clippy_lints/src/methods/suspicious_map.rs

use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::expr_or_init;
use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
use clippy_utils::usage::mutated_variables;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_span::sym;

use super::SUSPICIOUS_MAP;

pub fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, count_recv: &hir::Expr<'_>, map_arg: &hir::Expr<'_>) {
    if cx
        .ty_based_def(count_recv)
        .opt_parent(cx)
        .is_diag_item(cx, sym::Iterator)
        && let hir::ExprKind::Closure(closure) = expr_or_init(cx, map_arg).kind
        && let closure_body = cx.tcx.hir_body(closure.body)
        && !cx.typeck_results().expr_ty(closure_body.value).is_unit()
    {
        if let Some(map_mutated_vars) = mutated_variables(closure_body.value, cx)
            // A variable is used mutably inside of the closure. Suppress the lint.
            && !map_mutated_vars.is_empty()
        {
            return;
        }
        span_lint_and_help(
            cx,
            SUSPICIOUS_MAP,
            expr.span,
            "this call to `map()` won't have an effect on the call to `count()`",
            None,
            "make sure you did not confuse `map` with `filter`, `for_each` or `inspect`",
        );
    }
}