Rollup merge of rust-lang#81351 - lcnr:big-money-big-prices, r=oli-obk

combine: stop eagerly evaluating consts

`super_relate_consts` eagerly evaluates constants which doesn't seem too great.

I now also finally understand why all of the unused substs test passed. The reason being
that we just evaluated the constants in `super_relate_consts` 😆

While this change isn't strictly necessary as evaluating consts here doesn't hurt, it still feels a lot cleaner to do it this way

r? `@oli-obk` `@nikomatsakis`

Author: Dylan-DPC

compiler/rustc_infer/src/infer/combine.rs

@@ -543,10 +543,6 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         true
     }
 
-    fn visit_ct_substs(&self) -> bool {
-        true
-    }
-
     fn binders<T>(
         &mut self,
         a: ty::Binder<T>,
@@ -737,6 +733,16 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                     }
                 }
             }
+            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
+                if self.tcx().lazy_normalization() =>
+            {
+                assert_eq!(promoted, None);
+                let substs = self.relate_with_variance(ty::Variance::Invariant, substs, substs)?;
+                Ok(self.tcx().mk_const(ty::Const {
+                    ty: c.ty,
+                    val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
+                }))
+            }
             _ => relate::super_relate_consts(self, c, c),
         }
     }
@@ -822,10 +828,6 @@ impl TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         true
     }
 
-    fn visit_ct_substs(&self) -> bool {
-        true
-    }
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,
@@ -959,6 +961,16 @@ impl TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
                     }
                 }
             }
+            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
+                if self.tcx().lazy_normalization() =>
+            {
+                assert_eq!(promoted, None);
+                let substs = self.relate_with_variance(ty::Variance::Invariant, substs, substs)?;
+                Ok(self.tcx().mk_const(ty::Const {
+                    ty: c.ty,
+                    val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
+                }))
+            }
             _ => relate::super_relate_consts(self, c, c),
         }
     }

compiler/rustc_middle/src/ty/relate.rs

@@ -33,15 +33,6 @@ pub trait TypeRelation<'tcx>: Sized {
     /// relation. Just affects error messages.
     fn a_is_expected(&self) -> bool;
 
-    /// Whether we should look into the substs of unevaluated constants
-    /// even if `feature(const_evaluatable_checked)` is active.
-    ///
-    /// This is needed in `combine` to prevent accidentially creating
-    /// infinite types as we abuse `TypeRelation` to walk a type there.
-    fn visit_ct_substs(&self) -> bool {
-        false
-    }
-
     fn with_cause<F, R>(&mut self, _cause: Cause, f: F) -> R
     where
         F: FnOnce(&mut Self) -> R,
@@ -432,9 +423,9 @@ pub fn super_relate_tys<R: TypeRelation<'tcx>>(
                     let sz_a = sz_a.try_eval_usize(tcx, relation.param_env());
                     let sz_b = sz_b.try_eval_usize(tcx, relation.param_env());
                     match (sz_a, sz_b) {
-                        (Some(sz_a_val), Some(sz_b_val)) => Err(TypeError::FixedArraySize(
-                            expected_found(relation, sz_a_val, sz_b_val),
-                        )),
+                        (Some(sz_a_val), Some(sz_b_val)) if sz_a_val != sz_b_val => Err(
+                            TypeError::FixedArraySize(expected_found(relation, sz_a_val, sz_b_val)),
+                        ),
                         _ => Err(err),
                     }
                 }
@@ -532,7 +523,7 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
         }
 
         (ty::ConstKind::Unevaluated(au), ty::ConstKind::Unevaluated(bu))
-            if tcx.features().const_evaluatable_checked && !relation.visit_ct_substs() =>
+            if tcx.features().const_evaluatable_checked =>
         {
             tcx.try_unify_abstract_consts(((au.def, au.substs), (bu.def, bu.substs)))
         }

src/test/ui/const-generics/issues/issue-69654-run-pass.rs

@@ -1,4 +1,3 @@
-// run-pass
 #![feature(const_generics)]
 #![allow(incomplete_features, unused_braces)]
 
@@ -15,4 +14,5 @@ where
 
 fn main() {
     Foo::foo();
+    //~^ ERROR the function or associated item
 }

src/test/ui/const-generics/issues/issue-69654-run-pass.stderr

@@ -0,0 +1,15 @@
+error[E0599]: the function or associated item `foo` exists for struct `Foo<{_: usize}>`, but its trait bounds were not satisfied
+  --> $DIR/issue-69654-run-pass.rs:16:10
+   |
+LL | struct Foo<const N: usize> {}
+   | -------------------------- function or associated item `foo` not found for this
+...
+LL |     Foo::foo();
+   |          ^^^ function or associated item cannot be called on `Foo<{_: usize}>` due to unsatisfied trait bounds
+   |
+   = note: the following trait bounds were not satisfied:
+           `[u8; _]: Bar<[(); _]>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0599`.

src/test/ui/const-generics/issues/issue-69654.rs

@@ -15,4 +15,5 @@ where
 
 fn main() {
     Foo::foo();
+    //~^ ERROR the function or associated item
 }

src/test/ui/const-generics/issues/issue-69654.stderr

@@ -4,6 +4,19 @@ error[E0423]: expected value, found type parameter `T`
 LL | impl<T> Bar<T> for [u8; T] {}
    |                         ^ not a value
 
-error: aborting due to previous error
+error[E0599]: the function or associated item `foo` exists for struct `Foo<{_: usize}>`, but its trait bounds were not satisfied
+  --> $DIR/issue-69654.rs:17:10
+   |
+LL | struct Foo<const N: usize> {}
+   | -------------------------- function or associated item `foo` not found for this
+...
+LL |     Foo::foo();
+   |          ^^^ function or associated item cannot be called on `Foo<{_: usize}>` due to unsatisfied trait bounds
+   |
+   = note: the following trait bounds were not satisfied:
+           `[u8; _]: Bar<[(); _]>`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0423`.
+Some errors have detailed explanations: E0423, E0599.
+For more information about an error, try `rustc --explain E0423`.

src/test/ui/const-generics/occurs-check/unused-substs-1.rs

@@ -1,4 +1,3 @@
-// build-pass
 #![feature(const_generics)]
 #![allow(incomplete_features)]
 
@@ -10,5 +9,5 @@ where
     A<N>: Bar<N>;
 
 fn main() {
-    let _ = A;
+    let _ = A; //~ERROR the trait bound
 }

src/test/ui/const-generics/occurs-check/unused-substs-1.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `A<{_: usize}>: Bar<{_: usize}>` is not satisfied
+  --> $DIR/unused-substs-1.rs:12:13
+   |
+LL | / struct A<const N: usize>
+LL | | where
+LL | |     A<N>: Bar<N>;
+   | |_________________- required by `A`
+...
+LL |       let _ = A;
+   |               ^ the trait `Bar<{_: usize}>` is not implemented for `A<{_: usize}>`
+   |
+   = help: the following implementations were found:
+             <A<7_usize> as Bar<N>>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/const-generics/occurs-check/unused-substs-2.rs

@@ -1,4 +1,3 @@
-// check-pass
 #![feature(const_generics)]
 #![allow(incomplete_features)]
 
@@ -24,4 +23,6 @@ fn main() {
     // `t` is `ty::Infer(TyVar(_#1t))`
     // `foo` contains `ty::Infer(TyVar(_#1t))` in its substs
     t = foo;
+    //~^ ERROR mismatched types
+    //~| NOTE cyclic type
 }

src/test/ui/const-generics/occurs-check/unused-substs-2.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/unused-substs-2.rs:25:9
+   |
+LL |     t = foo;
+   |         ^^^ cyclic type of infinite size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.