Do not resolve associated const when there is no provided value

Author: compiler-errors

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -185,14 +185,20 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
         }
         let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
         match concrete {
-            Err(ErrorHandled::TooGeneric) => Err(if !uv.has_infer_types_or_consts() {
+            Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
+                NotConstEvaluatable::MentionsInfer
+            } else if uv.has_param_types_or_consts() {
                 infcx
                     .tcx
                     .sess
                     .delay_span_bug(span, &format!("unexpected `TooGeneric` for {:?}", uv));
                 NotConstEvaluatable::MentionsParam
             } else {
-                NotConstEvaluatable::MentionsInfer
+                let guar = infcx.tcx.sess.delay_span_bug(
+                    span,
+                    format!("Missing value for constant, but no error reported?"),
+                );
+                NotConstEvaluatable::Error(guar)
             }),
             Err(ErrorHandled::Linted) => {
                 let reported = infcx
@@ -240,8 +246,11 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
 
             Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
                 NotConstEvaluatable::MentionsInfer
-                } else {
+                } else if uv.has_param_types_or_consts() {
                 NotConstEvaluatable::MentionsParam
+            } else {
+                let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
+                NotConstEvaluatable::Error(guar)
             }),
             Err(ErrorHandled::Linted) => {
                 let reported =

compiler/rustc_ty_utils/src/instance.rs

@@ -280,6 +280,11 @@ fn resolve_associated_item<'tcx>(
                 return Ok(None);
             }
 
+            // If the item does not have a value, then we cannot return an instance.
+            if !leaf_def.item.defaultness.has_value() {
+                return Ok(None);
+            }
+
             let substs = tcx.erase_regions(substs);
 
             // Check if we just resolved an associated `const` declaration from

src/test/ui/const-generics/issues/issue-86530.rs

@@ -15,7 +15,6 @@ where
 fn unit_literals() {
     z(" ");
     //~^ ERROR: the trait bound `&str: X` is not satisfied
-    //~| ERROR: unconstrained generic constant
 }
 
 fn main() {}

src/test/ui/const-generics/issues/issue-86530.stderr

@@ -15,22 +15,6 @@ LL | where
 LL |     T: X,
    |        ^ required by this bound in `z`
 
-error: unconstrained generic constant
-  --> $DIR/issue-86530.rs:16:5
-   |
-LL |     z(" ");
-   |     ^
-   |
-   = help: try adding a `where` bound using this expression: `where [(); T::Y]:`
-note: required by a bound in `z`
-  --> $DIR/issue-86530.rs:11:10
-   |
-LL | fn z<T>(t: T)
-   |    - required by a bound in this
-...
-LL |     [(); T::Y]: ,
-   |          ^^^^ required by this bound in `z`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/const-generics/issues/issue-98629.rs

@@ -0,0 +1,15 @@
+#![feature(const_trait_impl)]
+
+trait Trait {
+    const N: usize;
+}
+
+impl const Trait for i32 {}
+//~^ ERROR not all trait items implemented, missing: `N`
+
+fn f()
+where
+    [(); <i32 as Trait>::N]:,
+{}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-98629.stderr

@@ -0,0 +1,12 @@
+error[E0046]: not all trait items implemented, missing: `N`
+  --> $DIR/issue-98629.rs:7:1
+   |
+LL |     const N: usize;
+   |     -------------- `N` from trait
+...
+LL | impl const Trait for i32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ missing `N` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

src/test/ui/issues/issue-77919.rs

@@ -1,6 +1,5 @@
 fn main() {
     [1; <Multiply<Five, Five>>::VAL];
-    //~^ ERROR: constant expression depends on a generic parameter
 }
 trait TypeVal<T> {
     const VAL: T;

src/test/ui/issues/issue-77919.stderr

@@ -1,5 +1,5 @@
 error[E0412]: cannot find type `PhantomData` in this scope
-  --> $DIR/issue-77919.rs:10:9
+  --> $DIR/issue-77919.rs:9:9
    |
 LL |     _n: PhantomData,
    |         ^^^^^^^^^^^ not found in this scope
@@ -10,31 +10,23 @@ LL | use std::marker::PhantomData;
    |
 
 error[E0412]: cannot find type `VAL` in this scope
-  --> $DIR/issue-77919.rs:12:63
+  --> $DIR/issue-77919.rs:11:63
    |
 LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
    |          -                                                    ^^^ not found in this scope
    |          |
    |          help: you might be missing a type parameter: `, VAL`
 
 error[E0046]: not all trait items implemented, missing: `VAL`
-  --> $DIR/issue-77919.rs:12:1
+  --> $DIR/issue-77919.rs:11:1
    |
 LL |     const VAL: T;
    |     ------------ `VAL` from trait
 ...
 LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation
 
-error: constant expression depends on a generic parameter
-  --> $DIR/issue-77919.rs:2:9
-   |
-LL |     [1; <Multiply<Five, Five>>::VAL];
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: this may fail depending on what value the parameter takes
-
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0046, E0412.
 For more information about an error, try `rustc --explain E0046`.