rust/clippy_lints/src/minmax.rs
2020-02-21 11:14:18 +01:00

103 lines
3.3 KiB
Rust

use crate::consts::{constant_simple, Constant};
use crate::utils::{match_def_path, paths, span_lint};
use rustc_hir::{Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use std::cmp::Ordering;
declare_clippy_lint! {
/// **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:**
/// ```ignore
/// 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`.
pub MIN_MAX,
correctness,
"`min(_, max(_, _))` (or vice versa) with bounds clamping the result to a constant"
}
declare_lint_pass!(MinMaxPass => [MIN_MAX]);
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) {
if let Some((inner_max, inner_c, ie)) = min_max(cx, oe) {
if outer_max == inner_max {
return;
}
match (
outer_max,
Constant::partial_cmp(cx.tcx, cx.tables.expr_ty(ie), &outer_c, &inner_c),
) {
(_, None) | (MinMax::Max, Some(Ordering::Less)) | (MinMax::Min, Some(Ordering::Greater)) => (),
_ => {
span_lint(
cx,
MIN_MAX,
expr.span,
"this `min`/`max` combination leads to constant result",
);
},
}
}
}
}
}
#[derive(PartialEq, Eq, Debug)]
enum MinMax {
Min,
Max,
}
fn min_max<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr<'a>) -> Option<(MinMax, Constant, &'a Expr<'a>)> {
if let ExprKind::Call(ref path, ref args) = expr.kind {
if let ExprKind::Path(ref qpath) = path.kind {
cx.tables.qpath_res(qpath, path.hir_id).opt_def_id().and_then(|def_id| {
if match_def_path(cx, def_id, &paths::CMP_MIN) {
fetch_const(cx, args, MinMax::Min)
} else if match_def_path(cx, def_id, &paths::CMP_MAX) {
fetch_const(cx, args, MinMax::Max)
} else {
None
}
})
} else {
None
}
} else {
None
}
}
fn fetch_const<'a>(
cx: &LateContext<'_, '_>,
args: &'a [Expr<'a>],
m: MinMax,
) -> Option<(MinMax, Constant, &'a Expr<'a>)> {
if args.len() != 2 {
return None;
}
if let Some(c) = constant_simple(cx, cx.tables, &args[0]) {
if constant_simple(cx, cx.tables, &args[1]).is_none() {
// otherwise ignore
Some((m, c, &args[1]))
} else {
None
}
} else if let Some(c) = constant_simple(cx, cx.tables, &args[1]) {
Some((m, c, &args[0]))
} else {
None
}
}