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diff --git a/xpcom/threads/StaticString.h b/xpcom/threads/StaticString.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef XPCOM_THREADS_STATICSTRING_H_
+#define XPCOM_THREADS_STATICSTRING_H_
+
+#include <cstddef>
+#include "mozilla/Attributes.h"
+
+// from "nsStringFwd.h"
+template <typename T>
+class nsTLiteralString;
+using nsLiteralCString = nsTLiteralString<char>;
+
+namespace mozilla {
+// class StaticString
+//
+// Wrapper type containing a C-style string which is guaranteed[*] to have been
+// created (potentially indirectly) from static data. Primarily intended for
+// text that may eventually be sent up via telemetry, to avoid the possibility
+// of accidentally exfiltrating PII.
+//
+// `StaticString`, like `template <size_t N> const char (&str)[N]`, can be
+// freely and implicitly converted to `const char *`, to simplify its use as a
+// drop-in replacement when detemplatizing functions.
+//
+// ### Comparison/contrast with `nsLiteralCString`
+//
+// Concretely, `StaticString` is smaller than `nsLiteralCString`, as it does not
+// store the string-length. It's also trivial to construct a `StaticString` from
+// the variable `__func__` or the preprocessor token `__FILE__`, which would
+// require additional work to be used with the latter's `_ns` constructor.
+//
+// Conventionally, the primary intended use case of `StaticString` is subtly
+// different from that of `nsLiteralCString`:
+//   * `StaticString` is intended for correctness (etc.) in contexts where the
+//     consumer of the string requires that it be static.
+//   * `nsLiteralCString` is more for efficiency in contexts where the source
+//     string _happens to be_ static, but in which the consumer does not care
+//     (and so accepts `nsACString const &` or similar).
+//
+// This is not a hard rule, however, and is notably bent in dom::Promise. (See
+// comments below.)
+//
+// Both are trivially-copyable/-movable/-destructible, guaranteed non-null, and
+// can only contain static data.
+//
+// #### Footnotes
+//
+// [*] ```
+//     CHORUS:   "What, never?"
+//     CAPTAIN:  "Well... hardly ever!"
+//     CHORUS:   "He's hardly ever sick of C!"
+//         -- Gilbert & Sullivan, _H.M.S. Pinafore_ (emended)
+// ```
+//
+class StaticString {
+  /* TODO(C++20): convert `constexpr` to `consteval` wherever possible. */
+  const char* mStr;  // guaranteed nonnull
+
+ public:
+  template <size_t N>
+  constexpr MOZ_IMPLICIT StaticString(const char (&str)[N]) : mStr(str) {}
+
+  // `nsLiteralCString` has the same guarantees as `StaticString` (both in being
+  // nonnull and containing only static data), so it's safe to construct either
+  // from the other.
+  //
+  // At present we only support construction of a `StaticString` from an
+  // `nsLiteralCString`, since this is zero-cost (the converse would not be),
+  // and is probably the simplest way to support dom::Promise's interoperation
+  // with MozPromise.
+  //
+  // (A more principled approach, in some sense, would be to create a third type
+  // `StaticStringWithLength` (or whatever) acting as the lattice-join of the
+  // two, which could then decay to either one as necessary. This is overkill
+  // for our current goals... but might be worthwhile if, _e.g._, you really
+  // need to get `__func__` into an `nsLiteralCString` rather than just an
+  // `nsDependentCString` for some reason.)
+  //
+  constexpr explicit StaticString(nsLiteralCString const& str);
+
+  constexpr StaticString(StaticString const&) = default;
+  constexpr StaticString(StaticString&&) = default;
+  ~StaticString() = default;
+
+  constexpr MOZ_IMPLICIT operator const char*() const { return mStr; }
+
+  // Not normally needed, but useful for variadic logging functions.
+  constexpr const char* get() const { return mStr; }
+};
+
+// Under the covers, StaticString is as lightweight as a single pointer: it does
+// not store the length of its deta.
+static_assert(sizeof(StaticString) == sizeof(const char*));
+static_assert(alignof(StaticString) == alignof(const char*));
+}  // namespace mozilla
+
+#endif