Implement From<char> for u32, and From<u8> for char

These fit with other From implementations between integer types.

This helps the coding style of avoiding the 'as' operator that sometimes
silently truncates, and signals that these specific conversions are
lossless and infaillible.

Author: SimonSapin

Diff for: src/libcore/char.rs

@@ -176,6 +176,22 @@ pub unsafe fn from_u32_unchecked(i: u32) -> char {
     transmute(i)
 }
 
+#[stable(feature = "char_convert", since = "1.12.0")]
+impl From<char> for u32 {
+    #[inline]
+    fn from(c: char) -> Self {
+        c as u32
+    }
+}
+
+#[stable(feature = "char_convert", since = "1.12.0")]
+impl From<u8> for char {
+    #[inline]
+    fn from(i: u8) -> Self {
+        i as char
+    }
+}
+
 /// Converts a digit in the given radix to a `char`.
 ///
 /// A 'radix' here is sometimes also called a 'base'. A radix of two

Diff for: src/libcoretest/char.rs

@@ -10,6 +10,14 @@
 
 use std::char;
 
+#[test]
+fn test_convert() {
+    assert_eq!(u32::from('a'), 0x61);
+    assert_eq!(char::from(b'\0'), '\0');
+    assert_eq!(char::from(b'a'), 'a');
+    assert_eq!(char::from(b'\xFF'), '\u{FF}');
+}
+
 #[test]
 fn test_is_lowercase() {
     assert!('a'.is_lowercase());