Add check against date placeholders

The date of publication often has to be adjusted at the last minute.
This can easily be forgotten. This commit introduces a new workflow:
Blog authors keep a placeholder as the date (9999/12/99) until shortly
before publication. Setting the date is aided with automation. Lastly,
A check triggered by the merge queue event is used to block PRs from
being merged where the placeholder hasn't been replaced yet.

Author: senekor

.github/workflows/merge_queue.yml

@@ -0,0 +1,61 @@
+name: CI
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+env:
+  # renovate: datasource=github-tags depName=rust lookupName=rust-lang/rust
+  RUST_VERSION: 1.86.0
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - run: rustup override set ${{ env.RUST_VERSION }}
+      - run: rustup component add clippy
+      - run: rustup component add rustfmt
+      - uses: Swatinem/rust-cache@9d47c6ad4b02e050fd481d890b2ea34778fd09d6 # v2.7.8
+
+      - run: cargo clippy --workspace -- -D warnings
+      - run: cargo fmt --check --all
+      - run: cargo test --package front_matter
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - run: rustup override set ${{ env.RUST_VERSION }}
+      - uses: Swatinem/rust-cache@9d47c6ad4b02e050fd481d890b2ea34778fd09d6 # v2.7.8
+
+      - name: Install Zola
+        run: cargo install --locked --git https://github.com/senekor/zola --rev 620bf3c46a39b41db30b1e91756a995bbff84d3a
+      - run: zola build
+      - run: cp CNAME ./public/
+      - run: touch public/.nojekyll
+
+      - uses: actions/upload-pages-artifact@56afc609e74202658d3ffba0e8f6dda462b719fa # v3.0.1
+        with:
+          path: public
+
+  deploy:
+    if: ${{ github.ref == 'refs/heads/master' }}
+
+    needs: build
+
+    permissions:
+      pages: write
+      id-token: write
+
+    runs-on: ubuntu-latest
+    steps:
+      - id: deployment
+        uses: actions/deploy-pages@d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e # v4.0.5
+
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}

README.md

@@ -33,10 +33,15 @@ You can store your main blog post in `content/<some-slug>/index.md`.
 Images go into the same directory: `content/<some-slug>/my_image.png`.
 Now you can reference that image with a simple relative path: `![alt text](my_image.png)`.
 
+A post's date of publication is embedded in the `path` key of the front matter.
+Keep the placeholder (`9999/12/99`) until the post is about to be merged.
+You can easily do so by adding a comment containing the string `publish=today` on the PR.
+Don't worry, there's a merge queue check to make sure you don't forget.
+
 Here is an example of the front matter format:
 ```md
 +++
-path = "2015/03/15/some-slug"
+path = "9999/12/99/some-slug"
 title = "Title of the blog post"
 authors = ["Blog post author (or on behalf of which team)"]
 description = "(optional)"

front_matter/src/lib.rs

@@ -212,4 +212,47 @@ The post {post} has abnormal front matter.
             };
         }
     }
+
+    /// This test is run by the merge queue check to make sure a blog post
+    /// isn't merged before its date is set. The date of a blog post is usually
+    /// a placeholder (path = "9999/12/99/...") until shortly before it's
+    /// published. Setting the date can be done manually or by commenting
+    /// "publish=today" on the PR.
+    #[test]
+    #[ignore]
+    fn date_is_set() {
+        let repo_root = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("..");
+
+        let posts = fs::read_dir(repo_root.join("content"))
+            .unwrap()
+            .chain(fs::read_dir(repo_root.join("content/inside-rust")).unwrap())
+            .map(|p| p.unwrap().path())
+            .filter(|p| p.is_file() && p.file_name() != Some("_index.md".as_ref()));
+
+        for post in posts {
+            let slug = post.file_stem().unwrap().to_str().unwrap();
+
+            let content = fs::read_to_string(&post).unwrap();
+            let (front_matter, _) = parse(&content).unwrap();
+
+            if front_matter.path.starts_with("9999/12/99/") {
+                panic!(
+                    "
+The post {slug} has a placeholder publication date.
+
+    ┌──────────────────────────────────────────────────────────────────────────┐
+    │                                                                          │
+    │       You can set the publication date automatically by commenting       │
+    │                                                                          │
+    │                              publish=today                               │
+    │                                                                          │
+    │                     on the pull request of the post.                     │
+    │     (You can also edit the 'path' key in the front matter directly.)     │
+    │                                                                          │
+    └──────────────────────────────────────────────────────────────────────────┘
+                    ",
+                );
+            }
+        }
+    }
 }