chore: add cargo-deny for license, duplicate and source checks

.github/workflows/ci.yml

@@ -99,3 +99,31 @@ jobs:
         # advisories (unsound / unmaintained) are reported as warnings only —
         # Dependabot does not surface those at all, so the log is the record.
         run: cargo audit
+
+  deny:
+    name: cargo deny
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo-deny binary
+        id: cache-deny
+        uses: actions/cache@v6
+        with:
+          path: ~/.cargo/bin/cargo-deny
+          key: ${{ runner.os }}-cargo-deny
+
+      - name: Install cargo-deny
+        if: steps.cache-deny.outputs.cache-hit != 'true'
+        run: cargo install cargo-deny --locked
+
+      - name: Check licenses, bans and sources
+        # `advisories` is intentionally omitted — the cargo audit job above
+        # already covers RustSec. This job guards the other three: license
+        # compatibility with GPL-3.0-only, duplicate crate versions (a second
+        # crypto trait generation in the graph), and dependencies resolving to
+        # anything other than crates.io.
+        run: cargo deny check licenses bans sources

CONTRIBUTING.md

@@ -103,8 +103,9 @@ benchmark/
 scripts/
   release.sh      Automated release + Homebrew formula update
 .github/
-  workflows/ci.yml  GitHub Actions CI (fmt, clippy, test, cargo audit)
+  workflows/ci.yml  GitHub Actions CI (fmt, clippy, test, cargo audit, cargo deny)
   dependabot.yml    Weekly cargo + github-actions dependency updates
+deny.toml           cargo-deny policy (licenses, duplicate versions, sources)
 ```
 
 ## Development Guidelines
@@ -115,6 +116,23 @@ scripts/
 - Run `cargo clippy` and fix any warnings
 - Follow standard Rust naming conventions
 
+### Dependencies
+
+CI enforces a dependency policy on top of the test suite. Both tools are
+optional locally (`cargo install cargo-audit cargo-deny --locked`), but a PR
+that trips either one will fail:
+
+- `cargo audit` fails on known RustSec vulnerabilities. Informational
+  advisories (unsound / unmaintained) are reported as warnings only.
+- `cargo deny check licenses bans sources` enforces `deny.toml`: dependency
+  licenses must be compatible with GPL-3.0-only, no crate may appear at two
+  versions, and every dependency must resolve to crates.io. The `advisories`
+  check is deliberately left to `cargo audit` rather than run twice.
+
+A duplicate-version failure matters most for the RustCrypto crates — two
+generations of the `cipher` / `digest` traits in one graph will not compile.
+That is why `.github/dependabot.yml` groups them into a single PR.
+
 ### Compatibility
 
 This is the most important constraint. Gitveil must remain **byte-compatible** with git-crypt:
@@ -157,7 +175,7 @@ This is the most important constraint. Gitveil must remain **byte-compatible** w
 1. Fork the repository
 2. Create a feature branch (`git checkout -b my-feature`)
 3. Make your changes
-4. Run `cargo fmt && cargo clippy && cargo test`
+4. Run `cargo fmt && cargo clippy && cargo test` (and `cargo deny check licenses bans sources` if you changed dependencies)
 5. Commit with a clear message
 6. Open a pull request
 

deny.toml

@@ -0,0 +1,47 @@
+# cargo-deny configuration.
+#
+# Scope note: the `advisories` check is deliberately NOT run in CI — the
+# separate `cargo audit` job already covers RustSec advisories and reports
+# informational (unsound / unmaintained) findings as warnings. Running both
+# would duplicate the signal. CI runs `cargo deny check licenses bans sources`.
+
+[graph]
+all-features = true
+
+[licenses]
+# GPL-3.0-only is this crate's own license. Everything below is either
+# permissive or GPL-3.0-compatible weak copyleft. Crates offering a choice
+# (e.g. rustix: "Apache-2.0 WITH LLVM-exception OR Apache-2.0 OR MIT")
+# resolve against the first allowed term, so only the terms actually
+# selected need listing here.
+allow = [
+    "Apache-2.0",
+    "GPL-3.0-only",
+    "MIT",
+    # `colored` is MPL-2.0: file-level copyleft, explicitly GPL-compatible.
+    "MPL-2.0",
+    # `unicode-ident` is "(MIT OR Apache-2.0) AND Unicode-3.0".
+    "Unicode-3.0",
+]
+confidence-threshold = 0.9
+
+[bans]
+# A second copy of a crypto crate in the graph means two incompatible
+# trait generations (see the `rustcrypto` group in .github/dependabot.yml).
+# Denying duplicates outright is what catches that.
+multiple-versions = "deny"
+wildcards = "deny"
+
+skip = [
+    # syn 2 and syn 3 coexist because zeroize_derive is still on 2 while
+    # clap_derive and thiserror-impl moved to 3. Both are proc-macro
+    # build-time only and never reach the shipped binary.
+    { crate = "syn" },
+]
+
+[sources]
+# Every dependency must come from crates.io. A git or alternate-registry
+# source appearing in the graph is the supply-chain case worth failing on.
+unknown-registry = "deny"
+unknown-git = "deny"
+allow-registry = ["https://github.com/rust-lang/crates.io-index"]