Do not trigger missing_const_for_fn for tests (#13945)

Close #13938

changelog: [`missing_const_for_fn`]: do not trigger for tests

clippy_lints/src/missing_const_for_fn.rs

@@ -2,7 +2,7 @@ use clippy_config::Conf;
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::qualify_min_const_fn::is_min_const_fn;
-use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, is_from_proc_macro, trait_ref_of_method};
+use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, is_from_proc_macro, is_in_test, trait_ref_of_method};
 use rustc_errors::Applicability;
 use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_hir::intravisit::FnKind;
@@ -97,6 +97,11 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
         span: Span,
         def_id: LocalDefId,
     ) {
+        let hir_id = cx.tcx.local_def_id_to_hir_id(def_id);
+        if is_in_test(cx.tcx, hir_id) {
+            return;
+        }
+
         if !self.msrv.meets(msrvs::CONST_IF_MATCH) {
             return;
         }
@@ -136,8 +141,6 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
             return;
         }
 
-        let hir_id = cx.tcx.local_def_id_to_hir_id(def_id);
-
         // Const fns are not allowed as methods in a trait.
         {
             let parent = cx.tcx.hir().get_parent_item(hir_id).def_id;

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -47,7 +47,34 @@ fn get_y() -> u32 {
     Y
 }
 
-// Don't lint entrypoint functions
+#[cfg(test)]
+mod with_test_fn {
+    #[derive(Clone, Copy)]
+    pub struct Foo {
+        pub n: u32,
+    }
+
+    impl Foo {
+        #[must_use]
+        pub const fn new(n: u32) -> Foo {
+            Foo { n }
+        }
+    }
+
+    #[test]
+    fn foo_is_copy() {
+        let foo = Foo::new(42);
+        let one = foo;
+        let two = foo;
+        _ = one;
+        _ = two;
+    }
+}
+
+// Allowing on this function, because it would lint, which we don't want in this case.
+// if we have `#[start]` and `#[test]` check `is_entrypoint_fn(cx, def_id.to_def_id())` is stopped
+// working
+#[allow(clippy::missing_const_for_fn)]
 #[start]
 fn init(num: isize, something: *const *const u8) -> isize {
     1