cli: config structure supports tagged unions

The syntax of tagged unions is `tag:value`. This matches the tagged
union parsing syntax for keybindings (i.e. `new_split:right`).

I'm adding this now on its own without a user-facing feature because
I can see some places we might use this and I want to separate this out.
There is already a PR open now that can utilize this (#2231).

src/cli/args.zig

@@ -268,7 +268,13 @@ fn parseIntoField(
                         value orelse return error.ValueRequired,
                     ),
 
-                    else => unreachable,
+                    .Union => try parseTaggedUnion(
+                        Field,
+                        alloc,
+                        value orelse return error.ValueRequired,
+                    ),
+
+                    else => @compileError("unsupported field type"),
                 },
             };
 
@@ -279,6 +285,44 @@ fn parseIntoField(
     return error.InvalidField;
 }
 
+fn parseTaggedUnion(comptime T: type, alloc: Allocator, v: []const u8) !T {
+    const info = @typeInfo(T).Union;
+    assert(@typeInfo(info.tag_type.?) == .Enum);
+
+    // Get the union tag that is being set.
+    const colon_idx = mem.indexOf(u8, v, ":") orelse return error.InvalidValue;
+    const tag_str = std.mem.trim(u8, v[0..colon_idx], whitespace);
+
+    // Find the field in the union that matches the tag.
+    inline for (info.fields) |field| {
+        if (mem.eql(u8, field.name, tag_str)) {
+            // We need to create a struct that looks like this union field.
+            // This lets us use parseIntoField as if its a dedicated struct.
+            const Target = @Type(.{ .Struct = .{
+                .layout = .auto,
+                .fields = &.{.{
+                    .name = field.name,
+                    .type = field.type,
+                    .default_value = null,
+                    .is_comptime = false,
+                    .alignment = @alignOf(field.type),
+                }},
+                .decls = &.{},
+                .is_tuple = false,
+            } });
+
+            // Parse the value into the struct
+            var t: Target = undefined;
+            try parseIntoField(Target, alloc, &t, field.name, v[colon_idx + 1 ..]);
+
+            // Build our union
+            return @unionInit(T, field.name, @field(t, field.name));
+        }
+    }
+
+    return error.InvalidValue;
+}
+
 fn parsePackedStruct(comptime T: type, v: []const u8) !T {
     const info = @typeInfo(T).Struct;
     assert(info.layout == .@"packed");
@@ -742,6 +786,89 @@ test "parseIntoField: struct with parse func with unsupported error tracking" {
     );
 }
 
+test "parseIntoField: tagged union" {
+    const testing = std.testing;
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+
+    var data: struct {
+        value: union(enum) {
+            a: u8,
+            b: u8,
+        } = undefined,
+    } = .{};
+
+    // Set one field
+    try parseIntoField(@TypeOf(data), alloc, &data, "value", "a:1");
+    try testing.expectEqual(1, data.value.a);
+
+    // Set another
+    try parseIntoField(@TypeOf(data), alloc, &data, "value", "b:2");
+    try testing.expectEqual(2, data.value.b);
+}
+
+test "parseIntoField: tagged union unknown filed" {
+    const testing = std.testing;
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+
+    var data: struct {
+        value: union(enum) {
+            a: u8,
+            b: u8,
+        } = undefined,
+    } = .{};
+
+    try testing.expectError(
+        error.InvalidValue,
+        parseIntoField(@TypeOf(data), alloc, &data, "value", "c:1"),
+    );
+}
+
+test "parseIntoField: tagged union invalid field value" {
+    const testing = std.testing;
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+
+    var data: struct {
+        value: union(enum) {
+            a: u8,
+            b: u8,
+        } = undefined,
+    } = .{};
+
+    try testing.expectError(
+        error.InvalidValue,
+        parseIntoField(@TypeOf(data), alloc, &data, "value", "a:hello"),
+    );
+}
+
+test "parseIntoField: tagged union missing tag" {
+    const testing = std.testing;
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+
+    var data: struct {
+        value: union(enum) {
+            a: u8,
+            b: u8,
+        } = undefined,
+    } = .{};
+
+    try testing.expectError(
+        error.InvalidValue,
+        parseIntoField(@TypeOf(data), alloc, &data, "value", "a"),
+    );
+    try testing.expectError(
+        error.InvalidValue,
+        parseIntoField(@TypeOf(data), alloc, &data, "value", ":a"),
+    );
+}
+
 /// Returns an iterator (implements "next") that reads CLI args by line.
 /// Each CLI arg is expected to be a single line. This is used to implement
 /// configuration files.