bazel/wasm: upgrade wasmtime from 32.0.0 to 42.0.2

[travisdowns]

Motivation: make the Bazel build of //:redpanda byte-for-byte reproducible. #30187 is related to handles for hwloc, openssl via local rules_foreign_cc build tweaks, but for the wasmtime / cranelift side we had two local reproducibility patches that were already fixed upstream and only required a version bump to pick up. Per the discussion on #30187, the cleaner path is to drop the patches and upgrade wasmtime instead — which is what this PR does.

The minimum wasmtime version that carries both cranelift fixes is v42, so this is a jump from v32.0.0 to v42.0.2. The upgrade brings along the following required adjustments:

• Bump Rust toolchain from 1.86.0 to 1.91.0 (wasmtime 42 MSRV).
• Add PULLEY and ALL_ARCH feature flags to the wasmtime.BUILD conf.h template substitutions (new in wasmtime 42).
• Drop the wasmtime_config_async_support_set() call — async support is implicit now, the separate toggle was deprecated and removed from the C API in bytecodealliance/wasmtime#12371.
• Bump rules_rust 0.60.0 → 0.70.0 and opt in to experimental_use_allocator_libraries_with_mangled_symbols so the mangled __rustc::* allocator shim symbols (introduced in rustc 1.87+) are provided when a Rust staticlib is linked into a cc_binary via cc_common.link. Without this flag the link fails with undefined references to __rustc::__rust_alloc, __rustc::__rust_dealloc, etc. Background:
  • Issue: cc_common_link is broken in rust 1.87.0 bazelbuild/rules_rust#3459
  • Fix PR: add allocator libraries compatible with the new rustc mangling bazelbuild/rules_rust#3403
  • Landed in rules_rust 0.63.0 as an opt-in setting.
• Mirror the rustc tarball through vectorized-public to reduce dependence on static.rust-lang.org.
• Incidental: bump bazel_skylib 1.8.1 → 1.8.2 and rules_shell 0.4.1 → 0.6.1 to silence direct-dependency version warnings pulled in by the new rules_rust.

Supersedes #30250, which took the opposite approach of carrying a local rust_alloc_shim workaround. Bumping rules_rust and flipping the flag is cleaner than maintaining a custom shim — per the approach suggested by fhanau in rules_rust#3459.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

• none

[travisdowns]

@claude review

[Copilot]

Pull request overview

This PR upgrades the Bazel-managed Wasmtime/Cranelift dependency stack to Wasmtime v42.0.2 to eliminate local reproducibility patches and move toward byte-for-byte reproducible //:redpanda builds.

Changes:

• Bump Wasmtime crates (wasmtime and wasmtime-c-api-impl) from 32.0.0 → 42.0.2 and refresh Cargo.lock accordingly.
• Update Bazel integration for the new Wasmtime version (new conf.h feature-template substitutions; remove deprecated C API call).
• Update Bazel module/tooling dependencies (notably rules_rust), bump Rust toolchain version, and add allocator-symbol setting needed for newer rustc.

Reviewed changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/v/wasm/wasmtime.cc	Removes the deprecated wasmtime_config_async_support_set call to match Wasmtime v42 C API behavior.
bazel/thirdparty/wasmtime.BUILD	Extends conf.h template substitutions for new Wasmtime v42 feature macros.
bazel/thirdparty/Cargo.toml	Bumps Wasmtime crate versions to 42.0.2.
bazel/thirdparty/Cargo.lock	Updates the resolved Rust dependency graph for Wasmtime 42.0.2.
MODULE.bazel	Bumps rules_rust (and other Bazel deps), updates Rust toolchain version, and adds mirrored rustc URL fallback.
.bazelrc	Enables the rules_rust allocator-library setting required for rustc 1.87+ allocator shim symbol mangling.

[StephanDollberg]

Does this refer to whatever below does or how the rust CDN files work?

[travisdowns]

I'm not quite sure I got the question.

Do you refer to the "fall through"? That's a bazel feature. It will try the urls in order.

[travisdowns]

Regardless I think I should just remove this.

[travisdowns]

I removed this as I think it's more a "me" problem.

[vbotbuildovich]

CI test results

test results on build#83568

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	UpgradeBackToBackTest	test_upgrade_with_all_workloads	{"single_upgrade": false}	integration	https://buildkite.com/redpanda/redpanda/builds/83568#019dbaa1-4c68-488c-8db1-0903433de6f0	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0000, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=UpgradeBackToBackTest&test_method=test_upgrade_with_all_workloads

[dotnwat]

async support is now implicit