rust-clippy/clippy_lints/src/minmax.rs

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use crate::consts::{constant_simple, Constant};
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use crate::utils::{match_def_path, opt_def_id, paths, span_lint};
use rustc::hir::*;
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use rustc::lint::*;
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use rustc::{declare_lint, lint_array};
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use std::cmp::Ordering;
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/// **What it does:** Checks for expressions where `std::cmp::min` and `max` are
/// used to clamp values, but switched so that the result is constant.
///
/// **Why is this bad?** This is in all probability not the intended outcome. At
/// the least it hurts readability of the code.
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// min(0, max(100, x))
/// ```
/// It will always be equal to `0`. Probably the author meant to clamp the value
/// between 0 and 100, but has erroneously swapped `min` and `max`.
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declare_clippy_lint! {
pub MIN_MAX,
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correctness,
"`min(_, max(_, _))` (or vice versa) with bounds clamping the result to a constant"
}
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#[allow(missing_copy_implementations)]
pub struct MinMaxPass;
impl LintPass for MinMaxPass {
fn get_lints(&self) -> LintArray {
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lint_array!(MIN_MAX)
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}
}
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MinMaxPass {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
if let Some((outer_max, outer_c, oe)) = min_max(cx, expr) {
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if let Some((inner_max, inner_c, ie)) = min_max(cx, oe) {
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if outer_max == inner_max {
return;
}
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match (
outer_max,
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Constant::partial_cmp(cx.tcx, &cx.tables.expr_ty(ie).sty, &outer_c, &inner_c),
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) {
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(_, None) | (MinMax::Max, Some(Ordering::Less)) | (MinMax::Min, Some(Ordering::Greater)) => (),
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_ => {
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span_lint(
cx,
MIN_MAX,
expr.span,
"this min/max combination leads to constant result",
);
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},
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}
}
}
}
}
#[derive(PartialEq, Eq, Debug)]
enum MinMax {
Min,
Max,
}
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fn min_max<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<(MinMax, Constant, &'a Expr)> {
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if let ExprKind::Call(ref path, ref args) = expr.node {
if let ExprKind::Path(ref qpath) = path.node {
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opt_def_id(cx.tables.qpath_def(qpath, path.hir_id)).and_then(|def_id| {
if match_def_path(cx.tcx, def_id, &paths::CMP_MIN) {
fetch_const(cx, args, MinMax::Min)
} else if match_def_path(cx.tcx, def_id, &paths::CMP_MAX) {
fetch_const(cx, args, MinMax::Max)
} else {
None
}
})
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} else {
None
}
} else {
None
}
}
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fn fetch_const<'a>(cx: &LateContext<'_, '_>, args: &'a [Expr], m: MinMax) -> Option<(MinMax, Constant, &'a Expr)> {
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if args.len() != 2 {
return None;
}
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if let Some(c) = constant_simple(cx, cx.tables, &args[0]) {
if constant_simple(cx, cx.tables, &args[1]).is_none() {
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// otherwise ignore
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Some((m, c, &args[1]))
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} else {
None
}
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} else if let Some(c) = constant_simple(cx, cx.tables, &args[1]) {
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Some((m, c, &args[0]))
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} else {
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None
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}
}