Scheduled to be released on 2021-10-21.

https://internals.rust-lang.org/t/rust-1-56-0-pre-release-testing/15459
https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1560-2021-10-21

Including changes to USES=cargo for https://bugs.freebsd.org/256581:

Uses/cargo: Rework git source support based on patch-in-config sections

Git sources from Cargo.lock are added to CARGO_CRATES through
the normal mechanism of make cargo-crates by the porter. They
are used to populate MASTER_SITES, DISTFILES with static
git-archive(1) tarballs a la USE_GITHUB, USE_GITLAB. In the
configure phase we generate [patch] sections in the config file
which will cause cargo update to auto-update Cargo.lock to point
to the appropriate extraction directories.

Normally cargo update would connect to the network to update all
Git sources but since rust-1.55.0 our cargo has been patched to
skip this when CARGO_FREEBSD_PORTS_SKIP_GIT_UPDATE is set in the
environment.

This replaces the old CARGO_USE_GITHUB, CARGO_USE_GITLAB hacks
where this was done by editing all Cargo.toml with sed(1) calls.

Additionally, we try to automatically infer the individiual crate
sub-directories inside the Git sources based on package.name in
Cargo.toml to remove the need for CARGO_GIT_SUBDIR.

USES=cargo also now sets WRKSRC_crate_$name for each crate to
point to the crate extraction directories.

poudriere 13.0 amd64 ok
ref13-aarch64 ok
ref13-i386 ok
ref13-ppc64 ok
ref12-amd64 ok
13.0 powerpc ok
13.0 powerpc64le ok
12.2 aarch64 ok
12.2 armv7 ok
12.2 armv6 fails to build, the problem with DOCS is back (armv7 is not affected)
Exp-run: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259251