feat: add action file

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  merge_group:
+    types: [checks_requested]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  msrv:
+    name: MSRV
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+
+      - name: Determine MSRV
+        id: msrv
+        uses: ./
+        with:
+          manifest-path: ./test-workspace/Cargo.toml
+
+    outputs:
+      msrv: "${{ steps.msrv.outputs.msrv }}"
+
+  build:
+    needs: msrv
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        toolchain:
+          - { name: msrv, version: "${{ needs.msrv.outputs.msrv }}" }
+          - { name: stable, version: stable }
+
+    name: Test / ${{ matrix.toolchain.name }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+        with:
+          toolchain: ${{ matrix.toolchain.version }}
+          cache-workspaces: ./test-workspace
+
+      - name: Test
+        run: cargo run --manifest-path=./test-workspace/Cargo.toml
+
+  combined:
+    runs-on: ubuntu-latest
+
+    name: Test Combined
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust (stable)
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+        with:
+          cache: false
+
+      - name: Determine MSRV
+        id: msrv
+        uses: ./
+        with:
+          manifest-path: ./test-workspace/Cargo.toml
+
+      - name: Install Rust (MSRV)
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+        with:
+          toolchain: ${{ steps.msrv.outputs.msrv }}
+          cache-workspaces: ./test-workspace
+
+      - name: Test
+        run: cargo run --manifest-path=./test-workspace/Cargo.toml

CHANGELOG.md

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+# Changelog
+
+## 0.1.0
+
+- Initial release.

action.yml

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+name: Determine minimum supported Rust version (MSRV) for workspace
+
+description: |
+  Determine minimum supported Rust version (MSRV) for workspace for use in a job matrix.
+
+inputs:
+  manifest-path:
+    description: Path to Cargo.toml manifest for workspace
+    required: false
+
+runs:
+  using: composite
+
+  steps:
+    - name: Read MSRV from workspace manifest
+      id: read_msrv
+      shell: bash
+      run: |
+        cargo metadata --format-version=1 --manifest-path=${{ inputs.manifest-path || 'Cargo.toml' }} \
+        | jq -r 'first(.packages[] | select(.source == null and .rust_version)) | .rust_version' \
+        | sed -E 's/^1\.([0-9]{2})$/1\.\1\.0/' \
+        | xargs -0 printf "msrv=%s" \
+        | tee /dev/stderr \
+        >> "$GITHUB_OUTPUT"
+
+outputs:
+  msrv:
+    description: |-
+      Minimum supported Rust version. E.g.: 1.72.1"
+    value: ${{ steps.read_msrv.outputs.msrv }}

test-workspace/Cargo.toml

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+[package]
+name = "crate"
+version = "1.0.0"
+publish = false
+edition = "2018"
+rust-version = "1.72"
+
+[dependencies]
+detrim = "0.1"

test-workspace/src/main.rs

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+fn main() {
+    println!("Hello, world!");
+}