ci: add code coverage workflow and badge

Signed-off-by: Brian McGee <[email protected]>

Author: brianmcgee

.github/workflows/coverage.yml

@@ -0,0 +1,26 @@
+name: Code Coverage
+
+on: [push]
+
+jobs:
+    coverage:
+        runs-on: "ubuntu-20.04"
+        name: Coverage
+        steps:
+            - uses: actions/checkout@v4
+              with:
+                  fetch-depth: 0
+            - name: Install Nix
+              uses: cachix/install-nix-action@v30
+              with:
+                  extra_nix_config: |
+                      accept-flake-config = true
+                      experimental-features = nix-command flakes
+            - name: Run coverage
+              run: nix build .#treefmt.tests.coverage
+            - name: Upload coverage
+              uses: coverallsapp/github-action@v2
+              with:
+                  github-token: ${{ secrets.github_token }}
+                  file: result
+                  format: golang

README.md

@@ -6,9 +6,10 @@
 
 **One CLI to format your repo**
 
-*A <a href="https://numtide.com/">numtide</a> project.*
+_A <a href="https://numtide.com/">numtide</a> project._
 
 <p>
+<a href='https://coveralls.io/github/numtide/treefmt?branch=new-README'><img src='https://coveralls.io/repos/github/numtide/treefmt/badge.svg?branch=new-README' alt='Coverage Status' /></a>
 <a href="https://github.com/numtide/treefmt/actions/workflows/release.yml"><img src="https://github.com/numtide/treefmt/actions/workflows/release.yml/badge.svg"/></a>
 <img alt="Static Badge" src="https://img.shields.io/badge/status-beta-yellow">
 <a href="https://app.element.io/#/room/#home:numtide.com"><img src="https://img.shields.io/badge/Support-%23numtide-blue"/></a>

nix/packages/treefmt/default.nix

@@ -63,8 +63,22 @@ in
       git config --global user.name "Treefmt Test"
     '';
 
-    passthru.tests = {
-      golangci-lint = perSystem.self.treefmt.overrideAttrs (old: {
+    passthru.tests = let
+      inherit (perSystem.self) treefmt;
+    in {
+      coverage = lib.optionalAttrs pkgs.stdenv.isx86_64 (treefmt.overrideAttrs (old: {
+        nativeBuildInputs = old.nativeBuildInputs ++ [pkgs.gcc];
+        CGO_ENABLED = 1;
+        buildPhase = ''
+          HOME=$TMPDIR
+          go test -race -covermode=atomic -coverprofile=coverage.out -v ./...
+        '';
+        installPhase = ''
+          mv coverage.out $out
+        '';
+      }));
+
+      golangci-lint = treefmt.overrideAttrs (old: {
         nativeBuildInputs = old.nativeBuildInputs ++ [pkgs.golangci-lint];
         buildPhase = ''
           HOME=$TMPDIR