Constants are printed as `_`, even when their value is known

[varkor]

error[E0308]: mismatched types
  --> $DIR/const-argument-cross-crate-mismatch.rs:8:39
   |
LL |     let _: const_generic_lib::Alias = const_generic_lib::Struct([0u8, 1u8, 2u8]);
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `2usize`, found `3usize`
   |
   = note: expected type `const_generic_lib::Struct<2usize>`
              found type `const_generic_lib::Struct<_: usize>`

Seen in https://github.com/rust-lang/rust/pull/65365/files#diff-5f3259ccaa0ac2f4805ec7379527c987.