From 9c3d81e186c36091b1609db88cbec961c2fc49ab Mon Sep 17 00:00:00 2001
From: Giles Cope <gilescope@gmail.com>
Date: Thu, 10 Jun 2021 20:16:35 +0100
Subject: [PATCH] Further simplification of to_digit

---
 library/core/src/char/methods.rs | 24 +++++++++---------------
 library/core/src/lib.rs          |  1 +
 library/core/tests/char.rs       | 12 +++++++++++-
 3 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index b3e1becf570a..7f7a1c21e4a2 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -1,6 +1,5 @@
 //! impl char {}
 
-use crate::intrinsics::likely;
 use crate::slice;
 use crate::str::from_utf8_unchecked_mut;
 use crate::unicode::printable::is_printable;
@@ -332,21 +331,16 @@ impl char {
     #[inline]
     pub fn to_digit(self, radix: u32) -> Option<u32> {
         assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");
-        // the code is split up here to improve execution speed for cases where
-        // the `radix` is constant and 10 or smaller
-        let val = if likely(radix <= 10) {
-            // If not a digit, a number greater than radix will be created.
-            (self as u32).wrapping_sub('0' as u32)
-        } else {
-            match self {
-                '0'..='9' => self as u32 - '0' as u32,
-                'a'..='z' => self as u32 - 'a' as u32 + 10,
-                'A'..='Z' => self as u32 - 'A' as u32 + 10,
-                _ => return None,
+        // If not a digit, a number greater than radix will be created.
+        let mut digit = (self as u32).wrapping_sub('0' as u32);
+        if radix > 10 {
+            if digit < 10 {
+                return Some(digit);
             }
-        };
-
-        if val < radix { Some(val) } else { None }
+            // 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)
     }
 
     /// Returns an iterator that yields the hexadecimal Unicode escape of a
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index e10e1738de55..4353d2cda8a4 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -66,6 +66,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(arbitrary_self_types)]
 #![feature(asm)]
+#![feature(bool_to_option)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_heap)]
 #![feature(const_alloc_layout)]
diff --git a/library/core/tests/char.rs b/library/core/tests/char.rs
index c16f54081ce0..51eca1e05d34 100644
--- a/library/core/tests/char.rs
+++ b/library/core/tests/char.rs
@@ -67,10 +67,20 @@ fn test_to_digit() {
     assert_eq!('A'.to_digit(16), Some(10));
     assert_eq!('b'.to_digit(16), Some(11));
     assert_eq!('B'.to_digit(16), Some(11));
+    assert_eq!('A'.to_digit(36), Some(10));
     assert_eq!('z'.to_digit(36), Some(35));
     assert_eq!('Z'.to_digit(36), Some(35));
-    assert_eq!(' '.to_digit(10), None);
+    assert_eq!('['.to_digit(36), None);
+    assert_eq!('`'.to_digit(36), None);
+    assert_eq!('{'.to_digit(36), None);
     assert_eq!('$'.to_digit(36), None);
+    assert_eq!('@'.to_digit(16), None);
+    assert_eq!('G'.to_digit(16), None);
+    assert_eq!('g'.to_digit(16), None);
+    assert_eq!(' '.to_digit(10), None);
+    assert_eq!('/'.to_digit(10), None);
+    assert_eq!(':'.to_digit(10), None);
+    assert_eq!(':'.to_digit(11), None);
 }
 
 #[test]