rustc: include proc-macro dep transitive rlibs in proc-macro link inputs

[walter-zeromatter]

Problem

When building a proc-macro target, rustc passes all transitively-reachable rlibs to the linker (clang++). This includes rlibs that arrive through proc-macro dependency chains — for example, if proc-macro A depends on proc-macro B which depends on rlib chrono, rustc's linker invocation for A.so includes chrono.rlib.

collect_deps in rustc.bzl skips dep_info.transitive_crate_outputs for proc-macro deps (the if not is_proc_macro guard at line 258). This is correct for non-proc-macro callers — they don't need those rlibs. But it means proc-macro targets don't declare those rlibs as Bazel action inputs.

Under --remote_download_outputs=minimal (remote execution with minimal downloads), Bazel only downloads declared action inputs. Undeclared rlibs are absent from the sandbox, causing:

clang++: error: no such file or directory: 'bazel-out/.../libchrono-2319700776.rlib'

Fix

Add a transitive_proc_macro_dep_outputs field to DepInfo that captures the transitive rlib outputs of proc-macro deps (exactly what the if not is_proc_macro guard was skipping). In collect_inputs, when the target being built is itself a proc-macro, merge this depset into the action inputs.

This is targeted: non-proc-macro callers are unaffected (no change to transitive_crate_outputs semantics for them), and the new inputs are only added for proc-macro link actions where they're actually needed.

Changes

• rust/private/providers.bzl: new transitive_proc_macro_dep_outputs field on DepInfo
• rust/private/rustc.bzl (collect_deps): for proc-macro deps, accumulate their transitive_crate_outputs into the new field
• rust/private/rustc.bzl (collect_inputs): for proc-macro targets, include transitive_proc_macro_dep_outputs in the action inputs depset

🤖 Generated with Claude Code

[yeswalrus]

Pushed follow-up 729e90b38 for the nested proc-macro case: proc-macro deps now carry their own transitive_proc_macro_dep_outputs, and there is a regression test for a two-hop proc-macro chain.

Validated with buildifier, targeted proc-macro/transitive-crate-output Bazel tests, bazel build //test/unit/proc_macro:nested_outer_macro, and git diff --check.