feat(rust): scaffold Cargo workspace and Rust CI (#1263)

Introduces the Rust toolchain skeleton for the upcoming port. Ships
nothing functional — the binary prints an identification line and
exits 0. The value of this commit is the groundwork: a reviewable
workspace layout, lint/test/release pipeline wired into CI, and a
clear toolchain policy so subsequent phases have a stable foundation.

## Layout

```
Cargo.toml              # workspace root
rust-toolchain.toml     # tracks latest stable + rustfmt/clippy
crates/
  mergify-cli/          # [[bin]] name = "mergify"; Phase 1.0 stub
  mergify-core/         # shared foundations (empty today; filled in 1.2)
```

Per the design, more crates (`mergify-py-shim`, one per command group)
land in later phases. The 2-crate start is deliberate: the workspace
exists, the lints and profile are set workspace-wide, but there is
nothing for a reviewer to miss.

## Conventions

- Edition 2024, MSRV `rust-version = 1.85` (edition-2024 minimum).
- ``rust-toolchain.toml`` uses ``channel = "stable"`` — tracks the
  current stable toolchain, not a specific pinned version. Renovate
  can bump it later if we want a narrower contract.
- Workspace-level lints: `unsafe_code = "forbid"`, `clippy::all`,
  `clippy::pedantic`. A small allow-list (`module_name_repetitions`,
  `missing_errors_doc`, `missing_panics_doc`) kills noise that would
  otherwise dominate a CLI with many small modules. Revisit per-PR.
- Release profile: thin LTO, single codegen unit, symbols stripped —
  reduces binary size for `cargo-dist` output in Phase 1.5.

## CI

New `rust` job on ubuntu-24.04: ``rustup toolchain install stable``
explicitly (so the build is deterministic against whatever the
runner image has cached), then ``cargo fmt --check``, ``cargo clippy
-D warnings``, ``cargo test``, ``cargo build --release``. Added to
``ci-gate`` so a red Rust job blocks merge just like linters and
tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jd

.github/workflows/ci.yaml

@@ -44,6 +44,32 @@ jobs:
         shell: bash
         run: uv run --locked poe compat-test
 
+  rust:
+    timeout-minutes: 10
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v6.0.2
+
+      - name: Install Rust toolchain
+        run: |
+          rustup toolchain install stable --profile minimal
+          rustup default stable
+          rustup component add rustfmt clippy --toolchain stable
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: cargo fmt
+        run: cargo fmt --all --check
+
+      - name: cargo clippy
+        run: cargo clippy --all-targets --workspace -- -D warnings
+
+      - name: cargo test
+        run: cargo test --workspace
+
+      - name: cargo build --release
+        run: cargo build --release
+
   test:
     timeout-minutes: 10
     strategy:
@@ -89,6 +115,7 @@ jobs:
       - test
       - linters
       - compat-tests
+      - rust
     runs-on: ubuntu-latest
     steps:
       - name: Verify all jobs succeeded

.gitignore

@@ -2,3 +2,4 @@ __pycache__
 .mypy_cache
 dist
 build
+target

Cargo.lock

@@ -0,0 +1,40 @@
+#
+# Copyright © 2021-2026 Mergify SAS
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+
+[workspace]
+resolver = "3"
+members = ["crates/*"]
+
+[workspace.package]
+edition = "2024"
+# MSRV: the minimum supported toolchain. Set to the rustc that
+# ships edition 2024, so the project works on any stable from
+# early 2025 onwards. CI uses whatever stable is current.
+rust-version = "1.85"
+license = "Apache-2.0"
+repository = "https://github.com/Mergifyio/mergify-cli"
+authors = ["Mergify Team <hello@mergify.com>"]
+
+[workspace.lints.rust]
+unsafe_code = "forbid"
+
+[workspace.lints.clippy]
+all = { level = "warn", priority = -1 }
+pedantic = { level = "warn", priority = -1 }
+# Allow-list: pedantic rules that fire too aggressively for a CLI.
+# Keep this list intentional and short; revisit per-PR as we learn.
+module_name_repetitions = "allow"
+missing_errors_doc = "allow"
+missing_panics_doc = "allow"
+
+[profile.release]
+lto = "thin"
+codegen-units = 1
+strip = "symbols"

Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "mergify-cli"
+version = "0.0.0"
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+authors.workspace = true
+description = "mergify CLI — Rust implementation."
+publish = false
+
+[[bin]]
+name = "mergify"
+path = "src/main.rs"
+
+[dependencies]
+mergify-core = { path = "../mergify-core" }
+
+[lints]
+workspace = true

crates/mergify-cli/Cargo.toml

@@ -0,0 +1,15 @@
+//! `mergify` binary entry point.
+//!
+//! This is the Phase 1.0 scaffolding stub: it prints an identification
+//! line and exits 0 so CI can verify the Rust toolchain, build, and
+//! release profile end-to-end. Subsequent phases replace this with the
+//! real clap-based dispatch that forwards commands to native Rust
+//! implementations or the embedded Python shim.
+
+fn main() {
+    println!(
+        "mergify {} — Rust scaffolding (Phase 1.0). \
+         Use the Python CLI for actual functionality.",
+        mergify_core::VERSION,
+    );
+}

crates/mergify-cli/src/main.rs

@@ -0,0 +1,13 @@
+[package]
+name = "mergify-core"
+version = "0.0.0"
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+authors.workspace = true
+description = "Shared foundations for mergify-cli: HTTP, auth, errors, output, git, config, prompts."
+publish = false
+
+[lints]
+workspace = true

crates/mergify-core/Cargo.toml

@@ -0,0 +1,9 @@
+//! Shared foundations for the mergify CLI Rust port.
+//!
+//! This crate is intentionally empty in Phase 1.0 — it is the container
+//! that subsequent phases fill with the HTTP client, auth, error types,
+//! output traits, git operations, config loading, and interactive prompts.
+
+/// Compile-time version string taken from the crate package metadata
+/// via ``CARGO_PKG_VERSION``.
+pub const VERSION: &str = env!("CARGO_PKG_VERSION");

crates/mergify-core/src/lib.rs

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "stable"
+components = ["rustfmt", "clippy"]