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Rollup merge of rust-lang#89258 - est31:const_char_convert, r=oli-obk
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Make char conversion functions unstably const

The char conversion functions like `char::from_u32` do trivial computations and can easily be converted into const fns. Only smaller tricks are needed to avoid non-const standard library functions like `Result::ok` or `bool::then_some`.

Tracking issue: rust-lang#89259
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matthiaskrgr authored Nov 18, 2021
2 parents b6f580a + 7272b6f commit 0e39dca
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Showing 3 changed files with 33 additions and 15 deletions.
32 changes: 22 additions & 10 deletions library/core/src/char/convert.rs
Original file line number Diff line number Diff line change
Expand Up @@ -51,8 +51,13 @@ use super::MAX;
#[must_use]
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn from_u32(i: u32) -> Option<char> {
char::try_from(i).ok()
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
pub const fn from_u32(i: u32) -> Option<char> {
// FIXME: once Result::ok is const fn, use it here
match char_try_from_u32(i) {
Ok(c) => Some(c),
Err(_) => None,
}
}

/// Converts a `u32` to a `char`, ignoring validity.
Expand Down Expand Up @@ -91,7 +96,8 @@ pub fn from_u32(i: u32) -> Option<char> {
#[inline]
#[must_use]
#[stable(feature = "char_from_unchecked", since = "1.5.0")]
pub unsafe fn from_u32_unchecked(i: u32) -> char {
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
pub const unsafe fn from_u32_unchecked(i: u32) -> char {
// SAFETY: the caller must guarantee that `i` is a valid char value.
if cfg!(debug_assertions) { char::from_u32(i).unwrap() } else { unsafe { transmute(i) } }
}
Expand Down Expand Up @@ -248,18 +254,23 @@ impl FromStr for char {
}
}

#[inline]
const fn char_try_from_u32(i: u32) -> Result<char, CharTryFromError> {
if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
Err(CharTryFromError(()))
} else {
// SAFETY: checked that it's a legal unicode value
Ok(unsafe { transmute(i) })
}
}

#[stable(feature = "try_from", since = "1.34.0")]
impl TryFrom<u32> for char {
type Error = CharTryFromError;

#[inline]
fn try_from(i: u32) -> Result<Self, Self::Error> {
if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
Err(CharTryFromError(()))
} else {
// SAFETY: checked that it's a legal unicode value
Ok(unsafe { transmute(i) })
}
char_try_from_u32(i)
}
}

Expand Down Expand Up @@ -327,7 +338,8 @@ impl fmt::Display for CharTryFromError {
#[inline]
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn from_digit(num: u32, radix: u32) -> Option<char> {
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
pub const fn from_digit(num: u32, radix: u32) -> Option<char> {
if radix > 36 {
panic!("from_digit: radix is too high (maximum 36)");
}
Expand Down
15 changes: 10 additions & 5 deletions library/core/src/char/methods.rs
Original file line number Diff line number Diff line change
Expand Up @@ -136,9 +136,10 @@ impl char {
/// assert_eq!(None, c);
/// ```
#[stable(feature = "assoc_char_funcs", since = "1.52.0")]
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
#[must_use]
#[inline]
pub fn from_u32(i: u32) -> Option<char> {
pub const fn from_u32(i: u32) -> Option<char> {
super::convert::from_u32(i)
}

Expand Down Expand Up @@ -178,9 +179,10 @@ impl char {
/// assert_eq!('❤', c);
/// ```
#[stable(feature = "assoc_char_funcs", since = "1.52.0")]
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
#[must_use]
#[inline]
pub unsafe fn from_u32_unchecked(i: u32) -> char {
pub const unsafe fn from_u32_unchecked(i: u32) -> char {
// SAFETY: the safety contract must be upheld by the caller.
unsafe { super::convert::from_u32_unchecked(i) }
}
Expand Down Expand Up @@ -235,9 +237,10 @@ impl char {
/// let _c = char::from_digit(1, 37);
/// ```
#[stable(feature = "assoc_char_funcs", since = "1.52.0")]
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
#[must_use]
#[inline]
pub fn from_digit(num: u32, radix: u32) -> Option<char> {
pub const fn from_digit(num: u32, radix: u32) -> Option<char> {
super::convert::from_digit(num, radix)
}

Expand Down Expand Up @@ -331,10 +334,11 @@ impl char {
/// let _ = '1'.to_digit(37);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub fn to_digit(self, radix: u32) -> Option<u32> {
pub const fn to_digit(self, radix: u32) -> Option<u32> {
assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");
// If not a digit, a number greater than radix will be created.
let mut digit = (self as u32).wrapping_sub('0' as u32);
Expand All @@ -345,7 +349,8 @@ impl char {
// Force the 6th bit to be set to ensure ascii is lower case.
digit = (self as u32 | 0b10_0000).wrapping_sub('a' as u32).saturating_add(10);
}
(digit < radix).then_some(digit)
// FIXME: once then_some is const fn, use it here
if digit < radix { Some(digit) } else { None }
}

/// Returns an iterator that yields the hexadecimal Unicode escape of a
Expand Down
1 change: 1 addition & 0 deletions library/core/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,7 @@
#![feature(const_bigint_helper_methods)]
#![feature(const_caller_location)]
#![feature(const_cell_into_inner)]
#![feature(const_char_convert)]
#![feature(const_discriminant)]
#![feature(const_eval_select)]
#![feature(const_float_bits_conv)]
Expand Down

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