rust-clippy/clippy_lints/src/identity_op.rs

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use clippy_utils::get_parent_expr;
use clippy_utils::source::snippet;
use rustc_hir::{BinOp, BinOpKind, Expr, ExprKind};
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use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
use clippy_utils::consts::{constant_full_int, constant_simple, Constant, FullInt};
use clippy_utils::diagnostics::span_lint;
use clippy_utils::{clip, unsext};
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declare_clippy_lint! {
/// ### What it does
/// Checks for identity operations, e.g., `x + 0`.
///
/// ### Why is this bad?
/// This code can be removed without changing the
/// meaning. So it just obscures what's going on. Delete it mercilessly.
///
/// ### Example
/// ```rust
/// # let x = 1;
/// x / 1 + 0 * 1 - 0 | 0;
/// ```
///
/// ### Known problems
/// False negatives: `f(0 + if b { 1 } else { 2 } + 3);` is reducible to
/// `f(if b { 1 } else { 2 } + 3);`. But the lint doesn't trigger for the code.
/// See [#8724](https://github.com/rust-lang/rust-clippy/issues/8724)
#[clippy::version = "pre 1.29.0"]
pub IDENTITY_OP,
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complexity,
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"using identity operations, e.g., `x + 0` or `y / 1`"
}
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declare_lint_pass!(IdentityOp => [IDENTITY_OP]);
impl<'tcx> LateLintPass<'tcx> for IdentityOp {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
if expr.span.from_expansion() {
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return;
}
if let ExprKind::Binary(cmp, left, right) = &expr.kind {
if !is_allowed(cx, *cmp, left, right) {
match cmp.node {
BinOpKind::Add | BinOpKind::BitOr | BinOpKind::BitXor => {
if reducible_to_right(cx, expr, right) {
check(cx, left, 0, expr.span, right.span);
}
check(cx, right, 0, expr.span, left.span);
},
BinOpKind::Shl | BinOpKind::Shr | BinOpKind::Sub => {
check(cx, right, 0, expr.span, left.span);
},
BinOpKind::Mul => {
if reducible_to_right(cx, expr, right) {
check(cx, left, 1, expr.span, right.span);
}
check(cx, right, 1, expr.span, left.span);
},
BinOpKind::Div => check(cx, right, 1, expr.span, left.span),
BinOpKind::BitAnd => {
if reducible_to_right(cx, expr, right) {
check(cx, left, -1, expr.span, right.span);
}
check(cx, right, -1, expr.span, left.span);
},
BinOpKind::Rem => {
// Don't call reducible_to_right because N % N is always reducible to 1
check_remainder(cx, left, right, expr.span, left.span);
},
_ => (),
}
}
}
}
}
/// Checks if `left op ..right` can be actually reduced to `right`
/// e.g. `0 + if b { 1 } else { 2 } + if b { 3 } else { 4 }`
/// cannot be reduced to `if b { 1 } else { 2 } + if b { 3 } else { 4 }`
/// See #8724
fn reducible_to_right(cx: &LateContext<'_>, binary: &Expr<'_>, right: &Expr<'_>) -> bool {
if let ExprKind::If(..) | ExprKind::Match(..) | ExprKind::Block(..) | ExprKind::Loop(..) = right.kind {
is_toplevel_binary(cx, binary)
} else {
true
}
}
fn is_toplevel_binary(cx: &LateContext<'_>, must_be_binary: &Expr<'_>) -> bool {
if let Some(parent) = get_parent_expr(cx, must_be_binary) && let ExprKind::Binary(..) = &parent.kind {
false
} else {
true
}
}
fn is_allowed(cx: &LateContext<'_>, cmp: BinOp, left: &Expr<'_>, right: &Expr<'_>) -> bool {
// This lint applies to integers
!cx.typeck_results().expr_ty(left).peel_refs().is_integral()
|| !cx.typeck_results().expr_ty(right).peel_refs().is_integral()
// `1 << 0` is a common pattern in bit manipulation code
|| (cmp.node == BinOpKind::Shl
&& constant_simple(cx, cx.typeck_results(), right) == Some(Constant::Int(0))
&& constant_simple(cx, cx.typeck_results(), left) == Some(Constant::Int(1)))
}
fn check_remainder(cx: &LateContext<'_>, left: &Expr<'_>, right: &Expr<'_>, span: Span, arg: Span) {
let lhs_const = constant_full_int(cx, cx.typeck_results(), left);
let rhs_const = constant_full_int(cx, cx.typeck_results(), right);
if match (lhs_const, rhs_const) {
(Some(FullInt::S(lv)), Some(FullInt::S(rv))) => lv.abs() < rv.abs(),
(Some(FullInt::U(lv)), Some(FullInt::U(rv))) => lv < rv,
_ => return,
} {
span_ineffective_operation(cx, span, arg);
}
}
fn check(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span) {
if let Some(Constant::Int(v)) = constant_simple(cx, cx.typeck_results(), e).map(Constant::peel_refs) {
let check = match *cx.typeck_results().expr_ty(e).peel_refs().kind() {
ty::Int(ity) => unsext(cx.tcx, -1_i128, ity),
ty::Uint(uty) => clip(cx.tcx, !0, uty),
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_ => return,
};
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if match m {
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0 => v == 0,
-1 => v == check,
1 => v == 1,
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_ => unreachable!(),
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} {
span_ineffective_operation(cx, span, arg);
}
}
}
fn span_ineffective_operation(cx: &LateContext<'_>, span: Span, arg: Span) {
span_lint(
cx,
IDENTITY_OP,
span,
&format!(
"the operation is ineffective. Consider reducing it to `{}`",
snippet(cx, arg, "..")
),
);
}