When the iterator nth() method was updated it was not in zip.
Zip needs to implement nth() for the trusted length specialised implementation.

Follow up to #80017.

Change filesearch::get_or_default_sysroot() to check if sysroot is found
using env::args().next() if rustc in argv[0] is a symlink; otherwise, or
if it is not found, use env::current_exe() to imply sysroot. This makes
the rustc binary able to locate Rust libraries in systems using
content-addressable storage (CAS).

Reverts PR #80830
Fixes taiki-e/pin-project#312

We can have an arbitrary number of `None`-delimited group frames pushed
on the stack due to proc-macro invocations, which can legally be exited.
Attempting to account for this would add a lot of complexity for a tiny
performance gain, so let's just use the original strategy.

- No Derefs in move closure, this will result in value behind a reference getting moved.
- No projections are applied to raw pointers, since these require unsafe blocks. We capture
  them completely.

Motivations for these are recorded here: https://hackmd.io/71qq-IOpTNqzMkPpAI1dVg?view

When `capture_disjoint_fields` is not enabled, checking if the root variable
binding is mutable would suffice.

However with the feature enabled, the captured place might be mutable
because it dereferences a mutable reference.

This PR computes the mutability of each capture after capture analysis
in rustc_typeck. We store this in `ty::CapturedPlace` and then use
`ty::CapturedPlace::mutability` in mir_build and borrow_check.

Fixes #81548.

The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.

refactor: removing custom nth Zip fn

Noticed `super_nth()` that seems very similar to `nth()` in `iterator.rs`.

If you look at `nth()` in `iterator.rs` before the commit `ecacc7534b6bf50205c37c89402565b82d95a257`  `super_nth()` looks exactly the same as `fn nth()` in `iterator.rs`.

I may be misunderstanding something, but I think `super_nth()` can just be removed.

Fix rustc sysroot in systems using CAS

Change filesearch::get_or_default_sysroot() to check if sysroot is found using env::args().next() if rustc in argv[0] is a symlink; otherwise, or if it is not found, use env::current_exe() to imply sysroot. This makes the rustc binary able to locate Rust libraries in systems using content-addressable storage (CAS).

2229: Fix issues with move closures and mutability

This PR fixes two issues when feature `capture_disjoint_fields` is used.

1. Can't mutate using a mutable reference
2. Move closures try to move value out through a reference.

To do so, we
1. Compute the mutability of the capture and store it as part of the `CapturedPlace`  that is written in TypeckResults
2. Restrict capture precision. Note this is temporary for now, to allow the feature to be used with move closures and ByValue captures and might change depending on discussions with the lang team.
    - No Derefs are captured for ByValue captures, since that will result in value behind a reference getting moved.
    - No projections are applied to raw pointers since these require unsafe blocks. We capture
    them completely.

r? ``@nikomatsakis``

Add Box::downcast() for dyn Any + Send + Sync

Looks like a plain omission, but unfortunately I just needed that in my code :)

Account for existing `_` field pattern when suggesting `..`

Follow up to #80017.

…enkov

Clone entire `TokenCursor` when collecting tokens

Reverts PR #80830
Fixes taiki-e/pin-project#312

We can have an arbitrary number of `None`-delimited group frames pushed
on the stack due to proc-macro invocations, which can legally be exited.
Attempting to account for this would add a lot of complexity for a tiny
performance gain, so let's just use the original strategy.

Optimize decimal formatting of 128-bit integers

## Description

This PR optimizes the `udivmod_1e19` function, which is used for formatting 128-bit integers, based on the algorithm provided in \[1\]. This optimization improves performance of formatting 128-bit integers, especially on 64-bit architectures. It also slightly reduces the output binary size.

## Assembler comparison

https://godbolt.org/z/YrG5zY

## Performance

#### previous results

```
test fmt::write_u128_max                                        ... bench:         552 ns/iter (+/- 4)
test fmt::write_u128_min                                        ... bench:         125 ns/iter (+/- 2)
```

#### new results

```
test fmt::write_u128_max                                        ... bench:         205 ns/iter (+/- 13)
test fmt::write_u128_min                                        ... bench:         129 ns/iter (+/- 5)
```

## Reference

\[1\] T. Granlund and P. Montgomery, “Division by Invariant Integers Using Multiplication” in Proc. of the SIGPLAN94 Conference on Programming Language Design and Implementation, 1994, pp. 61–72

…omez

Balance sidebar `Deref` cycle check with main content

The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.

Fixes #81395

r? ``@GuillaumeGomez``

…-schievink

Replace predecessor with range in collections documentation

Fixes #81548.