Rollup merge of #81309 - lcnr:lazy-norm-err-msgh, r=nikomatsakis

always eagerly eval consts in Relate

r? ```@nikomatsakis``` cc ```@varkor```

compiler/rustc_middle/src/ty/relate.rs

@@ -421,12 +421,14 @@ pub fn super_relate_tys<R: TypeRelation<'tcx>>(
             let t = relation.relate(a_t, b_t)?;
             match relation.relate(sz_a, sz_b) {
                 Ok(sz) => Ok(tcx.mk_ty(ty::Array(t, sz))),
-                // FIXME(#72219) Implement improved diagnostics for mismatched array
-                // length?
-                Err(err) if relation.tcx().lazy_normalization() => Err(err),
                 Err(err) => {
                     // Check whether the lengths are both concrete/known values,
                     // but are unequal, for better diagnostics.
+                    //
+                    // It might seem dubious to eagerly evaluate these constants here,
+                    // we however cannot end up with errors in `Relate` during both
+                    // `type_of` and `predicates_of`. This means that evaluating the
+                    // constants should not cause cycle errors here.
                     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) {

src/test/ui/const-generics/const-argument-cross-crate-mismatch.full.stderr

@@ -1,11 +1,11 @@
 error[E0308]: mismatched types
-  --> $DIR/const-argument-cross-crate-mismatch.rs:7:67
+  --> $DIR/const-argument-cross-crate-mismatch.rs:9:67
    |
 LL |     let _ = const_generic_lib::function(const_generic_lib::Struct([0u8, 1u8]));
    |                                                                   ^^^^^^^^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
 
 error[E0308]: mismatched types
-  --> $DIR/const-argument-cross-crate-mismatch.rs:9:65
+  --> $DIR/const-argument-cross-crate-mismatch.rs:11:65
    |
 LL |     let _: const_generic_lib::Alias = const_generic_lib::Struct([0u8, 1u8, 2u8]);
    |                                                                 ^^^^^^^^^^^^^^^ expected an array with a fixed size of 2 elements, found one with 3 elements

src/test/ui/const-generics/const-argument-cross-crate-mismatch.min.stderr

@@ -1,11 +1,11 @@
 error[E0308]: mismatched types
-  --> $DIR/const-argument-cross-crate-mismatch.rs:7:67
+  --> $DIR/const-argument-cross-crate-mismatch.rs:9:67
    |
 LL |     let _ = const_generic_lib::function(const_generic_lib::Struct([0u8, 1u8]));
    |                                                                   ^^^^^^^^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
 
 error[E0308]: mismatched types
-  --> $DIR/const-argument-cross-crate-mismatch.rs:9:65
+  --> $DIR/const-argument-cross-crate-mismatch.rs:11:65
    |
 LL |     let _: const_generic_lib::Alias = const_generic_lib::Struct([0u8, 1u8, 2u8]);
    |                                                                 ^^^^^^^^^^^^^^^ expected an array with a fixed size of 2 elements, found one with 3 elements

src/test/ui/const-generics/const-argument-cross-crate-mismatch.rs

@@ -1,5 +1,7 @@
 // aux-build:const_generic_lib.rs
 // revisions: full min
+#![cfg_attr(full, feature(const_generics))]
+#![cfg_attr(full, allow(incomplete_features))]
 
 extern crate const_generic_lib;