cargo xtask fuzz fails on aarch64 — LSE target feature dropped by cargo-fuzz RUSTFLAGS

cargo xtask fuzz run fails to compile on aarch64 (e.g. aarch64 WSL) because cargo-fuzz overrides .cargo/config.toml target features via its own RUSTFLAGS env var.

Symptoms

error: instruction requires: lse
 --> <inline asm>:2:1
  |
2 | casalb w21, w8, [x10]
  | ^

Root cause

.cargo/config.toml sets -Ctarget-feature=+lse,+neon for aarch64 targets. trycopy's inline asm depends on this — it uses casalb (an ARMv8.1 LSE atomic) for fault-recovering compare-and-swap.

cargo-fuzz constructs its own RUSTFLAGS (ASAN, coverage instrumentation) and sets it as an env var. Per Cargo's precedence rules, the env var completely overrides [target.*.rustflags] from config, silently dropping +lse. LLVM then rejects the LSE instructions.

Workaround

RUSTFLAGS="-Ctarget-feature=+lse,+neon" cargo xtask fuzz run fuzz_ide

cargo-fuzz prepends the user's RUSTFLAGS to its own, so the features are preserved.

Proposed fix

Detect aarch64 in xtask/src/tasks/fuzz/cargo_fuzz.rs and inject -Ctarget-feature=+lse,+neon into the environment before invoking cargo fuzz. This makes it automatic for all fuzz targets.

An alternative: make trycopy's LSE asm conditional on cfg!(target_feature = "lse") with an LL/SC fallback — but that's significantly more work.

Start in xtask/src/tasks/fuzz/cargo_fuzz.rs and trace how cargo xtask fuzz run invokes cargo-fuzz and constructs its environment. Reproduce the failure on an aarch64 target with cargo xtask fuzz run fuzz_ide, then verify that the LSE and NEON features are preserved and the fuzz target compiles successfully.