rust/tests/ui/used_underscore_binding.rs

//@aux-build:proc_macro_derive.rs
#![feature(rustc_private, lint_reasons)]
#![warn(clippy::used_underscore_binding)]
#![allow(clippy::disallowed_names, clippy::eq_op, clippy::uninlined_format_args)]

#[macro_use]
extern crate proc_macro_derive;

// This should not trigger the lint. There's underscore binding inside the external derive that
// would trigger the `used_underscore_binding` lint.
#[derive(DeriveSomething)]
struct Baz;

macro_rules! test_macro {
    () => {{
        let _foo = 42;
        _foo + 1
    }};
}

/// Tests that we lint if we use a binding with a single leading underscore
fn prefix_underscore(_foo: u32) -> u32 {
    _foo + 1
}

/// Tests that we lint if we use a `_`-variable defined outside within a macro expansion
fn in_macro_or_desugar(_foo: u32) {
    println!("{}", _foo);
    assert_eq!(_foo, _foo);

    test_macro!() + 1;
}

// Struct for testing use of fields prefixed with an underscore
struct StructFieldTest {
    _underscore_field: u32,
}

/// Tests that we lint the use of a struct field which is prefixed with an underscore
fn in_struct_field() {
    let mut s = StructFieldTest { _underscore_field: 0 };
    s._underscore_field += 1;
}

/// Tests that we do not lint if the struct field is used in code created with derive.
#[derive(Clone, Debug)]
pub struct UnderscoreInStruct {
    _foo: u32,
}

/// Tests that we do not lint if the underscore is not a prefix
fn non_prefix_underscore(some_foo: u32) -> u32 {
    some_foo + 1
}

/// Tests that we do not lint if we do not use the binding (simple case)
fn unused_underscore_simple(_foo: u32) -> u32 {
    1
}

/// Tests that we do not lint if we do not use the binding (complex case). This checks for
/// compatibility with the built-in `unused_variables` lint.
fn unused_underscore_complex(mut _foo: u32) -> u32 {
    _foo += 1;
    _foo = 2;
    1
}

/// Test that we do not lint for multiple underscores
fn multiple_underscores(__foo: u32) -> u32 {
    __foo + 1
}

// Non-variable bindings with preceding underscore
fn _fn_test() {}
struct _StructTest;
enum _EnumTest {
    _Empty,
    _Value(_StructTest),
}

/// Tests that we do not lint for non-variable bindings
fn non_variables() {
    _fn_test();
    let _s = _StructTest;
    let _e = match _EnumTest::_Value(_StructTest) {
        _EnumTest::_Empty => 0,
        _EnumTest::_Value(_st) => 1,
    };
    let f = _fn_test;
    f();
}

// Tests that we do not lint if the binding comes from await desugaring,
// but we do lint the awaited expression. See issue 5360.
async fn await_desugaring() {
    async fn foo() {}
    fn uses_i(_i: i32) {}

    foo().await;
    ({
        let _i = 5;
        uses_i(_i);
        foo()
    })
    .await
}

struct PhantomField<T> {
    _marker: std::marker::PhantomData<T>,
}

impl<T> std::fmt::Debug for PhantomField<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("PhantomField").field("_marker", &self._marker).finish()
    }
}

struct AllowedField {
    #[allow(clippy::used_underscore_binding)]
    _allowed: usize,
}

struct ExpectedField {
    #[expect(clippy::used_underscore_binding)]
    _expected: usize,
}

fn lint_levels(allowed: AllowedField, expected: ExpectedField) {
    let _ = allowed._allowed;
    let _ = expected._expected;
}

fn main() {
    let foo = 0u32;
    // tests of unused_underscore lint
    let _ = prefix_underscore(foo);
    in_macro_or_desugar(foo);
    in_struct_field();
    // possible false positives
    let _ = non_prefix_underscore(foo);
    let _ = unused_underscore_simple(foo);
    let _ = unused_underscore_complex(foo);
    let _ = multiple_underscores(foo);
    non_variables();
    await_desugaring();
}