Add allocator param to String

This is a third attempt at implementing adding Allocator support to
the std lib's `String`.  Still stuck on the same issue with type
inference failing on the newly generic `String<A>`, but I opted to do
even less than the previous WIP work, and have added no new functions
(`String<A>` can be constructed via `Vec<u8, A>` or `Box<str, A>`),
and have moved only the struct definition to its own mod to make
rebasing a bit easier if/when main changes underneath me.

Author: shua

library/alloc/src/boxed.rs

@@ -211,6 +211,8 @@ use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::raw_vec::RawVec;
 #[cfg(not(no_global_oom_handling))]
 use crate::str::from_boxed_utf8_unchecked;
+#[cfg(not(no_global_oom_handling))]
+use crate::vec::Vec;
 
 /// Conversion related impls for `Box<_>` (`From`, `downcast`, etc)
 mod convert;
@@ -1917,11 +1919,13 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
-impl Clone for Box<str> {
+impl<A: Allocator + Clone> Clone for Box<str, A> {
     fn clone(&self) -> Self {
-        // this makes a copy of the data
-        let buf: Box<[u8]> = self.as_bytes().into();
-        unsafe { from_boxed_utf8_unchecked(buf) }
+        let alloc = Box::allocator(self).clone();
+        let len = self.len();
+        let mut vec: Vec<u8, A> = Vec::with_capacity_in(len, alloc);
+        vec.extend_from_slice(self.as_bytes());
+        unsafe { from_boxed_utf8_unchecked(vec.into_boxed_slice()) }
     }
 }
 

library/alloc/src/str.rs

@@ -7,6 +7,7 @@
 // It's cleaner to just turn off the unused_imports warning than to fix them.
 #![allow(unused_imports)]
 
+use core::alloc::Allocator;
 use core::borrow::{Borrow, BorrowMut};
 use core::iter::FusedIterator;
 use core::mem::MaybeUninit;
@@ -52,7 +53,7 @@ use core::{mem, ptr};
 use crate::borrow::ToOwned;
 use crate::boxed::Box;
 use crate::slice::{Concat, Join, SliceIndex};
-use crate::string::String;
+use crate::string::generic::String;
 use crate::vec::Vec;
 
 /// Note: `str` in `Concat<str>` is not meaningful here.
@@ -186,15 +187,15 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl Borrow<str> for String {
+impl<A: Allocator> Borrow<str> for String<A> {
     #[inline]
     fn borrow(&self) -> &str {
         &self[..]
     }
 }
 
 #[stable(feature = "string_borrow_mut", since = "1.36.0")]
-impl BorrowMut<str> for String {
+impl<A: Allocator> BorrowMut<str> for String<A> {
     #[inline]
     fn borrow_mut(&mut self) -> &mut str {
         &mut self[..]
@@ -234,7 +235,7 @@ impl str {
     #[stable(feature = "str_box_extras", since = "1.20.0")]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
-    pub fn into_boxed_bytes(self: Box<Self>) -> Box<[u8]> {
+    pub fn into_boxed_bytes<A: Allocator>(self: Box<Self, A>) -> Box<[u8], A> {
         self.into()
     }
 
@@ -497,8 +498,8 @@ impl str {
     #[rustc_allow_incoherent_impl]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
-    pub fn into_string(self: Box<Self>) -> String {
-        let slice = Box::<[u8]>::from(self);
+    pub fn into_string<A: Allocator>(self: Box<Self, A>) -> String<A> {
+        let slice = Box::<[u8], A>::from(self);
         unsafe { String::from_utf8_unchecked(slice.into_vec()) }
     }
 
@@ -614,8 +615,9 @@ impl str {
 #[stable(feature = "str_box_extras", since = "1.20.0")]
 #[must_use]
 #[inline]
-pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
-    unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }
+pub unsafe fn from_boxed_utf8_unchecked<A: Allocator>(v: Box<[u8], A>) -> Box<str, A> {
+    let (ptr, alloc) = Box::into_raw_with_allocator(v);
+    unsafe { Box::from_raw_in(ptr as *mut str, alloc) }
 }
 
 /// Converts leading ascii bytes in `s` by calling the `convert` function.