rust/clippy_lints/src/bool_assert_comparison.rs

use clippy_utils::{diagnostics::span_lint_and_sugg, higher, is_direct_expn_of, ty::implements_trait};
use rustc_ast::ast::LitKind;
use rustc_errors::Applicability;
use rustc_hir::*;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::Ident;

declare_clippy_lint! {
    /// ### What it does
    /// This lint warns about boolean comparisons in assert-like macros.
    ///
    /// ### Why is this bad?
    /// It is shorter to use the equivalent.
    ///
    /// ### Example
    /// ```rust
    /// // Bad
    /// assert_eq!("a".is_empty(), false);
    /// assert_ne!("a".is_empty(), true);
    ///
    /// // Good
    /// assert!(!"a".is_empty());
    /// ```
    pub BOOL_ASSERT_COMPARISON,
    style,
    "Using a boolean as comparison value in an assert_* macro when there is no need"
}

declare_lint_pass!(BoolAssertComparison => [BOOL_ASSERT_COMPARISON]);

fn is_bool_lit(e: &Expr<'_>) -> bool {
    matches!(
        e.kind,
        ExprKind::Lit(Lit {
            node: LitKind::Bool(_),
            ..
        })
    ) && !e.span.from_expansion()
}

fn impl_not_trait_with_bool_out(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> bool {
    let ty = cx.typeck_results().expr_ty(e);

    cx.tcx
        .lang_items()
        .not_trait()
        .filter(|id| implements_trait(cx, ty, *id, &[]))
        .and_then(|id| {
            cx.tcx.associated_items(id).find_by_name_and_kind(
                cx.tcx,
                Ident::from_str("Output"),
                ty::AssocKind::Type,
                id,
            )
        })
        .map_or(false, |item| {
            let proj = cx.tcx.mk_projection(item.def_id, cx.tcx.mk_substs_trait(ty, &[]));
            let nty = cx.tcx.normalize_erasing_regions(cx.param_env, proj);

            nty.is_bool()
        })
}

impl<'tcx> LateLintPass<'tcx> for BoolAssertComparison {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        let macros = ["assert_eq", "debug_assert_eq"];
        let inverted_macros = ["assert_ne", "debug_assert_ne"];

        for mac in macros.iter().chain(inverted_macros.iter()) {
            if let Some(span) = is_direct_expn_of(expr.span, mac) {
                if let Some(args) = higher::extract_assert_macro_args(expr) {
                    if let [a, b, ..] = args[..] {
                        let nb_bool_args = is_bool_lit(a) as usize + is_bool_lit(b) as usize;

                        if nb_bool_args != 1 {
                            // If there are two boolean arguments, we definitely don't understand
                            // what's going on, so better leave things as is...
                            //
                            // Or there is simply no boolean and then we can leave things as is!
                            return;
                        }

                        if !impl_not_trait_with_bool_out(cx, a) || !impl_not_trait_with_bool_out(cx, b) {
                            // At this point the expression which is not a boolean
                            // literal does not implement Not trait with a bool output,
                            // so we cannot suggest to rewrite our code
                            return;
                        }

                        let non_eq_mac = &mac[..mac.len() - 3];
                        span_lint_and_sugg(
                            cx,
                            BOOL_ASSERT_COMPARISON,
                            span,
                            &format!("used `{}!` with a literal bool", mac),
                            "replace it with",
                            format!("{}!(..)", non_eq_mac),
                            Applicability::MaybeIncorrect,
                        );
                        return;
                    }
                }
            }
        }
    }
}