build: add support for skipping exercises

Currently, exercises not working with the current Zig compiler are
commented. This is problematic, since the exercise key and exercise
index are no longer consistent.

Add the skip field to the Excercise struct.  Update ZiglingStep to
support it and add a new SkipStep step to support it when using the
standard build step.

build.zig

@@ -33,6 +33,9 @@ const Exercise = struct {
     /// We need to keep track of this, so we compile with libc
     C: bool = false,
 
+    /// This exercise is not supported by the current Zig compiler.
+    skip: bool = false,
+
     /// Returns the name of the main file with .zig stripped.
     pub fn baseName(self: Exercise) []const u8 {
         assert(std.mem.endsWith(u8, self.main_file, ".zig"));
@@ -425,49 +428,59 @@ const exercises = [_]Exercise{
         .main_file = "083_anonymous_lists.zig",
         .output = "I say hello!",
     },
-    // disabled because of https://github.com/ratfactor/ziglings/issues/163
+
+    // Skipped because of https://github.com/ratfactor/ziglings/issues/163
     // direct link: https://github.com/ziglang/zig/issues/6025
-    // .{
+    .{
-    //     .main_file = "084_async.zig",
+        .main_file = "084_async.zig",
-    //     .output = "foo() A",
+        .output = "foo() A",
-    //     .hint = "Read the facts. Use the facts.",
+        .hint = "Read the facts. Use the facts.",
-    //     .@"async" = true,
+        .@"async" = true,
-    // },
+        .skip = true,
-    // .{
+    },
-    //     .main_file = "085_async2.zig",
+    .{
-    //     .output = "Hello async!",
+        .main_file = "085_async2.zig",
-    //     .@"async" = true,
+        .output = "Hello async!",
-    // },
+        .@"async" = true,
-    // .{
+        .skip = true,
-    //     .main_file = "086_async3.zig",
+    },
-    //     .output = "5 4 3 2 1",
+    .{
-    //     .@"async" = true,
+        .main_file = "086_async3.zig",
-    // },
+        .output = "5 4 3 2 1",
-    // .{
+        .@"async" = true,
-    //     .main_file = "087_async4.zig",
+        .skip = true,
-    //     .output = "1 2 3 4 5",
+    },
-    //     .@"async" = true,
+    .{
-    // },
+        .main_file = "087_async4.zig",
-    // .{
+        .output = "1 2 3 4 5",
-    //     .main_file = "088_async5.zig",
+        .@"async" = true,
-    //     .output = "Example Title.",
+        .skip = true,
-    //     .@"async" = true,
+    },
-    // },
+    .{
-    // .{
+        .main_file = "088_async5.zig",
-    //     .main_file = "089_async6.zig",
+        .output = "Example Title.",
-    //     .output = ".com: Example Title, .org: Example Title.",
+        .@"async" = true,
-    //     .@"async" = true,
+        .skip = true,
-    // },
+    },
-    // .{
+    .{
-    //     .main_file = "090_async7.zig",
+        .main_file = "089_async6.zig",
-    //     .output = "beef? BEEF!",
+        .output = ".com: Example Title, .org: Example Title.",
-    //     .@"async" = true,
+        .@"async" = true,
-    // },
+        .skip = true,
-    // .{
+    },
-    //     .main_file = "091_async8.zig",
+    .{
-    //     .output = "ABCDEF",
+        .main_file = "090_async7.zig",
-    //     .@"async" = true,
+        .output = "beef? BEEF!",
-    // },
+        .@"async" = true,
+        .skip = true,
+    },
+    .{
+        .main_file = "091_async8.zig",
+        .output = "ABCDEF",
+        .@"async" = true,
+        .skip = true,
+    },
+
     .{
         .main_file = "092_interfaces.zig",
         .output = "Daily Insect Report:\nAnt is alive.\nBee visited 17 flowers.\nGrasshopper hopped 32 meters.",
@@ -568,7 +581,12 @@ pub fn build(b: *Build) !void {
         const run_step = build_step.run();
 
         const test_step = b.step("test", b.fmt("Run {s} without checking output", .{ex.main_file}));
+        if (ex.skip) {
+            const skip_step = SkipStep.create(b, ex);
+            test_step.dependOn(&skip_step.step);
+        } else {
             test_step.dependOn(&run_step.step);
+        }
 
         const install_step = b.step("install", b.fmt("Install {s} to prefix path", .{ex.main_file}));
         install_step.dependOn(b.getInstallStep());
@@ -614,9 +632,13 @@ pub fn build(b: *Build) !void {
             build_step.install();
 
             const run_step = build_step.run();
-
+            if (ex.skip) {
+                const skip_step = SkipStep.create(b, ex);
+                test_step.dependOn(&skip_step.step);
+            } else {
                 test_step.dependOn(&run_step.step);
             }
+        }
 
         return;
     }
@@ -673,6 +695,12 @@ const ZiglingStep = struct {
     fn make(step: *Step, prog_node: *std.Progress.Node) anyerror!void {
         _ = prog_node;
         const self = @fieldParentPtr(@This(), "step", step);
+
+        if (self.exercise.skip) {
+            print("Skipping {s}\n\n", .{self.exercise.main_file});
+
+            return;
+        }
         self.makeInternal() catch {
             if (self.exercise.hint.len > 0) {
                 print("\n{s}HINT: {s}{s}", .{ bold_text, self.exercise.hint, reset_text });
@@ -863,3 +891,33 @@ const PrintStep = struct {
         try p.file.writeAll(p.message);
     }
 };
+
+// Skip an exercise.
+const SkipStep = struct {
+    step: Step,
+    exercise: Exercise,
+
+    pub fn create(owner: *Build, exercise: Exercise) *SkipStep {
+        const self = owner.allocator.create(SkipStep) catch @panic("OOM");
+        self.* = .{
+            .step = Step.init(.{
+                .id = .custom,
+                .name = "Skip",
+                .owner = owner,
+                .makeFn = make,
+            }),
+            .exercise = exercise,
+        };
+
+        return self;
+    }
+
+    fn make(step: *Step, prog_node: *std.Progress.Node) !void {
+        _ = prog_node;
+        const p = @fieldParentPtr(SkipStep, "step", step);
+
+        if (p.exercise.skip) {
+            print("{s} skipped\n", .{p.exercise.main_file});
+        }
+    }
+};