[leynos]

Part of DF12-110

Brings wildside to both Rust build rules in one PR, because they edit the same configuration files: the dev-build footprint (concordat rust-dev-build-footprint, DF-001 to DF-003) and the build standard (rust-build-defaults, BD-001 to BD-009).

Changes

• .cargo/config.toml: [profile.dev] gets incremental = false and debug = 0; [unstable] embed-metadata = false (the native key for dated nightlies from 2026-07-31, which the pin is on); -Zthreads=8 in [build] and the Linux target table, -Clink-arg=-fuse-ld=mold in the Linux table only (Cargo selects one rustflags source, so each repeats the frontend flag).
• Makefile: GATE_RUST_FLAGS_ENV composes -D warnings, any inherited RUSTFLAGS and the standard flags into lint-architecture and test-rust; it refuses CARGO_BUILD_TARGET, because the linker flag is chosen from the host. RUST_FLAGS_ENV (-D warnings only) stays on the Whitaker recipes, which build their lint driver on another toolchain and must not inherit the flags.
• CI: the build job's setup-rust step installs mold and exports RUSTFLAGS as -D warnings -Zthreads=8 -Clink-arg=-fuse-ld=mold, and the Dependabot Rust tests step restates it. The mutation workflow installs mold and its setup-commands restores the same value for cargo-mutants, because setup-rust exports -D warnings alone there. The coverage jobs are unchanged and keep -D warnings alone, so a measurement stays on the platform linker.
• Cranelift is not adopted; the developers' guide records the exception (BD-004/BD-006). make test-rust on nightly-2026-10-28 with Cranelift selected for the dev and test profiles was stopped at 1,218 of 1,849 tests with 52 SIGABRTs (Cranelift-compiled panics find no unwind handler; for example domain::ports::cache_key::tests::coordinate_bound_is_inclusive) and domain::user::tests::new_panics_when_invalid_id failing; the suite passes on LLVM. The pin has since moved to nightly-2026-10-31; the guide says so and sets a re-measurement by 2026-11-30.
• tests/workflow_contracts/build_standard_test.py holds the footprint settings, the configuration sources, the Makefile composition (Linux and non-Linux, via make -n), the target refusal, the Whitaker boundary, the build job's inputs and the mutation workflow. mutation_testing_test.py follows the new inputs.

Evidence

• Before, on main 9206b8f: rust-dev-build-footprint noncompliant (DF-001, DF-002, DF-003) and rust-build-defaults noncompliant (BD-001, BD-002, BD-004, BD-008, BD-009). After, on this tree: concordat artefact rule run reports both compliant.
• Gates through gate-slot.sh: make check-fmt, make lint (on nightly-2026-10-31), make test, make markdownlint and make nixie; make lint-makefile, make lint-actions, make lint-python, make check-fmt-python, make typecheck-python and make test-workflow-contracts also pass. Results are appended below once the run completes.
• Expect the first CI run to be cold: the flags are part of the sccache key, so every crate misses once (about 87 minutes for build on ubicloud-standard-4, under its 120-minute ceiling).

Open PRs touching the same files: Dependabot PRs #535 and #526.

Summary by Sourcery

Adopt the repository-wide Rust build standard and development footprint across Cargo, Makefile gates, and CI.

Enhancements:

• Adopt the Rust development-build footprint by disabling incremental state and debug information while using Cargo-managed metadata embedding.
• Adopt the Rust build standard with parallel compilation everywhere and the mold linker on Linux, including consistent Makefile and CI flag propagation with cross-target safeguards.
• Document the build defaults, platform-specific behavior, cache impact, and the decision not to adopt Cranelift.

CI:

• Update build, Dependabot test, and mutation-testing workflows to install mold and apply the standard Rust flags where appropriate.

Documentation:

• Document the Rust build standard, development footprint, linker and flag boundaries, and Cranelift exception in the developers guide.

Tests:

• Add workflow contract coverage for Cargo settings, Makefile flag composition, target refusal, Whitaker isolation, and CI linker provisioning; update mutation workflow contracts.

Chores:

• Update typo-checker exceptions for the new linker and workflow identifiers.

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[sourcery-ai]

Reviewer's Guide

This PR adopts the Rust build standard and development-build footprint by centralizing Cargo defaults, composing equivalent flags through Makefile and CI paths where RUSTFLAGS overrides Cargo configuration, provisioning mold on Linux runners, documenting the Cranelift exception and operational trade-offs, and enforcing the behavior with workflow contract tests.

Sequence diagram for Makefile Rust gate flag composition

sequenceDiagram
    participant Developer
    participant Makefile
    participant Cargo
    participant Rustc

    Developer->>Makefile: make test-rust
    Makefile->>Makefile: STANDARD_TARGET_GUARD
    Makefile->>Makefile: GATE_RUST_FLAGS_ENV
    Makefile->>Cargo: RUSTFLAGS=-D warnings plus inherited flags and STANDARD_RUSTFLAGS
    Cargo->>Rustc: cargo nextest run
    Rustc->>Rustc: -Zthreads=8
    alt Linux host
        Rustc->>Rustc: -Clink-arg=-fuse-ld=mold
    end

Loading

Flowchart for Cargo Rust build defaults

flowchart TD
    Start["Cargo build"] --> Profile["dev profile: incremental=false, debug=0"]
    Profile --> Metadata["embed-metadata=false"]
    Metadata --> Frontend["-Zthreads=8"]
    Frontend --> Platform{"target_os = linux?"}
    Platform -->|Yes| Mold["Use mold linker"]
    Platform -->|No| Native["Use platform linker"]

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File-Level Changes

Change	Details	Files
Adopts the Rust development-build footprint and build-standard defaults in repository Cargo configuration.	Disables dev incremental compilation and debug information, and disables embedded metadata using Cargo’s native unstable setting. Enables eight-thread rustc frontend compilation for all builds and mold linking on Linux only, while documenting the Cranelift exception.	.cargo/config.toml
Propagates the standard Rust flags through local build gates and preserves toolchain-specific boundaries.	Adds host-aware flag composition for architecture lint and Rust tests, including inherited RUSTFLAGS and a cross-target refusal. Keeps Whitaker recipes on warnings-only flags.	Makefile
Configures CI and mutation testing runners to provide the Linux linker and complete build-standard flags.	Installs mold and exports or restates the parallel frontend and linker flags in the build and Dependabot test paths. Restores the flags in mutation testing while leaving coverage behavior unchanged.	.github/workflows/ci.yml .github/workflows/mutation-testing.yml tests/workflow_contracts/mutation_testing_test.py
Documents the adopted defaults, operational trade-offs, and the measured Cranelift exception.	Explains Cargo flag-source precedence, platform behavior, cache impact, debugging overrides, and cross-compilation handling. Records Cranelift test failures, the current nightly pin, and a re-measurement deadline.	docs/developers-guide.md
Adds workflow contract coverage for configuration, Makefile behavior, and CI provisioning.	Validates footprint settings, flag sources, Linux-only mold usage, inherited flags, target refusal, and Whitaker isolation. Checks build and mutation workflow inputs and exact RUSTFLAGS composition.	tests/workflow_contracts/build_standard_test.py

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