From aa685bcccaa8fdb187e68de29f35db11aa99630f Mon Sep 17 00:00:00 2001
From: Marcondiro <cavenati.marco@gmail.com>
Date: Tue, 31 Dec 2024 15:57:46 +0100
Subject: [PATCH] char to_digit: avoid unnecessary casts to u64
---
library/core/src/char/methods.rs | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index 7d33765879f..fb8a740aced 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -394,17 +394,21 @@ impl char {
);
// check radix to remove letter handling code when radix is a known constant
let value = if self > '9' && radix > 10 {
- // convert ASCII letters to lowercase
- let lower = self as u32 | 0x20;
- // convert an ASCII letter to the corresponding value,
- // non-letters convert to values > 36
- lower.wrapping_sub('a' as u32) as u64 + 10
+ // mask to convert ASCII letters to uppercase
+ const TO_UPPERCASE_MASK: u32 = !0b0010_0000;
+ // Converts an ASCII letter to its corresponding integer value:
+ // A-Z => 10-35, a-z => 10-35. Other characters produce values >= 36.
+ //
+ // 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 {
// 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
- 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