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Auto merge of #134969 - Marcondiro:master, r=jhpratt,programmerjake

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char to_digit: avoid unnecessary casts to u64

Hello,
in the `char::to_digit` method there are a few `as u64` casts that are not strictly necessary.
I assume that the reason behind these casts is to avoid possible overflows in the `+ 10` add.

This PR removes the aforementioned casts, avoiding the overflow issue by slightly modifying the ASCII letter to int mapping.

Thanks,
Happy new year.
This commit is contained in:
bors 2025-01-01 10:54:12 +00:00
commit eeeff9a66c
1 changed files with 11 additions and 7 deletions
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18
library/core/src/char/methods.rs
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@ -394,17 +394,21 @@ impl char {
); );
// check radix to remove letter handling code when radix is a known constant // check radix to remove letter handling code when radix is a known constant
let value = if self > '9' && radix > 10 { let value = if self > '9' && radix > 10 {
// convert ASCII letters to lowercase // mask to convert ASCII letters to uppercase
let lower = self as u32 | 0x20; const TO_UPPERCASE_MASK: u32 = !0b0010_0000;
// convert an ASCII letter to the corresponding value, // Converts an ASCII letter to its corresponding integer value:
// non-letters convert to values > 36 // A-Z => 10-35, a-z => 10-35. Other characters produce values >= 36.
lower.wrapping_sub('a' as u32) as u64 + 10 //
// Add Overflow Safety:
// By applying the mask after the subtraction, the first addendum is
// constrained such that it never exceeds u32::MAX - 0x20.
((self as u32).wrapping_sub('A' as u32) & TO_UPPERCASE_MASK) + 10
} else { } else {
// convert digit to value, non-digits wrap to values > 36 // convert digit to value, non-digits wrap to values > 36
(self as u32).wrapping_sub('0' as u32) as u64 (self as u32).wrapping_sub('0' as u32)
}; };
// FIXME(const-hack): once then_some is const fn, use it here // FIXME(const-hack): once then_some is const fn, use it here
if value < radix as u64 { Some(value as u32) } else { None } if value < radix { Some(value) } else { None }
} }
/// Returns an iterator that yields the hexadecimal Unicode escape of a /// Returns an iterator that yields the hexadecimal Unicode escape of a