cargo check --workspace fails on the pinned stable toolchain (nightly-only x86_64 default feature + bootloader build script in the hv1 crates)

[wernerkasselman-au]

Summary

On a clean clone against the toolchain the repo itself pins
(rust-toolchain.toml -> channel = "stable", rust-version = "1.95"),
cargo check --workspace fails for me. Two dependencies of the Type-1
(hv1-*) crates require nightly and abort the check on a stable x86_64 host.
The rest of the workspace (all hv2-* crates plus hm-cli/hm-gui) compiles
cleanly once these two are addressed.

Environment

• rustc 1.95.0 stable, cargo 1.95.0 (matches rust-toolchain.toml and the
  workspace rust-version = "1.95")
• Host: x86_64 Linux
• protoc 35.1 present (so the hv2-api build is not the blocker)
• Current master (Cargo.lock: x86_64 v0.15.4, bootloader v0.11.15)

Reproduction

$ cargo check --workspace --all-targets

Fails with exit code 101. A minimal single-crate check reproduces the primary
blocker on its own:

$ cargo check -p hv1-core

Root cause 1: x86_64 = "0.15" enables the nightly abi_x86_interrupt gate

crates/hv1-core/Cargo.toml (and crates/hv1-boot/Cargo.toml) depend on
x86_64 = "0.15" with default features. In x86_64 0.15.4,
default = ["nightly", "instructions"] and nightly pulls in
abi_x86_interrupt, which activates a nightly-only feature gate:

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> x86_64-0.15.4/src/lib.rs:5
5 | #![cfg_attr(feature = "abi_x86_interrupt", feature(abi_x86_interrupt))]
error: could not compile `x86_64` (lib) due to 1 previous error

This hits the library build of hv1-core, so plain cargo check --workspace (not just --all-targets) fails. The x86_64 dependency is
target-gated to cfg(target_arch = "x86_64"), which does not help on an
x86_64 host: the gate is true, so the nightly-requiring dependency is pulled
in as normal.

Root cause 2: bootloader v0.11 build script uses nightly-only -Z flags

crates/hv1-boot has bootloader = "0.11" as a build-dependency. Its build
script invokes cargo with -Z flags, which stable cargo rejects:

error: failed to run custom build command for `bootloader v0.11.15`
  error: the `-Z` flag is only accepted on the nightly channel of Cargo, but
         this is the `stable` channel

This surfaces when --all-targets pulls in the hv1-boot bootable-image
target. Building an actual boot image is inherently a nightly task, so this
one is arguably by design; see the suggestion below.

Impact

• A plain workspace check against the pinned toolchain does not succeed out of
  the box. New contributors following the repo's own toolchain hit a hard
  error on first cargo check.
• Any stable CI job that runs a full cargo check --workspace on x86_64 would
  hit the same failure (target-gating does not skip the dep on an x86_64
  host).

Suggested minimal fix

Drop the x86_64 crate's nightly default feature and keep instructions (the
CPU-intrinsics / MSR-access feature the hv1 call sites I checked appear to
need; if you rely on the abi_x86_interrupt interrupt-handler surface
elsewhere you will want to keep that feature too):

# crates/hv1-core/Cargo.toml  and  crates/hv1-boot/Cargo.toml
- x86_64 = "0.15"
+ x86_64 = { version = "0.15", default-features = false, features = ["instructions"] }

For hv1-boot, since it is a genuinely nightly bootable-image crate, the
cleanest option is to keep it out of the default stable workspace build rather
than force it onto stable. Options, in order of least surprise (maintainers'
call):

1. Document cargo +nightly build -p hv1-boot and exclude it from the stable
   cargo check --workspace lane in CI, or
2. Move hv1-boot to exclude = [...] in the workspace with a
   # nightly-only bootimage note, or
3. Keep it a member but add a nightly CI job for it specifically.

Minor: crates/hv1-boot/Cargo.toml carries edition = "2024" # Requires nightly Rust. Edition 2024 has been stable since Rust 1.85, so that comment
is a bit out of date; the real nightly requirement here is the bootloader
build script.

Verification

With the x86_64 change applied and hv1-boot excluded:

$ cargo check --workspace --exclude hv1-boot --all-targets
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 10.67s

Finishes in about 11s on my machine (10.67s wall clock here), exit 0, zero
errors. All 13 remaining workspace crates compile on stable 1.95.0, including
hv1-core and hv1-arm (one pre-existing unused-Result warning at
crates/hv1-core/src/vm.rs:724, unrelated to this change).

Happy to open a PR for the x86_64 feature change once you have a preference
on how to handle hv1-boot (1 / 2 / 3 above), or to adjust it to match
whatever you prefer.