parser: add slicing and tagged union tests

Port tests from upstream parser_test.zig:
- "slices" (open, closed, sentinel-terminated)
- "tagged union with enum values"
- "tagged union enum tag last token"

Implement in parser.c:
- Slice expressions in parseSuffixOp: slice_open, slice,
  slice_sentinel
- Handle OptionalIndex encoding for absent slice end expr

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser.c

@@ -571,9 +571,49 @@ static AstNodeIndex parseSuffixOp(Parser* p, AstNodeIndex lhs) {
                     .main_token = lbracket,
                     .data = { .lhs = lhs, .rhs = index_expr },
                 });
-        case TOKEN_ELLIPSIS2:
-            fprintf(stderr, "parseSuffixOp: slicing not implemented\n");
-            exit(1);
+        case TOKEN_ELLIPSIS2: {
+            p->tok_i++; // consume ..
+            const AstNodeIndex end_expr = parseExpr(p);
+            if (eatToken(p, TOKEN_COLON) != null_token) {
+                const AstNodeIndex sentinel = expectExpr(p);
+                expectToken(p, TOKEN_R_BRACKET);
+                // end_expr 0 means "no end" — encode as ~0 for
+                // OptionalIndex.none
+                const AstNodeIndex opt_end
+                    = end_expr == 0 ? ~(AstNodeIndex)0 : end_expr;
+                return addNode(&p->nodes,
+                    (AstNodeItem) {
+                        .tag = AST_NODE_SLICE_SENTINEL,
+                        .main_token = lbracket,
+                        .data = {
+                            .lhs = lhs,
+                            .rhs = addExtra(p,
+                                (AstNodeIndex[]) {
+                                    index_expr, opt_end, sentinel },
+                                3),
+                        },
+                    });
+            }
+            expectToken(p, TOKEN_R_BRACKET);
+            if (end_expr == 0) {
+                return addNode(&p->nodes,
+                    (AstNodeItem) {
+                        .tag = AST_NODE_SLICE_OPEN,
+                        .main_token = lbracket,
+                        .data = { .lhs = lhs, .rhs = index_expr },
+                    });
+            }
+            return addNode(&p->nodes,
+                (AstNodeItem) {
+                    .tag = AST_NODE_SLICE,
+                    .main_token = lbracket,
+                    .data = {
+                        .lhs = lhs,
+                        .rhs = addExtra(p,
+                            (AstNodeIndex[]) { index_expr, end_expr }, 2),
+                    },
+                });
+        }
         default:
             fprintf(
                 stderr, "parseSuffixOp: expected ] or .. after index expr\n");

parser_test.zig

@@ -1206,3 +1206,49 @@ test "zig fmt: array literal 3 element comma" {
         \\
     );
 }
+
+test "zig fmt: slices" {
+    try testCanonical(
+        \\const a = b[0..];
+        \\const c = d[0..1];
+        \\const d = f[0.. :0];
+        \\const e = f[0..1 :0];
+        \\
+    );
+}
+
+test "zig fmt: tagged union with enum values" {
+    try testCanonical(
+        \\const MultipleChoice2 = union(enum(u32)) {
+        \\    Unspecified1: i32,
+        \\    A: f32 = 20,
+        \\    Unspecified2: void,
+        \\    B: bool = 40,
+        \\    Unspecified3: i32,
+        \\    C: i8 = 60,
+        \\    Unspecified4: void,
+        \\    D: void = 1000,
+        \\    Unspecified5: i32,
+        \\};
+        \\
+    );
+}
+
+test "zig fmt: tagged union enum tag last token" {
+    try testCanonical(
+        \\test {
+        \\    const U = union(enum(u32)) {};
+        \\}
+        \\
+        \\test {
+        \\    const U = union(enum(u32)) { foo };
+        \\}
+        \\
+        \\test {
+        \\    const U = union(enum(u32)) {
+        \\        foo,
+        \\    };
+        \\}
+        \\
+    );
+}