tests: add test-incremental step

This is contained in the `test` step, so is tested by CI.

This commit also includes some enhancements to the `incr-check` tool to
make this work correctly.

build.zig

@@ -577,6 +577,10 @@ pub fn build(b: *std.Build) !void {
     } else {
         update_mingw_step.dependOn(&b.addFail("The -Dmingw-src=... option is required for this step").step);
     }
+
+    const test_incremental_step = b.step("test-incremental", "Run the incremental compilation test cases");
+    try tests.addIncrementalTests(b, test_incremental_step);
+    test_step.dependOn(test_incremental_step);
 }
 
 fn addWasiUpdateStep(b: *std.Build, version: [:0]const u8) !void {

test/tests.zig

@@ -1509,3 +1509,31 @@ pub fn addDebuggerTests(b: *std.Build, options: DebuggerContext.Options) ?*Step
     });
     return step;
 }
+
+pub fn addIncrementalTests(b: *std.Build, test_step: *Step) !void {
+    const incr_check = b.addExecutable(.{
+        .name = "incr-check",
+        .root_source_file = b.path("tools/incr-check.zig"),
+        .target = b.graph.host,
+        .optimize = .Debug,
+    });
+
+    var dir = try b.build_root.handle.openDir("test/incremental", .{ .iterate = true });
+    defer dir.close();
+
+    var it = try dir.walk(b.graph.arena);
+    while (try it.next()) |entry| {
+        if (entry.kind != .file) continue;
+
+        const run = b.addRunArtifact(incr_check);
+        run.setName(b.fmt("incr-check '{s}'", .{entry.basename}));
+
+        run.addArg(b.graph.zig_exe);
+        run.addFileArg(b.path("test/incremental/").path(b, entry.path));
+        run.addArgs(&.{ "--zig-lib-dir", b.fmt("{}", .{b.graph.zig_lib_directory}) });
+
+        run.addCheck(.{ .expect_term = .{ .Exited = 0 } });
+
+        test_step.dependOn(&run.step);
+    }
+}

tools/incr-check.zig

@@ -68,7 +68,9 @@ pub fn main() !void {
     const debug_log_verbose = debug_zcu or debug_link;
 
     for (case.targets) |target| {
+        if (debug_log_verbose) {
             std.log.scoped(.status).info("target: '{s}-{s}'", .{ target.query, @tagName(target.backend) });
+        }
 
         var child_args: std.ArrayListUnmanaged([]const u8) = .empty;
         try child_args.appendSlice(arena, &.{