add test case for implicitly stable const fn

tests/ui/traits/const-traits/staged-api.rs

@@ -72,4 +72,17 @@ const fn stable_const_context() {
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
 }
 
+const fn implicitly_stable_const_context() {
+    Unstable::func();
+    //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
+    Foo::func();
+    //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
+    const_context_not_const_stable();
+    //~^ ERROR cannot use `#[feature(local_feature)]`
+    conditionally_const::<Foo>();
+    //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+}
+
 fn main() {}

tests/ui/traits/const-traits/staged-api.stderr

@@ -220,5 +220,108 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn stable_const_context() {
    |
 
-error: aborting due to 13 previous errors
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
+  --> $DIR/staged-api.rs:76:5
+   |
+LL |     Unstable::func();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:76:5
+   |
+LL |     Unstable::func();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
+  --> $DIR/staged-api.rs:79:5
+   |
+LL |     Foo::func();
+   |     ^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:79:5
+   |
+LL |     Foo::func();
+   |     ^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(local_feature)]`
+  --> $DIR/staged-api.rs:82:5
+   |
+LL |     const_context_not_const_stable();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: mark the callee as `#[rustc_const_stable_indirect]` if it does not itself require any unstable features
+help: if the caller is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(local_feature)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
+  --> $DIR/staged-api.rs:84:5
+   |
+LL |     conditionally_const::<Foo>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn implicitly_stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
+LL | const fn implicitly_stable_const_context() {
+   |
+
+error: aborting due to 19 previous errors