Rollup merge of rust-lang#99449 - compiler-errors:assoc-const-missing-item, r=lcnr

Do not resolve associated const when there is no provided value

Fixes rust-lang#98629, since now we just delay a bug when we're not able to evaluate a const item due to the value not actually being provided by anything. This means compilation proceeds forward to where the "missing item in impl" error is emitted.

----

The root issue here is that when we're looking for the defining `LeafDef` in `resolve_associated_item`, we end up getting the trait's AssocItem instead of the impl's AssocItem (which does not exist). This resolution "succeeds" even if the trait's item has no default value, and then since this item has no value to evaluate, it turns into a const eval error.

This root issue becomes problematic (as in rust-lang#98629) when this const eval error happens in wfcheck (for example, due to normalizing the param-env of something that references this const). Since this happens sooner than the check that an impl actually provides all of the items that a trait requires (which happens during later typecheck), we end up aborting compilation early with only this un-informative message.

I'm not exactly sure _why_ this bug arises due to rust-lang#96591 -- perhaps valtrees are evaluated more eagerly than in the old system?

r? ``@oli-obk`` or ``@lcnr`` since y'all are familiar with const eval and reviewed rust-lang#96591, though feel free to reassign.

This is a regression from stable to beta, so I would be open to considering this for beta backport. It seems correct to me, especially given the improvements in the other UI tests this PR touches, but may have some side-effects that I'm unaware of...?

Author: Dylan-DPC

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`.