Rollup merge of #135425 - compiler-errors:not-conditionally-const, r=RalfJung

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

This will improve error messages as we continue to constify traits, since we don't want to start calling things "conditionally const" if they aren't implemented with a const impl anyways.

The only case that this affects today is `Deref` since that's one of the only constified traits in the standard library :)

r? RalfJung

Author: matthiaskrgr

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`.