[SRU] Upgrade Rust to 1.76.0

[Zixing Liu]

[ Impact ]

* Due to blocking archive work in the 24.04 cycle, Rust 1.76 was not uploaded in time. However, web browsers and kernels may need a Rust version that is newer than 1.75 in the near future.
* Users anticipate a newer Rust release in 24.04, as the version was released two months before Noble's release.

[ Test Plan ]

* A Rust ecosystem rebuild was done in https://launchpad.net/~liushuyu-011/+archive/ubuntu/rust-updates-1.76-wrb-2/+packages to ensure compatibility.

[ Where problems could occur ]

* Rust libraries/applications using Rust internal interfaces not part of the Rust standard library public API will fail to build.
* Rust libraries/applications exploiting previous Rust soundness checking deficiencies will fail to build.
* Rust libraries/applications using nightly features may fail to build.

[ Other Info ]

* Upstream release note:
-----------------------------------------------------------------------------
Upgrade Rust to 1.76.0

Notable changes in 1.76:

* `char` and `u32` now have the same ABI size
* `std::any::type_name_of_val` is now stable and can be used to print out the type name

https://blog.rust-lang.org/2024/02/08/Rust-1.76.0.html

[Zixing Liu (liushuyu-011)]

Preliminary packages have been uploaded to https://launchpad.net/~liushuyu-011/+archive/ubuntu/rust-updates-1.76/+packages.
The complete corresponding source can be found at https://git.launchpad.net/~canonical-foundations/ubuntu/+source/rustc/tree/?h=merge-1.76.

--------
Lintian report:
E: rustc source: field-too-long Vendored-Sources-Rust (11622 chars > 5000)
W: rustc source: mismatched-override missing-license-paragraph-in-dep5-copyright debian/copyright mpl-2.0+ [debian/source/lintian-overrides:16]
W: rustc source: missing-license-paragraph-in-dep5-copyright mpl-2.0+ [debian/copyright:2333]
W: rustc source: unknown-field Vendored-Sources-Rust
N: 0 hints overridden; 1 unused override
N: Some overrides were ignored.
N: Use --verbose for more information.
Finished running lintian.
-------

Autopkgtest reports:

https://autopkgtest.ubuntu.com/results/autopkgtest-noble-liushuyu-011-rust-updates-1.76/?format=plain

[Zixing Liu (liushuyu-011)]

Converted to FFe bug since it is impossible to get Rust 1.76 uploaded before feature freeze due to armhf time_t transition.

[Steve Langasek (vorlon)]

Is the intention to make rust 1.76.0 the default rustc in noble (== 'rustc'), or to introduce it as an additional rustc-1.76 package?

If the former, what rebuild testing has been done on the reverse-dependencies, to judge the impact on releasability of noble?

[Zixing Liu (liushuyu-011)]

> Is the intention to make rust 1.76.0 the default rustc in noble (== 'rustc'), or to introduce it as an additional rustc-1.76 package?

It will be made default, and then LP: #2054748 will transition this package to rustc-1.76 instead of rustc.

> what rebuild testing has been done on the reverse-dependencies

Performing rebuild testing is currently very difficult because many micro-packages synchronized from Debian haven't yet formed a stable dependency tree. Many test failures suggest incomplete sync or incomplete work from Debian.