gitignore: allow target to be a symlink

Following rust-lang/cargo#4944.

Avoid tagging as I-nominated on toolstate breakage

r? @Mark-Simulacrum

Clean up E0458 explanation

r? @Dylan-DPC

Make x.py compatible with python 3.8.

Python 3.8 removes the `time.clock()` function, use `time.perf_counter()` instead.

Rename LayoutDetails to just Layout.

Following rust-lang/cargo#4944.

This reverts commit 46a8fcdf.

Rollup of 5 pull requests

Successful merges:

 - #70226 (use checked casts and arithmetic in Miri engine)
 - #70319 (correctly normalize constants)
 - #70352 (Add long error explanation for E0710 )
 - #70366 (Implement Fuse with Option)
 - #70379 (fix incorrect type name in doc comments)

Failed merges:

 - #70375 (avoid catching InterpError)

r? @ghost

fix incorrect type name in doc comments

Change : `InterpCtx` => `InterpCx`
(`rustc_mir::interpret::InterpCx`)

Implement Fuse with Option

The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.

The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.

r? @scottmcm

Add long error explanation for E0710

Add long explanation for the E0710 error code
Part of #61137

r? @GuillaumeGomez

correctly normalize constants

closes #70317

implements https://github.com/rust-lang/rust/issues/70125#issuecomment-602133708

r? eddyb cc @varkor

use checked casts and arithmetic in Miri engine

This is unfortunately pretty annoying because we have to cast back and forth between `u64` and `usize` more often that should be necessary, and that cast is considered fallible.

For example, should [this](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/value/enum.ConstValue.html) really be `usize`?

Also, `LayoutDetails` uses `usize` for field indices, but in Miri we use `u64` to be able to also handle array indexing. Maybe methods like `mplace_field` should be suitably generalized to accept both `u64` and `usize`?

r? @oli-obk Cc @EddyB

Python 3.8 removes the time.clock() function, use time.perf_counter() instead.

add helper method for ptr ops on Scalar; reduce unnecessary large operand of overflowing_signed_offset

Clean up debugging options

I found various sub-optimal things when I was looking at option handling.

Rollup of 7 pull requests

Successful merges:

 - #70331 (Increase verbosity when using update syntax with private fields)
 - #70349 (move `hir_id_validation` to `rustc_passes` + simplify `hir::map` code)
 - #70361 (Update backtrace crate to 0.3.46)
 - #70364 (resolve: Remove `rustc_attrs` as a standalone feature gate)
 - #70369 (Fix smaller issues with invalid placeholder type errors)
 - #70373 (normalize some imports & prefer direct ones)
 - #70376 (Add test for #66312)

Failed merges:

r? @ghost

Add test for #66312

Closes #66312. This issue was fixed by #68884.

r? @Zoxc

normalize some imports & prefer direct ones

r? @Mark-Simulacrum

Fix smaller issues with invalid placeholder type errors

Follow up to #70294.

- Fix placement of suggested generic param when bounds are present.
- Reduce error duplication for invalid placeholder types in `fn` types.

r? @Centril

resolve: Remove `rustc_attrs` as a standalone feature gate

Now it only gates specific built-in attributes.

So if you want to make a rustc attribute, make it a built-in (this was already the case in practice for some time).

Update backtrace crate to 0.3.46

* Support line-tables-only when using libbacktrace
* Update libbacktrace to latest master
* Define HAVE_KERN_PROC on FreeBSD to fix rust-lang/rust#54434

move `hir_id_validation` to `rustc_passes` + simplify `hir::map` code

r? @Zoxc

Increase verbosity when using update syntax with private fields

Resolves #70323

InterpCtx => InterpCx 
(rustc_mir::interpret::InterpCx)