Auto merge of #44275 - eddyb:deferred-ctfe, r=nikomatsakis

Evaluate fixed-length array length expressions lazily.

This is in preparation for polymorphic array lengths (aka `[T; T::A]`) and const generics.
We need deferred const-evaluation to break cycles when array types show up in positions which require knowing the array type to typeck the array length, e.g. the array type is in a `where` clause.

The final step - actually passing bounds in scope to array length expressions from the parent - is not done because it still produces cycles when *normalizing* `ParamEnv`s, and @nikomatsakis' in-progress lazy normalization work is needed to deal with that uniformly.

However, the changes here are still useful to unlock work on const generics, which @EpicatSupercell manifested interest in, and I might be mentoring them for that, but we need this baseline first.

r? @nikomatsakis cc @oli-obk

Author: bors

src/librustc/hir/mod.rs

@@ -611,7 +611,7 @@ pub enum BindingAnnotation {
   RefMut,
 }
 
-#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum RangeEnd {
     Included,
     Excluded,

src/librustc/ich/impls_mir.rs

@@ -493,10 +493,6 @@ impl<'a, 'gcx, 'tcx> HashStable<StableHashingContext<'a, 'gcx, 'tcx>> for mir::L
                                           hasher: &mut StableHasher<W>) {
         mem::discriminant(self).hash_stable(hcx, hasher);
         match *self {
-            mir::Literal::Item { def_id, substs } => {
-                def_id.hash_stable(hcx, hasher);
-                substs.hash_stable(hcx, hasher);
-            }
             mir::Literal::Value { ref value } => {
                 value.hash_stable(hcx, hasher);
             }