fix(mip): dump --stacks; remove the dead HostPath build dep variant - #842

Fixes #838
Routing-Key: inbox-route/I_kwDOSUhdos8AAAABJPmOPg

Problem

crates/mip/src/cmd_dump.rs had two reachable todo!() macros that panicked at runtime on advertised flag combinations:

1. The output dispatch in cmd_dump() only handled DumpKind { packages: true, stacks: false }; mip dump --stacks (the required-group alternative to -p/--packages) fell through to todo!("output not implemented: {:?}", args.kind).
2. pkg_ref_from_input() matched only Build, Local, Subset, and Source, so a BuildDep::HostPath — the one remaining BuildDep variant — hit _ => todo!("bsi variant {:?}", i). Any package with a host-path build dep panicked the dump.

Fix

This PR is two commits:

1. fix(mip): dump --stacks and HostPath deps instead of panicking
Adds a --stacks output branch that collects stacks via graph.iter_stacks(), applies the same --name/--exact filter the packages path uses, and sorts by name (stacks live in a HashMap, so deterministic ordering matters). The filtering/sorting is factored into a small pure select_stacks helper — mirroring how #146 extracted resolve_target so cmd_dump internals stay unit-testable without a Context.

2. refactor(decode,graph,mip)!: remove the unused HostPath build dep variant
Rather than handle the HostPath panic, this removes the variant entirely. BuildDep::HostPath was #[allow(dead_code)] throughout, was never materialized into a build sandbox, and only ever reached a todo!() when dumped — aspirational, never-functional surface. Removing it:

• Drops the HostPath type and its BuildDep contract arm from the Nickel stdlib (and 'Path from Tys).
• Removes ObjTy::Path and from_term_hostpath from decode.
• Removes the BuildDep::HostPath variant and every match arm across graph (loader, spec_hasher, transitives, planner, graph, wire). The matches over BuildDep are now exhaustive without a wildcard, so a future variant fails to compile instead of reintroducing a runtime panic.
• Removes NodeData::HostPath and the --hostpath-deps flag from mip dep, and reverts the PkgRef::HostPath mapping added in commit 1.

The two golden-hash tests in graph::spec_hasher that used a HostPath build dep as filler now construct their spec in Rust instead of via Nickel, so the hash is deterministic across architectures and no longer needs the arch_hash! macro.

BREAKING CHANGE: the HostPath build-dep type is removed from the Nickel stdlib; a .ncl using | HostPath no longer parses. It was non-functional (never built; dumping it panicked), so no working configuration is affected.

Verification

All commands run from the workspace root:

• cargo fmt --all — clean, no drift.
• cargo clippy --workspace --all-targets -- -D warnings — exit 0, no warnings.
• cargo test --workspace — exit 0; every test binary passed. The rewritten attrs_hash / subset_hash golden-hash tests pass with single architecture-independent hashes.

Generated by inbox-patch · ◷