cargo-rail: unify(msrv): write x.y.z, enforce inheritance option, and add fast CI MSRV gate

Author: loadingalias

.config/rail.toml

@@ -43,7 +43,7 @@ detect_unused = true  # Detect unused dependencies (default: true)
 remove_unused = true  # Auto-remove unused deps when applying (default: true)
 max_backups = 2  # Number of backup files to keep (default: 3)
 prune_dead_features = true  # Remove features never enabled in resolved graph (default: true)
-msrv_source = "max"  # How to compute MSRV: deps, workspace, max (default: max)
+msrv_source = "workspace"  # How to compute MSRV: deps, workspace, max (default: max)
 preserve_features = []  # Features to preserve from pruning (glob patterns)
 detect_undeclared_features = true  # Detect features borrowed via Cargo unification (default: true)
 fix_undeclared_features = true  # Auto-fix borrowed features (default: true)

.github/workflows/commit.yaml

@@ -55,11 +55,64 @@ jobs:
           since: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || github.event.before }}
 
   # ==========================================================================
-  # Job 2: CI (skipped if docs-only)
+  # Job 2: MSRV enforcement (fast fail)
+  # ==========================================================================
+  msrv:
+    name: MSRV Check
+    needs: detect
+    if: needs.detect.outputs.docs-only != 'true'
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8  # v5.0.0
+        with:
+          persist-credentials: false
+
+      - name: Detect MSRV
+        id: msrv
+        shell: bash
+        run: |
+          msrv="$(python3 - <<'PY'
+          import pathlib, tomllib
+
+          data = tomllib.loads(pathlib.Path("Cargo.toml").read_text())
+          msrv = (
+              data.get("workspace", {}).get("package", {}).get("rust-version")
+              or data.get("package", {}).get("rust-version")
+          )
+          if isinstance(msrv, dict):
+              msrv = None
+          if not msrv:
+              raise SystemExit("No rust-version found in Cargo.toml")
+          print(msrv)
+          PY
+          )"
+
+          echo "msrv=$msrv" >> "$GITHUB_OUTPUT"
+          echo "Using MSRV: $msrv"
+
+      - name: Install Rust MSRV Toolchain
+        uses: dtolnay/rust-toolchain@0b1efabc08b657293548b77fb76cc02d26091c7e  # master
+        with:
+          toolchain: ${{ steps.msrv.outputs.msrv }}
+
+      - name: Setup Rust Cache
+        uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1  # v2.8.1
+        with:
+          shared-key: "cargo-rail-msrv-v1"
+          key: msrv-${{ steps.msrv.outputs.msrv }}
+          cache-on-failure: true
+
+      - name: Compile (MSRV)
+        run: cargo check --workspace --all-targets --all-features --locked
+
+  # ==========================================================================
+  # Job 3: CI (skipped if docs-only)
   # ==========================================================================
   ci:
     name: CI (${{ matrix.target.name }})
-    needs: detect
+    needs: [msrv, detect]
     if: needs.detect.outputs.docs-only != 'true'
     runs-on: ${{ matrix.target.runner }}
     timeout-minutes: 30

Cargo.toml

@@ -2,7 +2,7 @@
 name = "cargo-rail"
 version = "0.7.3"
 edition = "2024"
-rust-version = "1.91.0"
+rust-version = { workspace = true }
 license = "MIT"
 authors = ["loadingalias"]
 description = "Graph-aware testing, dependency unification, and crate extraction for Rust monorepos"
@@ -27,6 +27,13 @@ exclude = [
     "backlog/",
 ]
 
+[workspace]
+members = ["."]
+resolver = "3"
+
+[workspace.package]
+rust-version = "1.91.0"
+
 [dependencies]
 clap = { version = "4.5.53", features = ["derive", "cargo"] }
 clap_complete = "4.5.62"

README.md

@@ -6,7 +6,7 @@
   <a href="https://crates.io/crates/cargo-rail"><img src="https://img.shields.io/crates/v/cargo-rail.svg" alt="Crates.io"></a>
   <a href="https://crates.io/crates/cargo-rail"><img src="https://img.shields.io/crates/d/cargo-rail.svg" alt="Downloads"></a>
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-MIT-blue.svg" alt="License: MIT"></a>
-  <a href="https://www.rust-lang.org"><img src="https://img.shields.io/badge/rust-1.85%2B-orange.svg" alt="Rust 1.85+"></a>
+  <a href="https://www.rust-lang.org"><img src="https://img.shields.io/crates/msrv/cargo-rail" alt="MSRV"></a>
 </p>
 
 <p align="center">
@@ -46,6 +46,15 @@ Optionally, install via the [pre-built binaries](https://github.com/loadingalias
 
 ---
 
+## MSRV policy
+
+- **MSRV source of truth**: `Cargo.toml` (`rust-version`, written as `major.minor.patch`)
+- **Cargo requirement**: Cargo shipped with that Rust release (newer Cargo is fine)
+- **CI**: builds on MSRV to prevent accidental bumps
+- **Workspaces**: `cargo rail unify` writes `[workspace.package].rust-version`; enable `[unify].enforce_msrv_inheritance = true` to set `[package].rust-version = { workspace = true }` in member crates
+
+---
+
 ## Quick Start
 
 ```bash
@@ -168,6 +177,7 @@ detect_unused = true
 prune_dead_features = true
 
 msrv = true
+enforce_msrv_inheritance = false
 msrv_source = "max"          # deps | workspace | max
 
 [release]

docs/config.md

@@ -102,7 +102,8 @@ Controls workspace dependency unification behavior. All options are optional wit
 
 | Option | Type | Default | Description |
 |--------|------|---------|-------------|
-| `msrv` | `bool` | `true` | Compute and write MSRV to `[workspace.package].rust-version`. The MSRV is determined by `msrv_source`. |
+| `msrv` | `bool` | `true` | Compute and write MSRV to `[workspace.package].rust-version` (written as `major.minor.patch`). The MSRV is determined by `msrv_source`. |
+| `enforce_msrv_inheritance` | `bool` | `false` | Ensure every workspace member inherits MSRV by setting `[package].rust-version = { workspace = true }` in each member's `Cargo.toml`. This makes `[workspace.package].rust-version` actually apply across the workspace. |
 | `msrv_source` | `enum` | `"max"` | How to compute the final MSRV:<br>• `"deps"` - Use maximum from dependencies only (original behavior)<br>• `"workspace"` - Preserve existing rust-version, warn if deps need higher<br>• `"max"` - Take max(workspace, deps) - your explicit setting wins if higher |
 | `detect_unused` | `bool` | `true` | Detect dependencies declared in manifests but absent from the resolved cargo graph. |
 | `remove_unused` | `bool` | `true` | Automatically remove unused dependencies during unification. Requires `detect_unused = true`. |
@@ -150,6 +151,7 @@ major_version_conflict = "bump"
 
 - In my experience, `major_version_conflict = "bump"` works in most cases; some may require code fixes
 - Use `"warn"` for safety, `"bump"` for the leanest build graph
+- If `[workspace.package].rust-version` is missing but root `[package].rust-version` is present, `unify` uses it as the baseline and writes it to `[workspace.package].rust-version` (consider enabling `enforce_msrv_inheritance` to avoid drift)
 
 #### Dependency Selection
 
@@ -193,6 +195,7 @@ transitive_host = "root"
 [unify]
 # Core options (defaults shown)
 msrv = true
+enforce_msrv_inheritance = false
 msrv_source = "max"  # "deps" | "workspace" | "max"
 detect_unused = true
 remove_unused = true

src/cargo/manifest_writer.rs

@@ -186,7 +186,7 @@ impl ManifestWriter {
   /// Write MSRV (rust-version) to workspace manifest
   ///
   /// Writes to [workspace.package].rust-version so that members can inherit it
-  /// via `rust-version.workspace = true`
+  /// via `rust-version = { workspace = true }`
   pub fn write_workspace_msrv(&self, workspace_toml_path: &Path, msrv: &semver::Version) -> RailResult<()> {
     // Read workspace Cargo.toml
     let mut doc = manifest_ops::read_toml_file(workspace_toml_path)?;
@@ -198,8 +198,8 @@ impl ManifestWriter {
     let ws_package = manifest_ops::get_or_create_table(&mut doc, "workspace.package")
       .context("Failed to create [workspace.package]")?;
 
-    // Format MSRV as "major.minor" (standard rust-version format)
-    let msrv_str = format!("{}.{}", msrv.major, msrv.minor);
+    // Format MSRV as "major.minor.patch" (explicit and unambiguous)
+    let msrv_str = format!("{}.{}.{}", msrv.major, msrv.minor, msrv.patch);
 
     // Insert or update rust-version
     ws_package.insert("rust-version", toml_edit::value(&msrv_str));
@@ -211,6 +211,25 @@ impl ManifestWriter {
     Ok(())
   }
 
+  /// Ensure a member manifest inherits `rust-version` from `[workspace.package]`.
+  ///
+  /// Sets `[package].rust-version = { workspace = true }`.
+  pub fn enforce_member_msrv_inheritance(&self, member_toml_path: &Path) -> RailResult<()> {
+    let mut doc = manifest_ops::read_toml_file(member_toml_path)?;
+
+    let Some(pkg) = doc.get_mut("package").and_then(|p| p.as_table_like_mut()) else {
+      return Ok(());
+    };
+
+    let mut tbl = toml_edit::InlineTable::new();
+    tbl.insert("workspace", true.into());
+    pkg.insert("rust-version", toml_edit::value(toml_edit::Value::InlineTable(tbl)));
+
+    self.formatter.format_manifest(&mut doc)?;
+    manifest_ops::write_toml_file(member_toml_path, &doc)?;
+    Ok(())
+  }
+
   /// Remove an unused dependency from a member's Cargo.toml
   ///
   /// # Arguments