[EXPERIMENT] Nullable From conversions

[clarfonthey]

After seeing rust-lang/libs-team#872, wanted to see if From impls for these would be possible without inference issues.

@rustbot label:S-experimental

r? ghost

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job test-pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- library/core/src/mem/mod.rs - mem::discriminant (line 1382) stdout ----
error[E0277]: the trait bound `*const _: From<&_doctest_main_library_core_src_mem_mod_rs_1382_0::Enum>` is not satisfied
    --> library/core/src/mem/mod.rs:1395:20
     |
1395 |         unsafe { *<*const _>::from(self).cast::<u8>() }
     |                    ^^^^^^^^ the trait `From<&_doctest_main_library_core_src_mem_mod_rs_1382_0::Enum>` is not implemented for `*const _`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
Couldn't compile the test.

[rust-bors]

☔ The latest upstream changes (presumably #129036) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.