rust-clippy/clippy_lints/src/neg_multiply.rs

use rustc::hir::*;
use rustc::lint::*;
use syntax::codemap::{Span, Spanned};

use consts::{self, Constant};
use utils::span_lint;

/// **What it does:** Checks for multiplication by -1 as a form of negation.
///
/// **Why is this bad?** It's more readable to just negate.
///
/// **Known problems:** This only catches integers (for now).
///
/// **Example:**
/// ```rust
/// x * -1
/// ```
declare_lint! {
    pub NEG_MULTIPLY,
    Warn,
    "multiplying integers with -1"
}

#[derive(Copy, Clone)]
pub struct NegMultiply;

impl LintPass for NegMultiply {
    fn get_lints(&self) -> LintArray {
        lint_array!(NEG_MULTIPLY)
    }
}

#[allow(match_same_arms)]
impl LateLintPass for NegMultiply {
    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
        if let ExprBinary(Spanned { node: BiMul, .. }, ref l, ref r) = e.node {
            match (&l.node, &r.node) {
                (&ExprUnary(..), &ExprUnary(..)) => (),
                (&ExprUnary(UnNeg, ref lit), _) => check_mul(cx, e.span, lit, r),
                (_, &ExprUnary(UnNeg, ref lit)) => check_mul(cx, e.span, lit, l),
                _ => (),
            }
        }
    }
}

fn check_mul(cx: &LateContext, span: Span, lit: &Expr, exp: &Expr) {
    if_let_chain!([
        let ExprLit(ref l) = lit.node,
        let Constant::Int(ref ci) = consts::lit_to_constant(&l.node),
        let Some(val) = ci.to_u64(),
        val == 1,
        cx.tcx.expr_ty(exp).is_integral()
    ], {
        span_lint(cx,
                  NEG_MULTIPLY,
                  span,
                  "Negation by multiplying with -1");
    })
}