This is an extremely marginal error, so the cost of properly threading
`Handler` everywhere just not seemed justified. However, it's useful
to panic when we create a file, and not when we slice strings with
overflown indexes somewhere in the guts of the compiler.

For this reason, while we provide safe `try_new_source_file`, we don't
change the existing public interface and just panic more or less
cleanly.

test more variants of enum-int-casting

As I learned in rust-lang#61673 (comment), there is a code path we are not testing yet. Looks like enum-int-casting with and without an intermediate let-binding is totally different.

EDIT: The reason for this is to get rid of the cycle in definitions such as:
```rust
enum Foo {
    A = 0,
    B = Foo::A as isize + 2,
}
```
This has historically been supported, so a hack adding special treatment to `Enum::Variant as _` was added to keep supporting it.

Replace some uses of NodeId with HirId

We are still using `NodeId` in some spots where we could use `HirId` instead; this PR targets some of these spots and removes some of the associated `hir::map` functions.

…ppe,RalfJung

Help LLVM better optimize slice::Iter(Mut)::len

r? @RalfJung

I've included a codegen test that fails without this change as a demonstration of usefulness.

make `Weak::ptr_eq`s into methods

This makes the `Weak::ptr_eq`s associated function into methods. There's no reason for methods on `Weak`s to be associated functions, as there is no `Dered` thus no possibility of a collision. Also: methods can be called using the associated function syntax.

follow up on rust-lang#55987
[Tracking issue for weak_ptr_eq](rust-lang#55981)

don't ICE on large files

This is an extremely marginal error, so the cost of properly threading
`Handler` everywhere just not seemed justified. However, it's useful
to panic when we create a file, and not when we slice strings with
overflown indexes somewhere in the guts of the compiler.

For this reason, while we provide safe `try_new_source_file`, we don't
change the existing public interface and just panic more or less
cleanly.

closes rust-lang#61904

r? @petrochenkov