address review by @eddyb

crates/hir_ty/src/consts.rs

@@ -4,7 +4,8 @@
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum ConstScalar {
     // for now, we only support the trivial case of constant evaluating the length of an array
-    Usize(usize),
+    // Note that this is u64 because the target usize may be bigger than our usize
+    Usize(u64),
 
     /// Case of an unknown value that rustc might know but we don't
     Unknown,

crates/hir_ty/src/infer/expr.rs

@@ -746,7 +746,7 @@ impl<'a> InferenceContext<'a> {
                     ty: TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner),
                     value: ConstValue::Concrete(chalk_ir::ConcreteConst {
                         interned: len
-                            .map(|len| ConstScalar::Usize(len))
+                            .map(|len| ConstScalar::Usize(len as u64))
                             .unwrap_or(ConstScalar::Unknown),
                     }),
                 };