Run direct-minimal-versions tests on a weekly basis

My understanding is that it's okay that we don't use these minimal
versions directly in our normal/committed Cargo.lock file, because we
may want to pick up more recent compatible versions that have addressed
security issues or improved performance, etc. That said, we still
want to periodically test the minimal versions to ensure that our own
project still compiles and runs as expected. This will help to increase
compatibility across the whole Rust ecosystem when there are shared
dependencies and someone is using our library.

I've also abstracted out the CUE _testRust steps here, so they can be
shared.

See:
- rust-lang/cargo#11688
- https://doc.rust-lang.org/beta/cargo/reference/unstable.html#minimal-versions
- https://users.rust-lang.org/t/psa-please-specify-precise-dependency-versions-in-cargo-toml/71277
- https://twitter.com/jonhoo/status/1571290371124260865
- cue-lang/cue#860

.github/cue/rust.cue

@@ -76,19 +76,7 @@ rust: _#useMergeQueue & {
 				_#checkoutCode,
 				_#installRust,
 				_#cacheRust & {with: "shared-key": "stable-${{ matrix.platform }}"},
-				_#installTool & {with: tool:       "cargo-nextest"},
-				{
-					name: "Compile tests"
-					run:  "cargo test --locked --no-run"
-				},
-				{
-					name: "Run tests"
-					run:  "cargo nextest run --locked"
-				},
-				{
-					name: "Run doctests"
-					run:  "cargo test --locked --doc"
-				},
+				for step in _testRust {step},
 			]
 		}
 

.github/cue/scheduled.cue

@@ -0,0 +1,44 @@
+package workflows
+
+scheduled: {
+	name: "scheduled"
+
+	on: {
+		// Run every Monday at 7:45am UTC.
+		schedule: [{cron: "45 7 * * 1"}]
+
+		// Allow manually running this workflow.
+		workflow_dispatch: null
+	}
+
+	concurrency: {
+		group:                "${{ github.workflow }}-${{ github.ref }}"
+		"cancel-in-progress": true
+	}
+
+	env: {
+		CARGO_INCREMENTAL: 0
+		CARGO_TERM_COLOR:  "always"
+		RUST_BACKTRACE:    1
+		RUSTFLAGS:         "-D warnings"
+	}
+
+	jobs: direct_minimal_versions: {
+		name:      "direct-minimal-versions / stable"
+		"runs-on": defaultRunner
+		steps: [
+			_#checkoutCode & {with: ref: defaultBranch},
+			_#installRust,
+			_#installRust & {with: toolchain: "nightly"},
+			{
+				name: "Default to stable Rust"
+				run:  "rustup default stable"
+			},
+			{
+				name: "Resolve minimal dependency versions instead of maximum"
+				run:  "cargo +nightly update -Zdirect-minimal-versions"
+			},
+			for step in _testRust {step},
+		]
+	}
+}

.github/cue/shared-steps.cue

@@ -86,3 +86,19 @@ _#prettier: _#step & {
 	uses: "creyD/prettier_action@31355f8eef017f8aeba2e0bc09d8502b13dbbad1"
 	with: prettier_version: "2.8.8"
 }
+
+_testRust: [
+	_#installTool & {with: tool: "cargo-nextest"},
+	_#step & {
+		name: "Compile tests"
+		run:  "cargo test --locked --no-run"
+	},
+	_#step & {
+		name: "Run tests"
+		run:  "cargo nextest run --locked --all-targets --all-features"
+	},
+	_#step & {
+		name: "Run doctests"
+		run:  "cargo test --locked --doc"
+	},
+]

.github/cue/workflows.cue

@@ -14,6 +14,7 @@ workflows: [
 	{workflow: githubPages},
 	{workflow: preloadCaches},
 	{workflow: rust},
+	{workflow: scheduled},
 	{workflow: wordsmith},
 ]
 

.github/workflows/rust.yml

@@ -144,7 +144,7 @@ jobs:
       - name: Compile tests
         run: cargo test --locked --no-run
       - name: Run tests
-        run: cargo nextest run --locked
+        run: cargo nextest run --locked --all-targets --all-features
       - name: Run doctests
         run: cargo test --locked --doc
   check_msrv:

.github/workflows/scheduled.yml

@@ -0,0 +1,46 @@
+# This file is generated by .github/cue/ci_tool.cue; DO NOT EDIT!
+
+name: scheduled
+"on":
+  schedule:
+    - cron: 45 7 * * 1
+  workflow_dispatch: null
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_TERM_COLOR: always
+  RUST_BACKTRACE: 1
+  RUSTFLAGS: -D warnings
+jobs:
+  direct_minimal_versions:
+    name: direct-minimal-versions / stable
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
+        with:
+          ref: main
+      - name: Install stable Rust toolchain
+        uses: dtolnay/rust-toolchain@b44cb146d03e8d870c57ab64b80f04586349ca5d
+        with:
+          toolchain: stable
+      - name: Install nightly Rust toolchain
+        uses: dtolnay/rust-toolchain@b44cb146d03e8d870c57ab64b80f04586349ca5d
+        with:
+          toolchain: nightly
+      - name: Default to stable Rust
+        run: rustup default stable
+      - name: Resolve minimal dependency versions instead of maximum
+        run: cargo +nightly update -Zdirect-minimal-versions
+      - name: Install cargo-nextest
+        uses: taiki-e/install-action@a775aaf2e8ed709f76ee019cb77e39fc50613631
+        with:
+          tool: cargo-nextest
+      - name: Compile tests
+        run: cargo test --locked --no-run
+      - name: Run tests
+        run: cargo nextest run --locked --all-targets --all-features
+      - name: Run doctests
+        run: cargo test --locked --doc