1 files changed, 36 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/methods/suspicious_map.rs b/src/tools/clippy/clippy_lints/src/methods/suspicious_map.rs
new file mode 100644
index 000000000..9c3375bf3
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/methods/suspicious_map.rs
@@ -0,0 +1,36 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::usage::mutated_variables;
+use clippy_utils::{expr_or_init, is_trait_method};
+use if_chain::if_chain;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::SUSPICIOUS_MAP;
+
+pub fn check<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, count_recv: &hir::Expr<'_>, map_arg: &hir::Expr<'_>) {
+    if_chain! {
+        if is_trait_method(cx, count_recv, sym::Iterator);
+        let closure = expr_or_init(cx, map_arg);
+        if let Some(def_id) = cx.tcx.hir().opt_local_def_id(closure.hir_id);
+        if let Some(body_id) = cx.tcx.hir().maybe_body_owned_by(def_id);
+        let closure_body = cx.tcx.hir().body(body_id);
+        if !cx.typeck_results().expr_ty(&closure_body.value).is_unit();
+        then {
+            if let Some(map_mutated_vars) = mutated_variables(&closure_body.value, cx) {
+                // A variable is used mutably inside of the closure. Suppress the lint.
+                if !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`",
+            );
+        }
+    }
+}