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mammothbanemammothbane
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liballoc: introduce String, Vec const-slicing
This change `const`-qualifies many methods on Vec and String, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice` with the following tracking issue: rust-lang#129041
1 parent 506f22b commit f07b0f9

8 files changed

+111
-133
lines changed

‎library/alloc/src/lib.rs

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -114,6 +114,9 @@
114114
#![feature(const_option)]
115115
#![feature(const_pin)]
116116
#![feature(const_size_of_val)]
117+
#![feature(const_slice_from_raw_parts_mut)]
118+
#![feature(const_str_from_utf8_unchecked_mut)]
119+
#![feature(const_vec_string_slice)]
117120
#![feature(core_intrinsics)]
118121
#![feature(deprecated_suggestion)]
119122
#![feature(deref_pure_trait)]

‎library/alloc/src/raw_vec.rs

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -280,7 +280,7 @@ impl<T, A: Allocator> RawVec<T, A> {
280280
/// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
281281
/// be careful.
282282
#[inline]
283-
pub fn ptr(&self) -> *mut T {
283+
pub const fn ptr(&self) -> *mut T {
284284
self.inner.ptr()
285285
}
286286

@@ -293,7 +293,7 @@ impl<T, A: Allocator> RawVec<T, A> {
293293
///
294294
/// This will always be `usize::MAX` if `T` is zero-sized.
295295
#[inline]
296-
pub fn capacity(&self) -> usize {
296+
pub const fn capacity(&self) -> usize {
297297
self.inner.capacity(size_of::<T>())
298298
}
299299

@@ -488,8 +488,8 @@ impl<A: Allocator> RawVecInner<A> {
488488
}
489489

490490
#[inline]
491-
fn ptr<T>(&self) -> *mut T {
492-
self.non_null::<T>().as_ptr()
491+
const fn ptr<T>(&self) -> *mut T {
492+
self.ptr.as_ptr() as _
493493
}
494494

495495
#[inline]
@@ -498,7 +498,7 @@ impl<A: Allocator> RawVecInner<A> {
498498
}
499499

500500
#[inline]
501-
fn capacity(&self, elem_size: usize) -> usize {
501+
const fn capacity(&self, elem_size: usize) -> usize {
502502
if elem_size == 0 { usize::MAX } else { self.cap.0 }
503503
}
504504

‎library/alloc/src/string.rs

Lines changed: 21 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -1059,7 +1059,8 @@ impl String {
10591059
#[inline]
10601060
#[must_use = "`self` will be dropped if the result is not used"]
10611061
#[stable(feature = "rust1", since = "1.0.0")]
1062-
pub fn into_bytes(self) -> Vec<u8> {
1062+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1063+
pub const fn into_bytes(self) -> Vec<u8> {
10631064
self.vec
10641065
}
10651066

@@ -1076,8 +1077,9 @@ impl String {
10761077
#[must_use]
10771078
#[stable(feature = "string_as_str", since = "1.7.0")]
10781079
#[cfg_attr(not(test), rustc_diagnostic_item = "string_as_str")]
1079-
pub fn as_str(&self) -> &str {
1080-
self
1080+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1081+
pub const fn as_str(&self) -> &str {
1082+
unsafe { str::from_utf8_unchecked(self.vec.as_slice()) }
10811083
}
10821084

10831085
/// Converts a `String` into a mutable string slice.
@@ -1096,8 +1098,9 @@ impl String {
10961098
#[must_use]
10971099
#[stable(feature = "string_as_str", since = "1.7.0")]
10981100
#[cfg_attr(not(test), rustc_diagnostic_item = "string_as_mut_str")]
1099-
pub fn as_mut_str(&mut self) -> &mut str {
1100-
self
1101+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1102+
pub const fn as_mut_str(&mut self) -> &mut str {
1103+
unsafe { str::from_utf8_unchecked_mut(self.vec.as_mut_slice()) }
11011104
}
11021105

11031106
/// Appends a given string slice onto the end of this `String`.
@@ -1168,7 +1171,8 @@ impl String {
11681171
#[inline]
11691172
#[must_use]
11701173
#[stable(feature = "rust1", since = "1.0.0")]
1171-
pub fn capacity(&self) -> usize {
1174+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1175+
pub const fn capacity(&self) -> usize {
11721176
self.vec.capacity()
11731177
}
11741178

@@ -1431,8 +1435,9 @@ impl String {
14311435
#[inline]
14321436
#[must_use]
14331437
#[stable(feature = "rust1", since = "1.0.0")]
1434-
pub fn as_bytes(&self) -> &[u8] {
1435-
&self.vec
1438+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1439+
pub const fn as_bytes(&self) -> &[u8] {
1440+
self.vec.as_slice()
14361441
}
14371442

14381443
/// Shortens this `String` to the specified length.
@@ -1784,7 +1789,8 @@ impl String {
17841789
/// ```
17851790
#[inline]
17861791
#[stable(feature = "rust1", since = "1.0.0")]
1787-
pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
1792+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1793+
pub const unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
17881794
&mut self.vec
17891795
}
17901796

@@ -1805,8 +1811,9 @@ impl String {
18051811
#[inline]
18061812
#[must_use]
18071813
#[stable(feature = "rust1", since = "1.0.0")]
1814+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
18081815
#[rustc_confusables("length", "size")]
1809-
pub fn len(&self) -> usize {
1816+
pub const fn len(&self) -> usize {
18101817
self.vec.len()
18111818
}
18121819

@@ -1824,7 +1831,8 @@ impl String {
18241831
#[inline]
18251832
#[must_use]
18261833
#[stable(feature = "rust1", since = "1.0.0")]
1827-
pub fn is_empty(&self) -> bool {
1834+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1835+
pub const fn is_empty(&self) -> bool {
18281836
self.len() == 0
18291837
}
18301838

@@ -2565,7 +2573,7 @@ impl ops::Deref for String {
25652573

25662574
#[inline]
25672575
fn deref(&self) -> &str {
2568-
unsafe { str::from_utf8_unchecked(&self.vec) }
2576+
self.as_str()
25692577
}
25702578
}
25712579

@@ -2576,7 +2584,7 @@ unsafe impl ops::DerefPure for String {}
25762584
impl ops::DerefMut for String {
25772585
#[inline]
25782586
fn deref_mut(&mut self) -> &mut str {
2579-
unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2587+
self.as_mut_str()
25802588
}
25812589
}
25822590

‎library/alloc/src/vec/mod.rs

Lines changed: 18 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -1240,7 +1240,8 @@ impl<T, A: Allocator> Vec<T, A> {
12401240
/// ```
12411241
#[inline]
12421242
#[stable(feature = "rust1", since = "1.0.0")]
1243-
pub fn capacity(&self) -> usize {
1243+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1244+
pub const fn capacity(&self) -> usize {
12441245
self.buf.capacity()
12451246
}
12461247

@@ -1548,8 +1549,9 @@ impl<T, A: Allocator> Vec<T, A> {
15481549
#[inline]
15491550
#[stable(feature = "vec_as_slice", since = "1.7.0")]
15501551
#[cfg_attr(not(test), rustc_diagnostic_item = "vec_as_slice")]
1551-
pub fn as_slice(&self) -> &[T] {
1552-
self
1552+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1553+
pub const fn as_slice(&self) -> &[T] {
1554+
unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
15531555
}
15541556

15551557
/// Extracts a mutable slice of the entire vector.
@@ -1566,8 +1568,9 @@ impl<T, A: Allocator> Vec<T, A> {
15661568
#[inline]
15671569
#[stable(feature = "vec_as_slice", since = "1.7.0")]
15681570
#[cfg_attr(not(test), rustc_diagnostic_item = "vec_as_mut_slice")]
1569-
pub fn as_mut_slice(&mut self) -> &mut [T] {
1570-
self
1571+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
1572+
pub const fn as_mut_slice(&mut self) -> &mut [T] {
1573+
unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
15711574
}
15721575

15731576
/// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
@@ -1622,9 +1625,10 @@ impl<T, A: Allocator> Vec<T, A> {
16221625
/// [`as_mut_ptr`]: Vec::as_mut_ptr
16231626
/// [`as_ptr`]: Vec::as_ptr
16241627
#[stable(feature = "vec_as_ptr", since = "1.37.0")]
1628+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
16251629
#[rustc_never_returns_null_ptr]
16261630
#[inline]
1627-
pub fn as_ptr(&self) -> *const T {
1631+
pub const fn as_ptr(&self) -> *const T {
16281632
// We shadow the slice method of the same name to avoid going through
16291633
// `deref`, which creates an intermediate reference.
16301634
self.buf.ptr()
@@ -1681,9 +1685,10 @@ impl<T, A: Allocator> Vec<T, A> {
16811685
/// [`as_mut_ptr`]: Vec::as_mut_ptr
16821686
/// [`as_ptr`]: Vec::as_ptr
16831687
#[stable(feature = "vec_as_ptr", since = "1.37.0")]
1688+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
16841689
#[rustc_never_returns_null_ptr]
16851690
#[inline]
1686-
pub fn as_mut_ptr(&mut self) -> *mut T {
1691+
pub const fn as_mut_ptr(&mut self) -> *mut T {
16871692
// We shadow the slice method of the same name to avoid going through
16881693
// `deref_mut`, which creates an intermediate reference.
16891694
self.buf.ptr()
@@ -2561,8 +2566,9 @@ impl<T, A: Allocator> Vec<T, A> {
25612566
/// ```
25622567
#[inline]
25632568
#[stable(feature = "rust1", since = "1.0.0")]
2569+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
25642570
#[rustc_confusables("length", "size")]
2565-
pub fn len(&self) -> usize {
2571+
pub const fn len(&self) -> usize {
25662572
self.len
25672573
}
25682574

@@ -2579,7 +2585,8 @@ impl<T, A: Allocator> Vec<T, A> {
25792585
/// ```
25802586
#[stable(feature = "rust1", since = "1.0.0")]
25812587
#[cfg_attr(not(test), rustc_diagnostic_item = "vec_is_empty")]
2582-
pub fn is_empty(&self) -> bool {
2588+
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
2589+
pub const fn is_empty(&self) -> bool {
25832590
self.len() == 0
25842591
}
25852592

@@ -3130,15 +3137,15 @@ impl<T, A: Allocator> ops::Deref for Vec<T, A> {
31303137

31313138
#[inline]
31323139
fn deref(&self) -> &[T] {
3133-
unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
3140+
self.as_slice()
31343141
}
31353142
}
31363143

31373144
#[stable(feature = "rust1", since = "1.0.0")]
31383145
impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
31393146
#[inline]
31403147
fn deref_mut(&mut self) -> &mut [T] {
3141-
unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
3148+
self.as_mut_slice()
31423149
}
31433150
}
31443151

‎tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir

Lines changed: 31 additions & 47 deletions
Original file line numberDiff line numberDiff line change
@@ -5,66 +5,48 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
55
let mut _0: &[u8];
66
scope 1 (inlined <Vec<u8> as Deref>::deref) {
77
debug self => _1;
8-
let mut _6: usize;
9-
scope 2 (inlined Vec::<u8>::as_ptr) {
8+
scope 2 (inlined Vec::<u8>::as_slice) {
109
debug self => _1;
11-
let mut _2: &alloc::raw_vec::RawVec<u8>;
12-
scope 3 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
13-
debug self => _2;
14-
let mut _3: &alloc::raw_vec::RawVecInner;
15-
scope 4 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
16-
debug self => _3;
17-
scope 5 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
10+
let mut _6: usize;
11+
scope 3 (inlined Vec::<u8>::as_ptr) {
12+
debug self => _1;
13+
let mut _2: &alloc::raw_vec::RawVec<u8>;
14+
scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
15+
debug self => _2;
16+
let mut _3: &alloc::raw_vec::RawVecInner;
17+
scope 5 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
1818
debug self => _3;
1919
let mut _4: std::ptr::NonNull<u8>;
20-
scope 6 (inlined Unique::<u8>::cast::<u8>) {
20+
scope 6 (inlined Unique::<u8>::as_ptr) {
2121
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
2222
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
23-
scope 7 (inlined NonNull::<u8>::cast::<u8>) {
23+
scope 7 (inlined NonNull::<u8>::as_ptr) {
2424
debug self => _4;
25-
scope 8 (inlined NonNull::<u8>::as_ptr) {
26-
debug self => _4;
27-
let mut _5: *const u8;
28-
}
29-
}
30-
}
31-
scope 9 (inlined #[track_caller] <Unique<u8> as Into<NonNull<u8>>>::into) {
32-
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
33-
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
34-
scope 10 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
35-
debug ((unique: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
36-
debug ((unique: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
37-
scope 11 (inlined Unique::<u8>::as_non_null_ptr) {
38-
debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
39-
debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
40-
}
25+
let mut _5: *const u8;
4126
}
4227
}
4328
}
44-
scope 12 (inlined NonNull::<u8>::as_ptr) {
45-
debug self => _4;
46-
}
47-
}
48-
}
49-
}
50-
scope 13 (inlined std::slice::from_raw_parts::<'_, u8>) {
51-
debug data => _5;
52-
debug len => _6;
53-
let _7: *const [u8];
54-
scope 14 (inlined core::ub_checks::check_language_ub) {
55-
scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
5629
}
5730
}
58-
scope 16 (inlined std::mem::size_of::<u8>) {
59-
}
60-
scope 17 (inlined align_of::<u8>) {
61-
}
62-
scope 18 (inlined slice_from_raw_parts::<u8>) {
31+
scope 8 (inlined std::slice::from_raw_parts::<'_, u8>) {
6332
debug data => _5;
6433
debug len => _6;
65-
scope 19 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
66-
debug data_pointer => _5;
67-
debug metadata => _6;
34+
let _7: *const [u8];
35+
scope 9 (inlined core::ub_checks::check_language_ub) {
36+
scope 10 (inlined core::ub_checks::check_language_ub::runtime) {
37+
}
38+
}
39+
scope 11 (inlined std::mem::size_of::<u8>) {
40+
}
41+
scope 12 (inlined align_of::<u8>) {
42+
}
43+
scope 13 (inlined slice_from_raw_parts::<u8>) {
44+
debug data => _5;
45+
debug len => _6;
46+
scope 14 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
47+
debug data_pointer => _5;
48+
debug metadata => _6;
49+
}
6850
}
6951
}
7052
}
@@ -75,8 +57,10 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
7557
_2 = &((*_1).0: alloc::raw_vec::RawVec<u8>);
7658
StorageLive(_3);
7759
_3 = &(((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner);
60+
StorageLive(_4);
7861
_4 = copy (((((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner).0: std::ptr::Unique<u8>).0: std::ptr::NonNull<u8>);
7962
_5 = copy (_4.0: *const u8);
63+
StorageDead(_4);
8064
StorageDead(_3);
8165
StorageDead(_2);
8266
StorageLive(_6);

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