add coverage support

Author: thsackos

build.zig

@@ -4,6 +4,8 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
+    const coverage = b.option(bool, "coverage", "Generate a coverage report with kcov") orelse false;
+
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
@@ -30,4 +32,38 @@ pub fn build(b: *std.Build) void {
     const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);
     const test_step = b.step("test", "Run unit tests");
     test_step.dependOn(&run_lib_unit_tests.step);
+
+    if (coverage) {
+        try addCoverageReporting();
+
+        var run_test_steps: std.ArrayListUnmanaged(*std.Build.Step.Run) = .empty;
+        run_test_steps.append(b.allocator, run_lib_unit_tests) catch @panic("OOM");
+
+        const kcov_bin = b.findProgram(&.{"kcov"}, &.{}) catch "kcov";
+
+        const merge_step = std.Build.Step.Run.create(b, "merge coverage");
+        merge_step.addArgs(&.{ kcov_bin, "--merge" });
+        merge_step.rename_step_with_output_arg = false;
+        const merged_coverage_output = merge_step.addOutputFileArg(".");
+
+        for (run_test_steps.items) |run_step| {
+            run_step.setName(b.fmt("{s} (collect coverage)", .{run_step.step.name}));
+
+            // prepend the kcov exec args
+            const argv = run_step.argv.toOwnedSlice(b.allocator) catch @panic("OOM");
+            run_step.addArgs(&.{ kcov_bin, "--collect-only" });
+            run_step.addPrefixedDirectoryArg("--include-pattern=", b.path("src"));
+            merge_step.addDirectoryArg(run_step.addOutputFileArg(run_step.producer.?.name));
+            run_step.argv.appendSlice(b.allocator, argv) catch @panic("OOM");
+        }
+
+        const install_coverage = b.addInstallDirectory(.{
+            .source_dir = merged_coverage_output,
+            .install_dir = .{ .custom = "coverage" },
+            .install_subdir = "",
+        });
+        test_step.dependOn(&install_coverage.step);
+    }
 }
+
+fn addCoverageReporting() !void {}