Rollup merge of rust-lang#54346 - eddyb:constant-horror, r=nikomatsakis

rustc: future-proof error reporting for polymorphic constants in types.

Currently, we have 3 categories of positions where a constant can be used (`const` and associated `const` can be considered "aliases" for an expression):
* runtime - if the function is polymorphic, we could even just warn and emit a panic
* `static` - always monomorphic, so we can error at definition site
* type-system - **must** *enforce* evaluation success where it was written

That last one is the tricky one, because we can't easily turn *the presence* a type with an erroring const into a runtime panic, and we'd have to do post-monomorphization errors (which we'd rather avoid).

<hr/>

The solution we came up with, as part of the plans for const generics, is to require successful evaluation wherever a constant shows up in a type (currently in array lengths, and values for const parameters in the future), *through* the WF system, which means that in certain situations (e.g. function signatures) we can assume evaluation *will* succeed, and require it of users (e.g. callers) instead (we've been doing this for lifetime bounds, for a long time now, and it's pretty ergonomic).

So once we do sth about rust-lang#43408, this example *should* work, by propagating the responsability, to callers of `foo::<X>`, of proving `std::mem::size_of::<X>()` succeeds (and those callers can do the same).
```rust
pub fn foo<T>(_: [u8; std::mem::size_of::<T>()]) {}
```
But this one *shouldn't*, as there is nothing in the signature/bounds to indicate it:
```rust
pub fn bar<T>() {
    let _: [u8; std::mem::size_of::<T>()];
}
```

<hr/>

I've come across some bit of code in the compiler that ignores const-evaluation errors *even when* they come from a constant in a type, and I've added an ICE *only when* there are no other reported errors (e.g. it's fine to ignore evaluation errors if the constant doesn't even type-check).

r? @nikomatsakis cc @oli-obk @RalfJung @Centril

src/librustc/traits/error_reporting.rs

@@ -839,7 +839,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     "could not evaluate constant expression",
                 ) {
                     Some(err) => err,
-                    None => return,
+                    None => {
+                        self.tcx.sess.delay_span_bug(span,
+                            &format!("constant in type had an ignored error: {:?}", err));
+                        return;
+                    }
                 }
             }