[beta-1.95] Add missing truncate when writing .crate files

[weihanglo]

Beta backports

• Add missing truncate when writing .crate files #16688

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⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against rust-1.95.0. Please double check that you specified the right target!

[weihanglo]

The failure was due to rust-lang/rust#152164, which worked correctly as ECHO_SCRIPT doesn't have any frontmatter.

thread 'script::cargo::test_no_build_rs' (9804054) panicked at tests/testsuite/script/cargo.rs:998:10:

---- expected: tests/testsuite/script/cargo.rs:991:27
++++ actual:   stderr
   1    1 | [WARNING] `package.edition` is unspecified, defaulting to the latest edition (currently `[..]`)
   2    2 | [COMPILING] script v0.0.0 ([ROOT]/foo/script.rs)
        3 + [WARNING] feature `frontmatter` is declared but not used
        4 +  --> <crate attribute>:1:12
        5 +   |
        6 + 1 | #![feature(frontmatter)]
        7 +   |            ^^^^^^^^^^^
        8 +   |
        9 +   = [NOTE] `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
       10 +
       11 + [WARNING] `script` (bin "script") generated 1 warning
   3   12 | [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
   4      - [RUNNING] `[ROOT]/home/.cargo/build/[HASH]/target/debug/script[EXE] --help`
       13 + [RUNNING] `[ROOT]/home/.cargo/build/[HASH]/target/debug/script --help`

Update with SNAPSHOTS=overwrite

A quick fix for our test suite can be adding empty frontmatter to ECHO_SCRIPT, but that won't help user's Cargo script.

Another quick fix is adding allow(unused_features) along with frontmatter. Since the stabilization FCP is complete, I guess we'll soon have no need of this feature.

[weihanglo]

BTW the fix should be ported to both master and rust-1.95.0 branch.

[ehuss]

Posted #16715 to update the tests.

[rustbot]

This PR was rebased onto a different rust-1.95.0 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.