Add m-esr52 at 52.6.0

1 files changed, 853 insertions, 0 deletions

diff --git a/xpcom/rust/nsstring/src/lib.rs b/xpcom/rust/nsstring/src/lib.rs
new file mode 100644
index 000000000..cd518f3c5
--- /dev/null
+++ b/xpcom/rust/nsstring/src/lib.rs
@@ -0,0 +1,853 @@
+//! This module provides rust bindings for the XPCOM string types.
+//!
+//! # TL;DR (what types should I use)
+//!
+//! Use `&{mut,} nsA[C]String` for functions in rust which wish to take or
+//! mutate XPCOM strings. The other string types `Deref` to this type.
+//!
+//! Use `ns[C]String<'a>` for string struct members which don't leave rust, and
+//! as an intermediate between rust string data structures (such as `String`,
+//! `Vec<u16>`, `&str`, and `&[u16]`) and `&{mut,} nsA[C]String` (using
+//! `ns[C]String::from(value)`). These conversions, when possible, will not
+//! perform any allocations.
+//!
+//! Use `nsFixed[C]String` or `ns_auto_[c]string!` for dynamic stack allocated
+//! strings which are expected to hold short string values.
+//!
+//! Use `*{const,mut} nsA[C]String` (`{const,} nsA[C]String*` in C++) for
+//! function arguments passed across the rust/C++ language boundary.
+//!
+//! Use `ns[C]StringRepr` for string struct members which are shared between
+//! rust and C++, but be careful, because this type lacks a `Drop`
+//! implementation.
+//!
+//! # String Types
+//!
+//! ## `nsA[C]String`
+//!
+//! The core types in this module are `nsAString` and `nsACString`. These types
+//! are zero-sized as far as rust is concerned, and are safe to pass around
+//! behind both references (in rust code), and pointers (in C++ code). They
+//! represent a handle to a XPCOM string which holds either `u16` or `u8`
+//! characters respectively. The backing character buffer is guaranteed to live
+//! as long as the reference to the `nsAString` or `nsACString`.
+//!
+//! These types in rust are simply used as dummy types. References to them
+//! represent a pointer to the beginning of a variable-sized `#[repr(C)]` struct
+//! which is common between both C++ and Rust implementations. In C++, their
+//! corresponding types are also named `nsAString` or `nsACString`, and they are
+//! defined within the `nsTSubstring.{cpp,h}` file.
+//!
+//! ### Valid Operations
+//!
+//! An `&nsA[C]String` acts like rust's `&str`, in that it is a borrowed
+//! reference to the backing data. When used as an argument to other functions
+//! on `&mut nsA[C]String`, optimizations can be performed to avoid copying
+//! buffers, as information about the backing storage is preserved.
+//!
+//! An `&mut nsA[C]String` acts like rust's `&mut Cow<str>`, in that it is a
+//! mutable reference to a potentially borrowed string, which when modified will
+//! ensure that it owns its own backing storage. This type can be appended to
+//! with the methods `.append`, `.append_utf{8,16}`, and with the `write!`
+//! macro, and can be assigned to with `.assign`.
+//!
+//! ## `ns[C]String<'a>`
+//!
+//! This type is an maybe-owned string type. It acts similarially to a
+//! `Cow<[{u8,u16}]>`. This type provides `Deref` and `DerefMut` implementations
+//! to `nsA[C]String`, which provides the methods for manipulating this type.
+//! This type's lifetime parameter, `'a`, represents the lifetime of the backing
+//! storage. When modified this type may re-allocate in order to ensure that it
+//! does not mutate its backing storage.
+//!
+//! `ns[C]String`s can be constructed either with `ns[C]String::new()`, which
+//! creates an empty `ns[C]String<'static>`, or through one of the provided
+//! `From` implementations. Both string types may be constructed `From<&'a
+//! str>`, with `nsCString` having a `'a` lifetime, as the storage is shared
+//! with the `str`, while `nsString` has a `'static` lifetime, as its storage
+//! has to be transcoded.
+//!
+//! When passing this type by reference, prefer passing a `&nsA[C]String` or
+//! `&mut nsA[C]String`. to passing this type.
+//!
+//! This type is _not_ `#[repr(C)]`, as it has a `Drop` impl, which in versions
+//! of `rustc < 1.13` adds drop flags to the struct, which messes up the layout,
+//! making it unsafe to pass across the FFI boundary. The rust compiler will
+//! warn if this type appears in `extern "C"` function definitions.
+//!
+//! When passing this type across the language boundary, pass it as `*const
+//! nsA[C]String` for an immutable reference, or `*mut nsA[C]String` for a
+//! mutable reference.
+//!
+//! This type is similar to the C++ type of the same name.
+//!
+//! ## `nsFixed[C]String<'a>`
+//!
+//! This type is a string type with fixed backing storage. It is created with
+//! `nsFixed[C]String::new(buffer)`, passing a mutable reference to a buffer as
+//! the argument. This buffer will be used as backing storage whenever the
+//! resulting string will fit within it, falling back to heap allocations only
+//! when the string size exceeds that of the backing buffer.
+//!
+//! Like `ns[C]String`, this type dereferences to `nsA[C]String` which provides
+//! the methods for manipulating the type, and is not `#[repr(C)]`.
+//!
+//! When passing this type by reference, prefer passing a `&nsA[C]String` or
+//! `&mut nsA[C]String`. to passing this type.
+//!
+//! This type is _not_ `#[repr(C)]`, as it has a `Drop` impl, which in versions
+//! of `rustc < 1.13` adds drop flags to the struct, which messes up the layout,
+//! making it unsafe to pass across the FFI boundary. The rust compiler will
+//! warn if this type appears in `extern "C"` function definitions.
+//!
+//! When passing this type across the language boundary, pass it as `*const
+//! nsA[C]String` for an immutable reference, or `*mut nsA[C]String` for a
+//! mutable reference.
+//!
+//! This type is similar to the C++ type of the same name.
+//!
+//! ## `ns_auto_[c]string!($name)`
+//!
+//! This is a helper macro which defines a fixed size, (currently 64 character),
+//! backing array on the stack, and defines a local variable with name `$name`
+//! which is a `nsFixed[C]String` using this buffer as its backing storage.
+//!
+//! Usage of this macro is similar to the C++ type `nsAuto[C]String`, but could
+//! not be implemented as a basic type due to the differences between rust and
+//! C++'s move semantics.
+//!
+//! ## `ns[C]StringRepr`
+//!
+//! This type represents a C++ `ns[C]String`. This type is `#[repr(C)]` and is
+//! safe to use in struct definitions which are shared across the language
+//! boundary. It automatically dereferences to `&{mut,} nsA[C]String`, and thus
+//! can be treated similarially to `ns[C]String`.
+//!
+//! If this type is dropped in rust, it will not free its backing storage. This
+//! is because types implementing `Drop` have a drop flag added, which messes up
+//! the layout of this type. When drop flags are removed, which should happen in
+//! `rustc 1.13` (see rust-lang/rust#35764), this type will likely be removed,
+//! and replaced with direct usage of `ns[C]String<'a>`, as its layout may be
+//! identical. This module provides rust bindings to our xpcom ns[C]String
+//! types.
+
+#![allow(non_camel_case_types)]
+
+use std::ops::{Deref, DerefMut};
+use std::marker::PhantomData;
+use std::slice;
+use std::ptr;
+use std::mem;
+use std::fmt;
+use std::cmp;
+use std::str;
+use std::u32;
+
+//////////////////////////////////
+// Internal Implemenation Flags //
+//////////////////////////////////
+
+const F_NONE: u32 = 0; // no flags
+
+// data flags are in the lower 16-bits
+const F_TERMINATED: u32 = 1 << 0; // IsTerminated returns true
+const F_VOIDED: u32 = 1 << 1; // IsVoid returns true
+const F_SHARED: u32 = 1 << 2; // mData points to a heap-allocated, shared buffer
+const F_OWNED: u32 = 1 << 3; // mData points to a heap-allocated, raw buffer
+const F_FIXED: u32 = 1 << 4; // mData points to a fixed-size writable, dependent buffer
+const F_LITERAL: u32 = 1 << 5; // mData points to a string literal; F_TERMINATED will also be set
+
+// class flags are in the upper 16-bits
+const F_CLASS_FIXED: u32 = 1 << 16; // indicates that |this| is of type nsTFixedString
+
+////////////////////////////////////
+// Generic String Bindings Macros //
+////////////////////////////////////
+
+macro_rules! define_string_types {
+    {
+        char_t = $char_t: ty;
+        AString = $AString: ident;
+        String = $String: ident;
+        FixedString = $FixedString: ident;
+
+        StringRepr = $StringRepr: ident;
+        FixedStringRepr = $FixedStringRepr: ident;
+        AutoStringRepr = $AutoStringRepr: ident;
+    } => {
+        /// The representation of a ns[C]String type in C++. This type is
+        /// used internally by our definition of ns[C]String to ensure layout
+        /// compatibility with the C++ ns[C]String type.
+        ///
+        /// This type may also be used in place of a C++ ns[C]String inside of
+        /// struct definitions which are shared with C++, as it has identical
+        /// layout to our ns[C]String type. Due to drop flags, our ns[C]String
+        /// type does not have identical layout. When drop flags are removed,
+        /// this type will likely be made a private implementation detail, and
+        /// its uses will be replaced with `ns[C]String`.
+        ///
+        /// This struct will leak its data if dropped from rust. See the module
+        /// documentation for more information on this type.
+        #[repr(C)]
+        pub struct $StringRepr {
+            data: *const $char_t,
+            length: u32,
+            flags: u32,
+        }
+
+        impl Deref for $StringRepr {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                unsafe {
+                    mem::transmute(self)
+                }
+            }
+        }
+
+        impl DerefMut for $StringRepr {
+            fn deref_mut(&mut self) -> &mut $AString {
+                unsafe {
+                    mem::transmute(self)
+                }
+            }
+        }
+
+        /// The representation of a nsFixed[C]String type in C++. This type is
+        /// used internally by our definition of nsFixed[C]String to ensure layout
+        /// compatibility with the C++ nsFixed[C]String type.
+        #[repr(C)]
+        struct $FixedStringRepr {
+            base: $StringRepr,
+            capacity: u32,
+            buffer: *mut $char_t,
+        }
+
+        /// This type is the abstract type which is used for interacting with
+        /// strings in rust. Each string type can derefence to an instance of
+        /// this type, which provides the useful operations on strings.
+        ///
+        /// NOTE: Rust thinks this type has a size of 0, because the data
+        /// associated with it is not necessarially safe to move. It is not safe
+        /// to construct a nsAString yourself, unless it is received by
+        /// dereferencing one of these types.
+        ///
+        /// NOTE: The `[u8; 0]` member is zero sized, and only exists to prevent
+        /// the construction by code outside of this module. It is used instead
+        /// of a private `()` member because the `improper_ctypes` lint complains
+        /// about some ZST members in `extern "C"` function declarations.
+        #[repr(C)]
+        pub struct $AString {
+            _prohibit_constructor: [u8; 0],
+        }
+
+        impl Deref for $AString {
+            type Target = [$char_t];
+            fn deref(&self) -> &[$char_t] {
+                unsafe {
+                    // This is legal, as all $AString values actually point to a
+                    // $StringRepr
+                    let this: &$StringRepr = mem::transmute(self);
+                    if this.data.is_null() {
+                        debug_assert!(this.length == 0);
+                        // Use an arbitrary non-null value as the pointer
+                        slice::from_raw_parts(0x1 as *const $char_t, 0)
+                    } else {
+                        slice::from_raw_parts(this.data, this.length as usize)
+                    }
+                }
+            }
+        }
+
+        impl cmp::PartialEq for $AString {
+            fn eq(&self, other: &$AString) -> bool {
+                &self[..] == &other[..]
+            }
+        }
+
+        impl cmp::PartialEq<[$char_t]> for $AString {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                &self[..] == other
+            }
+        }
+
+        impl<'a> cmp::PartialEq<$String<'a>> for $AString {
+            fn eq(&self, other: &$String<'a>) -> bool {
+                self.eq(&**other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<$FixedString<'a>> for $AString {
+            fn eq(&self, other: &$FixedString<'a>) -> bool {
+                self.eq(&**other)
+            }
+        }
+
+        pub struct $String<'a> {
+            hdr: $StringRepr,
+            _marker: PhantomData<&'a [$char_t]>,
+        }
+
+        impl $String<'static> {
+            pub fn new() -> $String<'static> {
+                $String {
+                    hdr: $StringRepr {
+                        data: ptr::null(),
+                        length: 0,
+                        flags: F_NONE,
+                    },
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl<'a> Deref for $String<'a> {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                &self.hdr
+            }
+        }
+
+        impl<'a> DerefMut for $String<'a> {
+            fn deref_mut(&mut self) -> &mut $AString {
+                &mut self.hdr
+            }
+        }
+
+        impl<'a> From<&'a String> for $String<'a> {
+            fn from(s: &'a String) -> $String<'a> {
+                $String::from(&s[..])
+            }
+        }
+
+        impl<'a> From<&'a Vec<$char_t>> for $String<'a> {
+            fn from(s: &'a Vec<$char_t>) -> $String<'a> {
+                $String::from(&s[..])
+            }
+        }
+
+        impl<'a> From<&'a [$char_t]> for $String<'a> {
+            fn from(s: &'a [$char_t]) -> $String<'a> {
+                assert!(s.len() < (u32::MAX as usize));
+                $String {
+                    hdr: $StringRepr {
+                        data: s.as_ptr(),
+                        length: s.len() as u32,
+                        flags: F_NONE,
+                    },
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl From<Box<[$char_t]>> for $String<'static> {
+            fn from(s: Box<[$char_t]>) -> $String<'static> {
+                assert!(s.len() < (u32::MAX as usize));
+                // SAFETY NOTE: This method produces an F_OWNED ns[C]String from
+                // a Box<[$char_t]>. this is only safe because in the Gecko
+                // tree, we use the same allocator for Rust code as for C++
+                // code, meaning that our box can be legally freed with
+                // libc::free().
+                let length = s.len() as u32;
+                let ptr = s.as_ptr();
+                mem::forget(s);
+                $String {
+                    hdr: $StringRepr {
+                        data: ptr,
+                        length: length,
+                        flags: F_OWNED,
+                    },
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl From<Vec<$char_t>> for $String<'static> {
+            fn from(s: Vec<$char_t>) -> $String<'static> {
+                s.into_boxed_slice().into()
+            }
+        }
+
+        impl<'a> From<&'a $AString> for $String<'static> {
+            fn from(s: &'a $AString) -> $String<'static> {
+                let mut string = $String::new();
+                string.assign(s);
+                string
+            }
+        }
+
+        impl<'a> fmt::Write for $String<'a> {
+            fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+                $AString::write_str(self, s)
+            }
+        }
+
+        impl<'a> fmt::Display for $String<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Display>::fmt(self, f)
+            }
+        }
+
+        impl<'a> fmt::Debug for $String<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Debug>::fmt(self, f)
+            }
+        }
+
+        impl<'a> cmp::PartialEq for $String<'a> {
+            fn eq(&self, other: &$String<'a>) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<[$char_t]> for $String<'a> {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b [$char_t]> for $String<'a> {
+            fn eq(&self, other: &&'b [$char_t]) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<str> for $String<'a> {
+            fn eq(&self, other: &str) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b str> for $String<'a> {
+            fn eq(&self, other: &&'b str) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl<'a> Drop for $String<'a> {
+            fn drop(&mut self) {
+                unsafe {
+                    self.finalize();
+                }
+            }
+        }
+
+        /// A nsFixed[C]String is a string which uses a fixed size mutable
+        /// backing buffer for storing strings which will fit within that
+        /// buffer, rather than using heap allocations.
+        pub struct $FixedString<'a> {
+            hdr: $FixedStringRepr,
+            _marker: PhantomData<&'a mut [$char_t]>,
+        }
+
+        impl<'a> $FixedString<'a> {
+            pub fn new(buf: &'a mut [$char_t]) -> $FixedString<'a> {
+                let len = buf.len();
+                assert!(len < (u32::MAX as usize));
+                let buf_ptr = buf.as_mut_ptr();
+                $FixedString {
+                    hdr: $FixedStringRepr {
+                        base: $StringRepr {
+                            data: ptr::null(),
+                            length: 0,
+                            flags: F_CLASS_FIXED,
+                        },
+                        capacity: len as u32,
+                        buffer: buf_ptr,
+                    },
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl<'a> Deref for $FixedString<'a> {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                &self.hdr.base
+            }
+        }
+
+        impl<'a> DerefMut for $FixedString<'a> {
+            fn deref_mut(&mut self) -> &mut $AString {
+                &mut self.hdr.base
+            }
+        }
+
+        impl<'a> fmt::Write for $FixedString<'a> {
+            fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+                $AString::write_str(self, s)
+            }
+        }
+
+        impl<'a> fmt::Display for $FixedString<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Display>::fmt(self, f)
+            }
+        }
+
+        impl<'a> fmt::Debug for $FixedString<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Debug>::fmt(self, f)
+            }
+        }
+
+        impl<'a> cmp::PartialEq for $FixedString<'a> {
+            fn eq(&self, other: &$FixedString<'a>) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<[$char_t]> for $FixedString<'a> {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b [$char_t]> for $FixedString<'a> {
+            fn eq(&self, other: &&'b [$char_t]) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<str> for $FixedString<'a> {
+            fn eq(&self, other: &str) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b str> for $FixedString<'a> {
+            fn eq(&self, other: &&'b str) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl<'a> Drop for $FixedString<'a> {
+            fn drop(&mut self) {
+                unsafe {
+                    self.finalize();
+                }
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////
+// Bindings for nsCString (u8 char type) //
+///////////////////////////////////////////
+
+define_string_types! {
+    char_t = u8;
+
+    AString = nsACString;
+    String = nsCString;
+    FixedString = nsFixedCString;
+
+    StringRepr = nsCStringRepr;
+    FixedStringRepr = nsFixedCStringRepr;
+    AutoStringRepr = nsAutoCStringRepr;
+}
+
+impl nsACString {
+    /// Leaves the nsACString in an unstable state with a dangling data pointer.
+    /// Should only be used in drop implementations of rust types which wrap
+    /// this type.
+    unsafe fn finalize(&mut self) {
+        Gecko_FinalizeCString(self);
+    }
+
+    pub fn assign(&mut self, other: &nsACString) {
+        unsafe {
+            Gecko_AssignCString(self as *mut _, other as *const _);
+        }
+    }
+
+    pub fn assign_utf16(&mut self, other: &nsAString) {
+        self.assign(&nsCString::new());
+        self.append_utf16(other);
+    }
+
+    pub fn append(&mut self, other: &nsACString) {
+        unsafe {
+            Gecko_AppendCString(self as *mut _, other as *const _);
+        }
+    }
+
+    pub fn append_utf16(&mut self, other: &nsAString) {
+        unsafe {
+            Gecko_AppendUTF16toCString(self as *mut _, other as *const _);
+        }
+    }
+
+    pub unsafe fn as_str_unchecked(&self) -> &str {
+        str::from_utf8_unchecked(self)
+    }
+}
+
+impl<'a> From<&'a str> for nsCString<'a> {
+    fn from(s: &'a str) -> nsCString<'a> {
+        s.as_bytes().into()
+    }
+}
+
+impl From<Box<str>> for nsCString<'static> {
+    fn from(s: Box<str>) -> nsCString<'static> {
+        s.into_string().into()
+    }
+}
+
+impl From<String> for nsCString<'static> {
+    fn from(s: String) -> nsCString<'static> {
+        s.into_bytes().into()
+    }
+}
+
+// Support for the write!() macro for appending to nsACStrings
+impl fmt::Write for nsACString {
+    fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+        self.append(&nsCString::from(s));
+        Ok(())
+    }
+}
+
+impl fmt::Display for nsACString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Display::fmt(&String::from_utf8_lossy(&self[..]), f)
+    }
+}
+
+impl fmt::Debug for nsACString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Debug::fmt(&String::from_utf8_lossy(&self[..]), f)
+    }
+}
+
+impl cmp::PartialEq<str> for nsACString {
+    fn eq(&self, other: &str) -> bool {
+        &self[..] == other.as_bytes()
+    }
+}
+
+#[macro_export]
+macro_rules! ns_auto_cstring {
+    ($name:ident) => {
+        let mut buf: [u8; 64] = [0; 64];
+        let mut $name = $crate::nsFixedCString::new(&mut buf);
+    }
+}
+
+///////////////////////////////////////////
+// Bindings for nsString (u16 char type) //
+///////////////////////////////////////////
+
+define_string_types! {
+    char_t = u16;
+
+    AString = nsAString;
+    String = nsString;
+    FixedString = nsFixedString;
+
+    StringRepr = nsStringRepr;
+    FixedStringRepr = nsFixedStringRepr;
+    AutoStringRepr = nsAutoStringRepr;
+}
+
+impl nsAString {
+    /// Leaves the nsAString in an unstable state with a dangling data pointer.
+    /// Should only be used in drop implementations of rust types which wrap
+    /// this type.
+    unsafe fn finalize(&mut self) {
+        Gecko_FinalizeString(self);
+    }
+
+    pub fn assign(&mut self, other: &nsAString) {
+        unsafe {
+            Gecko_AssignString(self as *mut _, other as *const _);
+        }
+    }
+
+    pub fn assign_utf8(&mut self, other: &nsACString) {
+        self.assign(&nsString::new());
+        self.append_utf8(other);
+    }
+
+    pub fn append(&mut self, other: &nsAString) {
+        unsafe {
+            Gecko_AppendString(self as *mut _, other as *const _);
+        }
+    }
+
+    pub fn append_utf8(&mut self, other: &nsACString) {
+        unsafe {
+            Gecko_AppendUTF8toString(self as *mut _, other as *const _);
+        }
+    }
+}
+
+// NOTE: The From impl for a string slice for nsString produces a <'static>
+// lifetime, as it allocates.
+impl<'a> From<&'a str> for nsString<'static> {
+    fn from(s: &'a str) -> nsString<'static> {
+        s.encode_utf16().collect::<Vec<u16>>().into()
+    }
+}
+
+// Support for the write!() macro for writing to nsStrings
+impl fmt::Write for nsAString {
+    fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+        // Directly invoke gecko's routines for appending utf8 strings to
+        // nsAString values, to avoid as much overhead as possible
+        self.append_utf8(&nsCString::from(s));
+        Ok(())
+    }
+}
+
+impl fmt::Display for nsAString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Display::fmt(&String::from_utf16_lossy(&self[..]), f)
+    }
+}
+
+impl fmt::Debug for nsAString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Debug::fmt(&String::from_utf16_lossy(&self[..]), f)
+    }
+}
+
+impl cmp::PartialEq<str> for nsAString {
+    fn eq(&self, other: &str) -> bool {
+        other.encode_utf16().eq(self.iter().cloned())
+    }
+}
+
+#[macro_export]
+macro_rules! ns_auto_string {
+    ($name:ident) => {
+        let mut buf: [u16; 64] = [0; 64];
+        let mut $name = $crate::nsFixedString::new(&mut buf);
+    }
+}
+
+// NOTE: These bindings currently only expose infallible operations. Perhaps
+// consider allowing for fallible methods?
+extern "C" {
+    // Gecko implementation in nsSubstring.cpp
+    fn Gecko_FinalizeCString(this: *mut nsACString);
+    fn Gecko_AssignCString(this: *mut nsACString, other: *const nsACString);
+    fn Gecko_AppendCString(this: *mut nsACString, other: *const nsACString);
+
+    fn Gecko_FinalizeString(this: *mut nsAString);
+    fn Gecko_AssignString(this: *mut nsAString, other: *const nsAString);
+    fn Gecko_AppendString(this: *mut nsAString, other: *const nsAString);
+
+    // Gecko implementation in nsReadableUtils.cpp
+    fn Gecko_AppendUTF16toCString(this: *mut nsACString, other: *const nsAString);
+    fn Gecko_AppendUTF8toString(this: *mut nsAString, other: *const nsACString);
+}
+
+//////////////////////////////////////
+// Repr Validation Helper Functions //
+//////////////////////////////////////
+
+pub mod test_helpers {
+    //! This module only exists to help with ensuring that the layout of the
+    //! structs inside of rust and C++ are identical.
+    //!
+    //! It is public to ensure that these testing functions are avaliable to
+    //! gtest code.
+
+    use super::{
+        nsFixedCStringRepr,
+        nsFixedStringRepr,
+        nsCStringRepr,
+        nsStringRepr,
+        F_NONE,
+        F_TERMINATED,
+        F_VOIDED,
+        F_SHARED,
+        F_OWNED,
+        F_FIXED,
+        F_LITERAL,
+        F_CLASS_FIXED,
+    };
+    use std::mem;
+
+    /// Generates an #[no_mangle] extern "C" function which returns the size and
+    /// alignment of the given type with the given name.
+    macro_rules! size_align_check {
+        ($T:ty, $fname:ident) => {
+            #[no_mangle]
+            #[allow(non_snake_case)]
+            pub extern fn $fname(size: *mut usize, align: *mut usize) {
+                unsafe {
+                    *size = mem::size_of::<$T>();
+                    *align = mem::align_of::<$T>();
+                }
+            }
+        }
+    }
+
+    size_align_check!(nsStringRepr, Rust_Test_ReprSizeAlign_nsString);
+    size_align_check!(nsCStringRepr, Rust_Test_ReprSizeAlign_nsCString);
+    size_align_check!(nsFixedStringRepr, Rust_Test_ReprSizeAlign_nsFixedString);
+    size_align_check!(nsFixedCStringRepr, Rust_Test_ReprSizeAlign_nsFixedCString);
+
+    /// Generates a $[no_mangle] extern "C" function which returns the size,
+    /// alignment and offset in the parent struct of a given member, with the
+    /// given name.
+    ///
+    /// This method can trigger Undefined Behavior if the accessing the member
+    /// $member on a given type would use that type's `Deref` implementation.
+    macro_rules! member_check {
+        ($T:ty, $member:ident, $method:ident) => {
+            #[no_mangle]
+            #[allow(non_snake_case)]
+            pub extern fn $method(size: *mut usize,
+                                  align: *mut usize,
+                                  offset: *mut usize) {
+                unsafe {
+                    // Create a temporary value of type T to get offsets, sizes
+                    // and aligns off of
+                    let tmp: $T = mem::zeroed();
+                    *size = mem::size_of_val(&tmp.$member);
+                    *align = mem::align_of_val(&tmp.$member);
+                    *offset =
+                        (&tmp.$member as *const _ as usize) -
+                        (&tmp as *const _ as usize);
+                    mem::forget(tmp);
+                }
+            }
+        }
+    }
+
+    member_check!(nsStringRepr, data, Rust_Test_Member_nsString_mData);
+    member_check!(nsStringRepr, length, Rust_Test_Member_nsString_mLength);
+    member_check!(nsStringRepr, flags, Rust_Test_Member_nsString_mFlags);
+    member_check!(nsCStringRepr, data, Rust_Test_Member_nsCString_mData);
+    member_check!(nsCStringRepr, length, Rust_Test_Member_nsCString_mLength);
+    member_check!(nsCStringRepr, flags, Rust_Test_Member_nsCString_mFlags);
+    member_check!(nsFixedStringRepr, capacity, Rust_Test_Member_nsFixedString_mFixedCapacity);
+    member_check!(nsFixedStringRepr, buffer, Rust_Test_Member_nsFixedString_mFixedBuf);
+    member_check!(nsFixedCStringRepr, capacity, Rust_Test_Member_nsFixedCString_mFixedCapacity);
+    member_check!(nsFixedCStringRepr, buffer, Rust_Test_Member_nsFixedCString_mFixedBuf);
+
+    #[no_mangle]
+    #[allow(non_snake_case)]
+    pub extern fn Rust_Test_NsStringFlags(f_none: *mut u32,
+                                          f_terminated: *mut u32,
+                                          f_voided: *mut u32,
+                                          f_shared: *mut u32,
+                                          f_owned: *mut u32,
+                                          f_fixed: *mut u32,
+                                          f_literal: *mut u32,
+                                          f_class_fixed: *mut u32) {
+        unsafe {
+            *f_none = F_NONE;
+            *f_terminated = F_TERMINATED;
+            *f_voided = F_VOIDED;
+            *f_shared = F_SHARED;
+            *f_owned = F_OWNED;
+            *f_fixed = F_FIXED;
+            *f_literal = F_LITERAL;
+            *f_class_fixed = F_CLASS_FIXED;
+        }
+    }
+}