Test char formatting, replace mem::uninitialized with MaybeUninit

tests/vs-std-write.rs

@@ -364,3 +364,13 @@ fn hex() {
 
     // <i8 as std::fmt::Display>::fmt(-128)
 }
+
+#[test]
+fn char() {
+    // Miri is slow, so step over the range of valid chars sparsely
+    let step = if cfg!(miri) { 1 << 16 } else { 1 };
+
+    for c in ('\0'..=char::MAX).step_by(step) {
+        cmp!("{}", c);
+    }
+}

write/src/lib.rs

@@ -9,10 +9,7 @@
 #[cfg(feature = "std")]
 use core::convert::Infallible;
 
-#[allow(deprecated)]
-unsafe fn uninitialized<T>() -> T {
-    core::mem::uninitialized()
-}
+use core::mem::MaybeUninit;
 
 /// A collection of methods that are required / used to format a message into a stream.
 #[allow(non_camel_case_types)]
@@ -32,8 +29,9 @@ pub trait uWrite {
     /// entire byte sequence was successfully written, and this method will not return until all
     /// data has been written or an error occurs.
     fn write_char(&mut self, c: char) -> Result<(), Self::Error> {
-        let mut buf: [u8; 4] = unsafe { uninitialized() };
-        self.write_str(c.encode_utf8(&mut buf))
+        let mut buf: MaybeUninit<[u8; 4]> = MaybeUninit::uninit();
+        let encoded = unsafe { encode_utf8_raw(c as u32, &mut buf) };
+        self.write_str(encoded)
     }
 }
 
@@ -46,3 +44,62 @@ impl uWrite for String {
         Ok(())
     }
 }
+
+#[inline]
+fn encode_utf8_raw(code: u32, dst: &mut MaybeUninit<[u8; 4]>) -> &str {
+    let len = len_utf8(code);
+
+    unsafe {
+        let dst = dst.as_mut_ptr();
+        let a = dst as *mut u8;
+        let b = a.add(1);
+        let c = a.add(2);
+        let d = a.add(3);
+        match len {
+            1 => {
+                *a = code as u8;
+            }
+            2 => {
+                *a = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
+                *b = (code & 0x3F) as u8 | TAG_CONT;
+            }
+            3 => {
+                *a = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
+                *b = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+                *c = (code & 0x3F) as u8 | TAG_CONT;
+            }
+            4 => {
+                *a = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
+                *b = (code >> 12 & 0x3F) as u8 | TAG_CONT;
+                *c = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+                *d = (code & 0x3F) as u8 | TAG_CONT;
+            }
+            _ => unreachable!(),
+        };
+
+        let bytes = core::slice::from_raw_parts(dst as *const u8, len);
+        core::str::from_utf8_unchecked(bytes)
+    }
+}
+
+#[inline]
+fn len_utf8(code: u32) -> usize {
+    if code < MAX_ONE_B {
+        1
+    } else if code < MAX_TWO_B {
+        2
+    } else if code < MAX_THREE_B {
+        3
+    } else {
+        4
+    }
+}
+
+// UTF-8 ranges and tags for encoding characters
+const TAG_CONT: u8 = 0b1000_0000;
+const TAG_TWO_B: u8 = 0b1100_0000;
+const TAG_THREE_B: u8 = 0b1110_0000;
+const TAG_FOUR_B: u8 = 0b1111_0000;
+const MAX_ONE_B: u32 = 0x80;
+const MAX_TWO_B: u32 = 0x800;
+const MAX_THREE_B: u32 = 0x10000;