FIX: Support uninitalized data in encode_utf8, and update try_push

We were using &mut [u8] in encode_utf8, but this is not right according
to the developing unsafe coding guidelines. We need to use raw pointers
to write to possibly uninit memory.

We use a raw pointer form for encode_utf8. It was first attempted to
encapsulate the trusted-to-be-valid raw pointer in a simple { *mut u8,
usize } struct, but the current way of passing ptr and len separately
was the only way to not regress performance.

This impl maintains the same performance in arraystring benches.

Add exhaustive-style tests for encode_utf8

src/array_string.rs

@@ -171,7 +171,9 @@ impl<A> ArrayString<A>
     pub fn try_push(&mut self, c: char) -> Result<(), CapacityError<char>> {
         let len = self.len();
         unsafe {
-            match encode_utf8(c, &mut self.raw_mut_bytes()[len..]) {
+            let ptr = self.xs.ptr_mut().add(len);
+            let remaining_cap = self.capacity() - len;
+            match encode_utf8(c, ptr, remaining_cap) {
                 Ok(n) => {
                     self.set_len(len + n);
                     Ok(())
@@ -346,11 +348,6 @@ impl<A> ArrayString<A>
     pub fn as_str(&self) -> &str {
         self
     }
-
-    /// Return a mutable slice of the whole string’s buffer
-    unsafe fn raw_mut_bytes(&mut self) -> &mut [u8] {
-        slice::from_raw_parts_mut(self.xs.ptr_mut(), self.capacity())
-    }
 }
 
 impl<A> Deref for ArrayString<A>

src/char.rs

@@ -10,6 +10,8 @@
 //
 // Original authors: alexchrichton, bluss
 
+use std::ptr;
+
 // UTF-8 ranges and tags for encoding characters
 const TAG_CONT: u8    = 0b1000_0000;
 const TAG_TWO_B: u8   = 0b1100_0000;
@@ -22,33 +24,75 @@ const MAX_THREE_B: u32 =  0x10000;
 /// Placeholder
 pub struct EncodeUtf8Error;
 
+#[inline]
+unsafe fn write(ptr: *mut u8, index: usize, byte: u8) {
+    ptr::write(ptr.add(index), byte)
+}
+
 /// Encode a char into buf using UTF-8.
 ///
 /// On success, return the byte length of the encoding (1, 2, 3 or 4).<br>
 /// On error, return `EncodeUtf8Error` if the buffer was too short for the char.
+///
+/// Safety: `ptr` must be writable for `len` bytes.
 #[inline]
-pub fn encode_utf8(ch: char, buf: &mut [u8]) -> Result<usize, EncodeUtf8Error>
+pub unsafe fn encode_utf8(ch: char, ptr: *mut u8, len: usize) -> Result<usize, EncodeUtf8Error>
 {
     let code = ch as u32;
-    if code < MAX_ONE_B && buf.len() >= 1 {
-        buf[0] = code as u8;
+    if code < MAX_ONE_B && len >= 1 {
+        write(ptr, 0, code as u8);
         return Ok(1);
-    } else if code < MAX_TWO_B && buf.len() >= 2 {
-        buf[0] = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
-        buf[1] = (code & 0x3F) as u8 | TAG_CONT;
+    } else if code < MAX_TWO_B && len >= 2 {
+        write(ptr, 0, (code >> 6 & 0x1F) as u8 | TAG_TWO_B);
+        write(ptr, 1, (code & 0x3F) as u8 | TAG_CONT);
         return Ok(2);
-    } else if code < MAX_THREE_B && buf.len() >= 3 {
-        buf[0] = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
-        buf[1] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
-        buf[2] = (code & 0x3F) as u8 | TAG_CONT;
+    } else if code < MAX_THREE_B && len >= 3 {
+        write(ptr, 0, (code >> 12 & 0x0F) as u8 | TAG_THREE_B);
+        write(ptr, 1, (code >>  6 & 0x3F) as u8 | TAG_CONT);
+        write(ptr, 2, (code & 0x3F) as u8 | TAG_CONT);
         return Ok(3);
-    } else if buf.len() >= 4 {
-        buf[0] = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
-        buf[1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
-        buf[2] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
-        buf[3] = (code & 0x3F) as u8 | TAG_CONT;
+    } else if len >= 4 {
+        write(ptr, 0, (code >> 18 & 0x07) as u8 | TAG_FOUR_B);
+        write(ptr, 1, (code >> 12 & 0x3F) as u8 | TAG_CONT);
+        write(ptr, 2, (code >>  6 & 0x3F) as u8 | TAG_CONT);
+        write(ptr, 3, (code & 0x3F) as u8 | TAG_CONT);
         return Ok(4);
     };
     Err(EncodeUtf8Error)
 }
 
+
+#[test]
+fn test_encode_utf8() {
+    // Test that all codepoints are encoded correctly
+    let mut data = [0u8; 16];
+    for codepoint in 0..=(std::char::MAX as u32) {
+        if let Some(ch) = std::char::from_u32(codepoint) {
+            for elt in &mut data { *elt = 0; }
+            let ptr = data.as_mut_ptr();
+            let len = data.len();
+            unsafe {
+                let res = encode_utf8(ch, ptr, len).ok().unwrap();
+                assert_eq!(res, ch.len_utf8());
+            }
+            let string = std::str::from_utf8(&data).unwrap();
+            assert_eq!(string.chars().next(), Some(ch));
+        }
+    }
+}
+
+#[test]
+fn test_encode_utf8_oob() {
+    // test that we report oob if the buffer is too short
+    let mut data = [0u8; 16];
+    let chars = ['a', 'α', '�', '𐍈'];
+    for (len, &ch) in (1..=4).zip(&chars) {
+        assert_eq!(len, ch.len_utf8(), "Len of ch={}", ch);
+        let ptr = data.as_mut_ptr();
+        unsafe {
+            assert!(matches::matches!(encode_utf8(ch, ptr, len - 1), Err(_)));
+            assert!(matches::matches!(encode_utf8(ch, ptr, len), Ok(_)));
+        }
+    }
+}
+