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use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet;
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;
declare_clippy_lint! {
/// ### What it does
/// Detects `().hash(_)`.
///
/// ### Why is this bad?
/// Hashing a unit value doesn't do anything as the implementation of `Hash` for `()` is a no-op.
///
/// ### Example
/// ```rust
/// # use std::hash::Hash;
/// # use std::collections::hash_map::DefaultHasher;
/// # enum Foo { Empty, WithValue(u8) }
/// # use Foo::*;
/// # let mut state = DefaultHasher::new();
/// # let my_enum = Foo::Empty;
/// match my_enum {
/// Empty => ().hash(&mut state),
/// WithValue(x) => x.hash(&mut state),
/// }
/// ```
/// Use instead:
/// ```rust
/// # use std::hash::Hash;
/// # use std::collections::hash_map::DefaultHasher;
/// # enum Foo { Empty, WithValue(u8) }
/// # use Foo::*;
/// # let mut state = DefaultHasher::new();
/// # let my_enum = Foo::Empty;
/// match my_enum {
/// Empty => 0_u8.hash(&mut state),
/// WithValue(x) => x.hash(&mut state),
/// }
/// ```
#[clippy::version = "1.58.0"]
pub UNIT_HASH,
correctness,
"hashing a unit value, which does nothing"
}
declare_lint_pass!(UnitHash => [UNIT_HASH]);
impl<'tcx> LateLintPass<'tcx> for UnitHash {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
if_chain! {
if let ExprKind::MethodCall(name_ident, _, args, _) = &expr.kind;
if name_ident.ident.name == sym::hash;
if let [recv, state_param] = args;
if cx.typeck_results().expr_ty(recv).is_unit();
then {
span_lint_and_then(
cx,
UNIT_HASH,
expr.span,
"this call to `hash` on the unit type will do nothing",
|diag| {
diag.span_suggestion(
expr.span,
"remove the call to `hash` or consider using",
format!(
"0_u8.hash({})",
snippet(cx, state_param.span, ".."),
),
Applicability::MaybeIncorrect,
);
diag.note("the implementation of `Hash` for `()` is a no-op");
}
);
}
}
}
}
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