Say what unstable feature is being used in a const-stable function

Author: clarfonthey

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -323,17 +323,19 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         let gate = match op.status_in_item(self.ccx) {
             Status::Allowed => return,
 
-            Status::Unstable(gate) if self.tcx.features().active(gate) => {
+            Status::Unstable(gate) => {
                 let unstable_in_stable = self.ccx.is_const_stable_const_fn()
                     && !super::rustc_allow_const_fn_unstable(self.tcx, self.def_id(), gate);
                 if unstable_in_stable {
                     emit_unstable_in_stable_error(self.ccx, span, gate);
+                    return;
+                } else if self.tcx.features().declared(gate) && self.tcx.features().active(gate) {
+                    return;
+                } else {
+                    Some(gate)
                 }
-
-                return;
             }
 
-            Status::Unstable(gate) => Some(gate),
             Status::Forbidden => None,
         };
 

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -332,6 +332,10 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
 pub struct FnCallUnstable(pub DefId, pub Option<Symbol>);
 
 impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
+    fn status_in_item(&self, _: &ConstCx<'_, 'tcx>) -> Status {
+        if let Some(symbol) = self.1 { Status::Unstable(symbol) } else { Status::Forbidden }
+    }
+
     fn build_error(
         &self,
         ccx: &ConstCx<'_, 'tcx>,
@@ -345,7 +349,11 @@ impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
             .create_err(errors::UnstableConstFn { span, def_path: ccx.tcx.def_path_str(def_id) });
 
         if ccx.is_const_stable_const_fn() {
-            err.help("const-stable functions can only call other const-stable functions");
+            if self.1.is_some() {
+                bug!("this should be triggering the UnstableInStable lint instead");
+            } else {
+                err.help("const-stable functions can only call other const-stable functions");
+            }
         } else if ccx.tcx.sess.is_nightly_build() {
             if let Some(feature) = feature {
                 err.help(format!("add `#![feature({feature})]` to the crate attributes to enable"));

tests/ui/consts/min_const_fn/min_const_fn_libstd_stability.stderr

@@ -1,10 +1,19 @@
-error: `foo` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(foo)]`
   --> $DIR/min_const_fn_libstd_stability.rs:16:25
    |
 LL | const fn bar() -> u32 { foo() }
    |                         ^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn bar() -> u32 { foo() }
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(foo)]
+LL | const fn bar() -> u32 { foo() }
+   |
 
 error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_fn_libstd_stability.rs:24:26

tests/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.stderr

@@ -1,10 +1,19 @@
-error: `foo` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(foo)]`
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:16:41
    |
 LL | const unsafe fn bar() -> u32 { unsafe { foo() } }
    |                                         ^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const unsafe fn bar() -> u32 { unsafe { foo() } }
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(foo)]
+LL | const unsafe fn bar() -> u32 { unsafe { foo() } }
+   |
 
 error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:24:42

tests/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.stderr

@@ -1,10 +1,19 @@
-error: `foo` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(foo)]`
   --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:16:32
    |
 LL | const unsafe fn bar() -> u32 { foo() }
    |                                ^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const unsafe fn bar() -> u32 { foo() }
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(foo)]
+LL | const unsafe fn bar() -> u32 { foo() }
+   |
 
 error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:24:33

tests/ui/intrinsics/const-eval-select-stability.stderr

@@ -1,10 +1,19 @@
-error: `const_eval_select` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(const_eval_select)]`
   --> $DIR/const-eval-select-stability.rs:17:5
    |
 LL |     const_eval_select((), nothing, log);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | pub const unsafe fn hey() {
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(const_eval_select)]
+LL | pub const unsafe fn hey() {
+   |
 
 error: aborting due to previous error
 

tests/ui/rfcs/rfc-2632-const-trait-impl/staged-api.unstable.stderr

@@ -14,29 +14,56 @@ LL |     Foo::func();
    |
    = help: add `#![feature(foo)]` to the crate attributes to enable
 
-error: `<staged_api::Unstable as staged_api::MyTrait>::func` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(unstable)]`
   --> $DIR/staged-api.rs:55:5
    |
 LL |     Unstable::func();
    |     ^^^^^^^^^^^^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn stable_const_context() {
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn stable_const_context() {
+   |
 
-error: `<Foo as staged_api::MyTrait>::func` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(foo)]`
   --> $DIR/staged-api.rs:57:5
    |
 LL |     Foo::func();
    |     ^^^^^^^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn stable_const_context() {
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(foo)]
+LL | const fn stable_const_context() {
+   |
 
-error: `const_context_not_const_stable` is not yet stable as a const fn
+error: const-stable function cannot use `#[feature(foo)]`
   --> $DIR/staged-api.rs:59:5
    |
 LL |     const_context_not_const_stable()
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: const-stable functions can only call other const-stable functions
+help: if it is not part of the public API, make this function unstably const
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn stable_const_context() {
+   |
+help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
+   |
+LL + #[rustc_allow_const_fn_unstable(foo)]
+LL | const fn stable_const_context() {
+   |
 
 error: aborting due to 5 previous errors