feat(elixir): add run-prod-compile to catch production-build breakage at PR time

.github/workflows/elixir.yml

@@ -66,6 +66,29 @@ on:
           itself queries. Pair it with run-hex-advisory-check.
         type: boolean
         default: false
+      run-prod-compile:
+        description: |
+          Additionally compile with `MIX_ENV=prod`, to catch breakage that only
+          exists in the production build.
+
+          Worth turning on for anything that ships a Mix release. A release
+          evaluates `config/config.exs` at *build* time under `MIX_ENV=prod`,
+          which is a path the normal dev/test compile never touches — a missing
+          `config/prod.exs`, a bad `import_config`, or code that only compiles
+          under dev/test all pass CI and then fail when the image is built.
+
+          Where a repo builds its image on `main` only (a common pattern, to
+          keep PR-controlled Dockerfiles off self-hosted runners), that failure
+          lands *after* merge, so `main` goes red on a diff that already passed
+          review. This check moves it to PR time for a fraction of the cost of
+          a full image build.
+
+          Not `--warnings-as-errors`: the goal is "does the production build
+          work", not warning hygiene, and prod-only warnings in a dependency
+          shouldn't red-line a consumer. `_build` is already cached, so the
+          extra prod tree costs a full compile only when the lockfile changes.
+        type: boolean
+        default: false
       run-hex-advisory-check:
         description: |
           Fail the build when Hex's own resolver reports a security advisory
@@ -212,6 +235,17 @@ jobs:
       - name: Compile (warnings as errors)
         run: mix compile --warnings-as-errors
 
+      - name: Compile (MIX_ENV=prod)
+        # Exercises the production build path: config.exs is re-evaluated with
+        # config_env() == :prod, so a missing config/prod.exs or a broken
+        # import_config fails here rather than in a post-merge image build.
+        if: inputs.run-prod-compile
+        env:
+          MIX_ENV: prod
+        run: |
+          mix deps.get
+          mix compile
+
       - name: Format check
         if: inputs.run-format-check
         run: mix format --check-formatted

CHANGELOG.md

@@ -6,6 +6,36 @@ project uses [SemVer](https://semver.org/) for the `vMAJOR.MINOR.PATCH` tags.
 
 ## [Unreleased]
 
+### Added
+
+- `elixir.yml` — `run-prod-compile`, which additionally compiles with
+  `MIX_ENV=prod`.
+
+  A Mix release evaluates `config/config.exs` at *build* time under
+  `config_env() == :prod`, a path the normal dev/test compile never touches.
+  So a missing `config/prod.exs`, a broken `import_config`, or code that only
+  compiles under dev/test passes CI green and then fails when the image is
+  built.
+
+  Found in a consumer repo where the image build had **never** succeeded —
+  every merge to `main` for months failed with
+  `could not read file "/app/config/prod.exs"`, while every PR was green. The
+  image build was gated `if: github.ref == 'refs/heads/main'` (a reasonable
+  pattern, keeping PR-controlled Dockerfiles off self-hosted runners), so the
+  failure always landed post-merge on a diff that had already passed review,
+  at the end of the run behind `needs:`. Nothing was watching the one place it
+  showed up.
+
+  This check moves that class of failure to PR time for a fraction of the cost
+  of a full image build. Deliberately not `--warnings-as-errors` — the
+  question is whether the production build *works*, not warning hygiene, and a
+  prod-only warning in a dependency shouldn't red-line a consumer. `_build` is
+  already cached, so the extra prod tree costs a full compile only when the
+  lockfile changes.
+
+  Off by default, like the other opt-in gates. Worth enabling for anything
+  that ships a release.
+
 ## [2.16.0] - 2026-08-24
 
 ### Added