On illumos and Solaris systems, Rust will use GCC as the link editor.
Rust does this by invoking "cc", which on many (Linux and perhaps BSD)
systems is generally either GCC or a GCC-compatible front-end.  On
historical Solaris systems, "cc" was often the Sun Studio compiler.
This history casts a long shadow, and as such, even most modern
illumos-based operating systems tend to install GCC as "gcc", without
also making it available as "cc".

We should invoke GCC as "gcc" on such systems to ensure we get the right
compiler driver.

- Added `Iterator::fold_first`, which is like `fold`, but uses the first element in the iterator as the initial accumulator
- Includes doc and doctest
- Rebase commit; see rust-lang#65222 for details

Co-Authored-By: Tim Vermeulen <tvermeulen@me.com>

Proposal: `fold_self` and `try_fold_self` for Iterators

This pull request proposes & implements two new methods on Iterators: `fold_self` and `try_fold_self`. These are variants of `fold` and `try_fold` that use the first element in the iterator as the initial accumulator.

Let me know if a public feature like this requires an RFC, or if this pull request is sufficient as place for discussion.

…an-DPC

clean up E0404 explanation

r? @Dylan-DPC

use "gcc" instead of "cc" on *-sun-solaris systems when linking

On illumos and Solaris systems, Rust will use GCC as the link editor.
Rust does this by invoking "cc", which on many (Linux and perhaps BSD)
systems is generally either GCC or a GCC-compatible front-end.  On
historical Solaris systems, "cc" was often the Sun Studio compiler.
This history casts a long shadow, and as such, even most modern
illumos-based operating systems tend to install GCC as "gcc", without
also making it available as "cc".

We should invoke GCC as "gcc" on such systems to ensure we get the right
compiler driver.

…an-DPC

Clean up E0463 explanation

r? @Dylan-DPC