Auto merge of #132709 - programmerjake:optimize-charto_digit, r=joshtriplett

optimize char::to_digit and assert radix is at least 2
approved by t-libs: rust-lang/libs-team#475 (comment)
let me know if this needs an assembly test or similar.

Author: bors

library/core/src/char/methods.rs

@@ -301,7 +301,7 @@ impl char {
 ///
 /// # Panics
 ///
-/// Panics if given a radix larger than 36.
+/// Panics if given a radix smaller than 2 or larger than 36.
 ///
 /// # Examples
 ///
@@ -319,6 +319,13 @@ impl char {
 /// // this panics
 /// '1'.is_digit(37);
 /// ```
+///
+/// Passing a small radix, causing a panic:
+///
+/// ```should_panic
+/// // this panics
+/// '1'.is_digit(1);
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_char_classify", issue = "132241")]
 #[inline]
@@ -345,7 +352,7 @@ impl char {
 ///
 /// # Panics
 ///
-/// Panics if given a radix larger than 36.
+/// Panics if given a radix smaller than 2 or larger than 36.
 ///
 /// # Examples
 ///
@@ -369,24 +376,35 @@ impl char {
 /// // this panics
 /// let _ = '1'.to_digit(37);
 /// ```
+/// Passing a small radix, causing a panic:
+///
+/// ```should_panic
+/// // this panics
+/// let _ = '1'.to_digit(1);
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "const_char_convert", since = "1.67.0")]
 #[must_use = "this returns the result of the operation, \
  without modifying the original"]
 #[inline]
 pubconstfnto_digit(self,radix:u32) -> Option<u32>{
-// If not a digit, a number greater than radix will be created.
-letmut digit = (selfasu32).wrapping_sub('0'asu32);
-if radix > 10{
-assert!(radix <= 36,"to_digit: radix is too high (maximum 36)");
-if digit < 10{
-returnSome(digit);
-}
-// Force the 6th bit to be set to ensure ascii is lower case.
- digit = (selfasu32 | 0b10_0000).wrapping_sub('a'asu32).saturating_add(10);
-}
+assert!(
+ radix >= 2 && radix <= 36,
+"to_digit: invalid radix -- radix must be in the range 2 to 36 inclusive"
+);
+// check radix to remove letter handling code when radix is a known constant
+let value = ifself > '9' && radix > 10{
+// convert ASCII letters to lowercase
+let lower = selfasu32 | 0x20;
+// convert an ASCII letter to the corresponding value,
+// non-letters convert to values > 36
+ lower.wrapping_sub('a'asu32)asu64 + 10
+}else{
+// convert digit to value, non-digits wrap to values > 36
+(selfasu32).wrapping_sub('0'asu32)asu64
+};
 // FIXME(const-hack): once then_some is const fn, use it here
-ifdigit < radix {Some(digit)}else{None}
+ifvalue < radix asu64{Some(value asu32)}else{None}
 }
 
 /// Returns an iterator that yields the hexadecimal Unicode escape of a