Do not consider traits that have unsatisfied const conditions to be conditionally const

Author: compiler-errors

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -35,6 +35,12 @@ use crate::errors;
 type QualifResults<'mir, 'tcx, Q> =
     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum ConstConditionsHold {
+    Yes,
+    No,
+}
+
 #[derive(Default)]
 pub(crate) struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
@@ -376,15 +382,15 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         callee: DefId,
         callee_args: ty::GenericArgsRef<'tcx>,
         call_span: Span,
-    ) -> bool {
+    ) -> Option<ConstConditionsHold> {
         let tcx = self.tcx;
         if !tcx.is_conditionally_const(callee) {
-            return false;
+            return None;
         }
 
         let const_conditions = tcx.const_conditions(callee).instantiate(tcx, callee_args);
         if const_conditions.is_empty() {
-            return false;
+            return None;
         }
 
         let (infcx, param_env) = tcx.infer_ctxt().build_with_typing_env(self.body.typing_env(tcx));
@@ -413,12 +419,13 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         }));
 
         let errors = ocx.select_all_or_error();
-        if !errors.is_empty() {
+        if errors.is_empty() {
+            Some(ConstConditionsHold::Yes)
+        } else {
             tcx.dcx()
                 .span_delayed_bug(call_span, "this should have reported a ~const error in HIR");
+            Some(ConstConditionsHold::No)
         }
-
-        true
     }
 
     pub fn check_drop_terminator(
@@ -706,7 +713,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     trace!("attempting to call a trait method");
                     let trait_is_const = tcx.is_const_trait(trait_did);
 
-                    if trait_is_const {
+                    // Only consider a trait to be const if the const conditions hold.
+                    // Otherwise, it's really misleading to call something "conditionally"
+                    // const when it's very obviously not conditionally const.
+                    if trait_is_const && has_const_conditions == Some(ConstConditionsHold::Yes) {
                         // Trait calls are always conditionally-const.
                         self.check_op(ops::ConditionallyConstCall {
                             callee,
@@ -730,7 +740,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
 
                 // Even if we know the callee, ensure we can use conditionally-const calls.
-                if has_const_conditions {
+                if has_const_conditions.is_some() {
                     self.check_op(ops::ConditionallyConstCall {
                         callee,
                         args: fn_args,

tests/ui/issues/issue-25901.rs

@@ -2,7 +2,7 @@ struct A;
 struct B;
 
 static S: &'static B = &A;
-//~^ ERROR cannot perform conditionally-const deref coercion
+//~^ ERROR cannot perform non-const deref coercion
 
 use std::ops::Deref;
 

tests/ui/issues/issue-25901.stderr

@@ -1,4 +1,4 @@
-error[E0658]: cannot perform conditionally-const deref coercion on `A` in statics
+error[E0015]: cannot perform non-const deref coercion on `A` in statics
   --> $DIR/issue-25901.rs:4:24
    |
 LL | static S: &'static B = &A;
@@ -10,11 +10,14 @@ note: deref defined here
    |
 LL |     type Target = B;
    |     ^^^^^^^^^^^
+note: impl defined here, but it is not `const`
+  --> $DIR/issue-25901.rs:9:1
+   |
+LL | impl Deref for A {
+   | ^^^^^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
-   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
-   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0015`.

tests/ui/self/arbitrary-self-from-method-substs-ice.rs

@@ -11,7 +11,7 @@ impl Foo {
         //~^ ERROR invalid generic `self` parameter type
         //~| ERROR destructor of `R` cannot be evaluated at compile-time
         self.0
-        //~^ ERROR cannot perform conditionally-const deref coercion on `R` in constant functions
+        //~^ ERROR cannot perform non-const deref coercion on `R` in constant functions
     }
 }
 

tests/ui/self/arbitrary-self-from-method-substs-ice.stderr

@@ -1,14 +1,11 @@
-error[E0658]: cannot perform conditionally-const deref coercion on `R` in constant functions
+error[E0015]: cannot perform non-const deref coercion on `R` in constant functions
   --> $DIR/arbitrary-self-from-method-substs-ice.rs:13:9
    |
 LL |         self.0
    |         ^^^^^^
    |
    = note: attempting to deref into `Foo`
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
-   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0493]: destructor of `R` cannot be evaluated at compile-time
   --> $DIR/arbitrary-self-from-method-substs-ice.rs:10:43
@@ -30,5 +27,5 @@ LL |     const fn get<R: Deref<Target = Self>>(self: R) -> u32 {
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0493, E0658, E0801.
-For more information about an error, try `rustc --explain E0493`.
+Some errors have detailed explanations: E0015, E0493, E0801.
+For more information about an error, try `rustc --explain E0015`.

tests/ui/traits/const-traits/cross-crate.stocknc.stderr

@@ -1,13 +1,10 @@
-error[E0658]: cannot call conditionally-const method `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
+error[E0015]: cannot call non-const method `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:19:14
    |
 LL |     NonConst.func();
    |              ^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
-   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: cannot call conditionally-const method `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:22:11
@@ -22,4 +19,5 @@ LL |     Const.func();
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0658`.
+Some errors have detailed explanations: E0015, E0658.
+For more information about an error, try `rustc --explain E0015`.