Update and fix clippy tests

clippy_lints/src/missing_const_for_fn.rs

@@ -3,6 +3,7 @@ use clippy_utils::qualify_min_const_fn::is_min_const_fn;
 use clippy_utils::ty::has_drop;
 use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, meets_msrv, msrvs, trait_ref_of_method};
 use rustc_hir as hir;
+use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
 use rustc_lint::{LateContext, LateLintPass};
@@ -131,6 +132,18 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
             FnKind::Closure => return,
         }
 
+        // Const fns are not allowed as methods in a trait.
+        {
+            let parent = cx.tcx.hir().get_parent_item(hir_id);
+            if parent != CRATE_DEF_ID {
+                if let hir::Node::Item(item) = cx.tcx.hir().get_by_def_id(parent) {
+                    if let hir::ItemKind::Trait(..) = &item.kind {
+                        return;
+                    }
+                }
+            }
+        }
+
         let mir = cx.tcx.optimized_mir(def_id);
 
         if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, self.msrv.as_ref()) {

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -49,8 +49,6 @@ fn sub(x: u32) -> usize {
     unsafe { transmute(&x) }
 }
 
-// NOTE: This is currently not yet allowed to be const
-// Once implemented, Clippy should be able to suggest this as const, too.
 fn generic_arr<T: Copy>(t: [T; 1]) -> T {
     t[0]
 }

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -58,7 +58,15 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:67:9
+  --> $DIR/could_be_const.rs:52:1
+   |
+LL | / fn generic_arr<T: Copy>(t: [T; 1]) -> T {
+LL | |     t[0]
+LL | | }
+   | |_^
+
+error: this could be a `const fn`
+  --> $DIR/could_be_const.rs:65:9
    |
 LL | /         pub fn b(self, a: &A) -> B {
 LL | |             B
@@ -66,12 +74,12 @@ LL | |         }
    | |_________^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:77:5
+  --> $DIR/could_be_const.rs:75:5
    |
 LL | /     fn const_fn_stabilized_before_msrv(byte: u8) {
 LL | |         byte.is_ascii_digit();
 LL | |     }
    | |_____^
 
-error: aborting due to 9 previous errors
+error: aborting due to 10 previous errors