let ref mut x = 2;
    x = &mut 5;
  //^^^^^^^^^^ error: cannot mutate immutable variable `x`
}
"#,
        );
    }

    #[test]
    fn field_mutate() {
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let mut x = (2, 7);
      //^^^^^ 💡 weak: remove this `mut`
    f(x.1);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let mut x = (2, 7);
    x.0 = 5;
    f(x.1);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let x = (2, 7);
    x.0 = 5;
  //^^^^^^^ 💡 error: cannot mutate immutable variable `x`
    f(x.1);
}
"#,
        );
    }

    #[test]
    fn mutable_reference() {
        check_diagnostics(
            r#"
fn main() {
    let mut x = &mut 2;
      //^^^^^ 💡 weak: remove this `mut`
    *x = 5;
}
"#,
        );
        check_diagnostics(
            r#"
fn main() {
    let x = 2;
    &mut x;
  //^^^^^^ 💡 error: cannot mutate immutable variable `x`
}
"#,
        );
        check_diagnostics(
            r#"
fn main() {
    let x_own = 2;
    let ref mut x_ref = x_own;
      //^^^^^^^^^^^^^ 💡 error: cannot mutate immutable variable `x_own`
}
"#,
        );
        check_diagnostics(
            r#"
struct Foo;
impl Foo {
    fn method(&mut self, x: i32) {}
}
fn main() {
    let x = Foo;
    x.method(2);
  //^ 💡 error: cannot mutate immutable variable `x`
}
"#,
        );
    }

    #[test]
    fn match_bindings() {
        check_diagnostics(
            r#"
fn main() {
    match (2, 3) {
        (x, mut y) => {
          //^^^^^ 💡 weak: remove this `mut`
            x = 7;
          //^^^^^ 💡 error: cannot mutate immutable variable `x`
        }
    }
}
"#,
        );
    }

    #[test]
    fn mutation_in_dead_code() {
        // This one is interesting. Dead code is not represented at all in the MIR, so
        // there would be no mutablility error for locals in dead code. Rustc tries to
        // not emit `unused_mut` in this case, but since it works without `mut`, and
        // special casing it is not trivial, we emit it.
        check_diagnostics(
            r#"
fn main() {
    return;
    let mut x = 2;
      //^^^^^ 💡 weak: remove this `mut`
    &mut x;
}
"#,
        );
        check_diagnostics(
            r#"
fn main() {
    loop {}
    let mut x = 2;
      //^^^^^ 💡 weak: remove this `mut`
    &mut x;
}
"#,
        );
        check_diagnostics(
            r#"
enum X {}
fn g() -> X {
    loop {}
}
fn f() -> ! {
    loop {}
}
fn main(b: bool) {
    if b {
        f();
    } else {
        g();
    }
    let mut x = 2;
      //^^^^^ 💡 weak: remove this `mut`
    &mut x;
}
"#,
        );
        check_diagnostics(
            r#"
fn main(b: bool) {
    if b {
        loop {}
    } else {
        return;
    }
    let mut x = 2;
      //^^^^^ 💡 weak: remove this `mut`
    &mut x;
}
"#,
        );
    }

    #[test]
    fn initialization_is_not_mutation() {
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let mut x;
      //^^^^^ 💡 weak: remove this `mut`
    x = 5;
    f(x);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main(b: bool) {
    let mut x;
      //^^^^^ 💡 weak: remove this `mut`
    if b {
        x = 1;
    } else {
        x = 3;
    }
    f(x);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main(b: bool) {
    let x;
    if b {
        x = 1;
    }
    x = 3;
  //^^^^^ 💡 error: cannot mutate immutable variable `x`
    f(x);
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    let x;
    loop {
        x = 1;
      //^^^^^ 💡 error: cannot mutate immutable variable `x`
        f(x);
    }
}
"#,
        );
        check_diagnostics(
            r#"
fn f(_: i32) {}
fn main() {
    loop {
        let mut x = 1;
          //^^^^^ 💡 weak: remove this `mut`
        f(x);
        if let mut y = 2 {
             //^^^^^ 💡 weak: remove this `mut`
            f(y);
        }
        match 3 {
            mut z => f(z),
          //^^^^^ 💡 weak: remove this `mut`
        }
    }
}
"#,
        );
    }

    #[test]
    fn function_arguments_are_initialized() {
        check_diagnostics(
            r#"
fn f(mut x: i32) {
   //^^^^^ 💡 weak: remove this `mut`
}
"#,
        );
        check_diagnostics(
            r#"
fn f(x: i32) {
   x = 5;
 //^^^^^ 💡 error: cannot mutate immutable variable `x`
}
"#,
        );
    }
}