rust-clippy/clippy_lints/src/minmax.rs

use crate::consts::{constant_simple, Constant};
use crate::utils::{match_def_path, opt_def_id, paths, span_lint};
use rustc::hir::*;
use rustc::lint::*;
use rustc::{declare_lint, lint_array};
use std::cmp::Ordering;

/// **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`.
declare_clippy_lint! {
    pub MIN_MAX,
    correctness,
    "`min(_, max(_, _))` (or vice versa) with bounds clamping the result to a constant"
}

#[allow(missing_copy_implementations)]
pub struct MinMaxPass;

impl LintPass for MinMaxPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(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).sty, &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) -> Option<(MinMax, Constant, &'a Expr)> {
    if let ExprKind::Call(ref path, ref args) = expr.node {
        if let ExprKind::Path(ref qpath) = path.node {
            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
                }
            })
        } else {
            None
        }
    } else {
        None
    }
}

fn fetch_const<'a>(cx: &LateContext<'_, '_>, args: &'a [Expr], m: MinMax) -> Option<(MinMax, Constant, &'a Expr)> {
    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
    }
}