Reduce number of allocations for serializing error messages

src/string_builder.zig

@@ -0,0 +1,32 @@
+usingnamespace @import("string_types.zig");
+const Allocator = @import("std").mem.Allocator;
+const assert = @import("std").debug.assert;
+const copy = @import("std").mem.copy;
+
+const StringBuilder = @This();
+
+len: usize = 0,
+cap: usize = 0,
+ptr: ?[*]u8 = null,
+
+pub fn count(this: *StringBuilder, slice: string) void {
+    this.cap += slice.len;
+}
+
+pub fn allocate(this: *StringBuilder, allocator: *Allocator) !void {
+    var slice = try allocator.alloc(u8, this.cap);
+    this.ptr = slice.ptr;
+    this.len = 0;
+}
+
+pub fn append(this: *StringBuilder, slice: string) string {
+    assert(this.len <= this.cap); // didn't count everything
+    assert(this.ptr != null); // must call allocate first
+
+    copy(u8, this.ptr.?[this.len..this.cap], slice);
+    const result = this.ptr.?[this.len..this.cap][0..slice.len];
+    this.len += slice.len;
+
+    assert(this.len <= this.cap);
+    return result;
+}