Add MAX_UTF{8, 16}_LEN constants

library/alloc/src/string.rs

@@ -43,7 +43,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 #[cfg(not(no_global_oom_handling))]
-use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
+use core::char::{decode_utf16, MAX_UTF8_LEN, REPLACEMENT_CHARACTER};
 use core::fmt;
 use core::hash;
 #[cfg(not(no_global_oom_handling))]
@@ -1221,7 +1221,7 @@ impl String {
     pub fn push(&mut self, ch: char) {
         match ch.len_utf8() {
             1 => self.vec.push(ch as u8),
-            _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
+            _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; MAX_UTF8_LEN]).as_bytes()),
         }
     }
 
@@ -1528,7 +1528,7 @@ impl String {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn insert(&mut self, idx: usize, ch: char) {
         assert!(self.is_char_boundary(idx));
-        let mut bits = [0; 4];
+        let mut bits = [0; MAX_UTF8_LEN];
         let bits = ch.encode_utf8(&mut bits).as_bytes();
 
         unsafe {
@@ -2497,7 +2497,7 @@ impl<T: fmt::Display + ?Sized> ToString for T {
 impl ToString for char {
     #[inline]
     fn to_string(&self) -> String {
-        String::from(self.encode_utf8(&mut [0; 4]))
+        String::from(self.encode_utf8(&mut [0; MAX_UTF8_LEN]))
     }
 }
 

library/alloc/tests/str.rs

@@ -1217,7 +1217,7 @@ fn test_to_uppercase_rev_iterator() {
 #[test]
 #[cfg_attr(miri, ignore)] // Miri is too slow
 fn test_chars_decoding() {
-    let mut bytes = [0; 4];
+    let mut bytes = [0; std::char::MAX_UTF8_LEN];
     for c in (0..0x110000).filter_map(std::char::from_u32) {
         let s = c.encode_utf8(&mut bytes);
         if Some(c) != s.chars().next() {
@@ -1229,7 +1229,7 @@ fn test_chars_decoding() {
 #[test]
 #[cfg_attr(miri, ignore)] // Miri is too slow
 fn test_chars_rev_decoding() {
-    let mut bytes = [0; 4];
+    let mut bytes = [0; std::char::MAX_UTF8_LEN];
     for c in (0..0x110000).filter_map(std::char::from_u32) {
         let s = c.encode_utf8(&mut bytes);
         if Some(c) != s.chars().rev().next() {

library/core/src/char/methods.rs

@@ -45,6 +45,14 @@ impl char {
     #[stable(feature = "assoc_char_consts", since = "1.52.0")]
     pub const UNICODE_VERSION: (u8, u8, u8) = crate::unicode::UNICODE_VERSION;
 
+    /// Maximum length required to store a UTF8 byte.
+    #[stable(feature = "max_len", since = "1.63.0")]
+    pub const MAX_UTF8_LEN: usize = 4;
+
+    /// Maximum length required to store a UTF16 byte.
+    #[stable(feature = "max_len", since = "1.63.0")]
+    pub const MAX_UTF16_LEN: usize = 2;
+
     /// Creates an iterator over the UTF-16 encoded code points in `iter`,
     /// returning unpaired surrogates as `Err`s.
     ///

library/core/src/char/mod.rs

@@ -97,6 +97,14 @@ pub const REPLACEMENT_CHARACTER: char = char::REPLACEMENT_CHARACTER;
 #[stable(feature = "unicode_version", since = "1.45.0")]
 pub const UNICODE_VERSION: (u8, u8, u8) = char::UNICODE_VERSION;
 
+/// Maximum length required to store a UTF8 byte.
+#[stable(feature = "max_len", since = "1.63.0")]
+pub const MAX_UTF8_LEN: usize = char::MAX_UTF8_LEN;
+
+/// Maximum length required to store a UTF16 byte.
+#[stable(feature = "max_len", since = "1.63.0")]
+pub const MAX_UTF16_LEN: usize = char::MAX_UTF16_LEN;
+
 /// Creates an iterator over the UTF-16 encoded code points in `iter`, returning
 /// unpaired surrogates as `Err`s. Use [`char::decode_utf16`] instead.
 #[stable(feature = "decode_utf16", since = "1.9.0")]

library/core/src/fmt/mod.rs

@@ -3,7 +3,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cell::{Cell, Ref, RefCell, RefMut, SyncUnsafeCell, UnsafeCell};
-use crate::char::EscapeDebugExtArgs;
+use crate::char::{EscapeDebugExtArgs, MAX_UTF8_LEN};
 use crate::marker::PhantomData;
 use crate::mem;
 use crate::num::fmt as numfmt;
@@ -161,7 +161,7 @@ pub trait Write {
     /// ```
     #[stable(feature = "fmt_write_char", since = "1.1.0")]
     fn write_char(&mut self, c: char) -> Result {
-        self.write_str(c.encode_utf8(&mut [0; 4]))
+        self.write_str(c.encode_utf8(&mut [0; MAX_UTF8_LEN]))
     }
 
     /// Glue for usage of the [`write!`] macro with implementors of this trait.
@@ -2225,7 +2225,7 @@ impl Display for char {
         if f.width.is_none() && f.precision.is_none() {
             f.write_char(*self)
         } else {
-            f.pad(self.encode_utf8(&mut [0; 4]))
+            f.pad(self.encode_utf8(&mut [0; MAX_UTF8_LEN]))
         }
     }
 }

library/core/src/str/iter.rs

@@ -1410,7 +1410,7 @@ impl<'a> Iterator for EncodeUtf16<'a> {
             return Some(tmp);
         }
 
-        let mut buf = [0; 2];
+        let mut buf = [0; char::MAX_UTF16_LEN];
         self.chars.next().map(|ch| {
             let n = ch.encode_utf16(&mut buf).len();
             if n == 2 {

library/core/src/str/pattern.rs

@@ -38,6 +38,7 @@
     issue = "27721"
 )]
 
+use crate::char::MAX_UTF8_LEN;
 use crate::cmp;
 use crate::fmt;
 use crate::slice::memchr;
@@ -540,7 +541,7 @@ impl<'a> Pattern<'a> for char {
 
     #[inline]
     fn into_searcher(self, haystack: &'a str) -> Self::Searcher {
-        let mut utf8_encoded = [0; 4];
+        let mut utf8_encoded = [0; MAX_UTF8_LEN];
         let utf8_size = self.encode_utf8(&mut utf8_encoded).len();
         CharSearcher {
             haystack,

library/core/tests/char.rs

@@ -258,7 +258,7 @@ fn test_escape_unicode() {
 #[test]
 fn test_encode_utf8() {
     fn check(input: char, expect: &[u8]) {
-        let mut buf = [0; 4];
+        let mut buf = [0; char::MAX_UTF8_LEN];
         let ptr = buf.as_ptr();
         let s = input.encode_utf8(&mut buf);
         assert_eq!(s.as_ptr() as usize, ptr as usize);
@@ -275,7 +275,7 @@ fn test_encode_utf8() {
 #[test]
 fn test_encode_utf16() {
     fn check(input: char, expect: &[u16]) {
-        let mut buf = [0; 2];
+        let mut buf = [0; char::MAX_UTF16_LEN];
         let ptr = buf.as_mut_ptr();
         let b = input.encode_utf16(&mut buf);
         assert_eq!(b.as_mut_ptr() as usize, ptr as usize);

library/std/src/fs/tests.rs

@@ -175,7 +175,7 @@ fn file_test_io_non_positional_read() {
 #[test]
 fn file_test_io_seek_and_tell_smoke_test() {
     let message = "ten-four";
-    let mut read_mem = [0; 4];
+    let mut read_mem = [0; char::MAX_UTF8_LEN];
     let set_cursor = 4 as u64;
     let tell_pos_pre_read;
     let tell_pos_post_read;

library/std/src/sys/windows/stdio.rs

@@ -1,6 +1,6 @@
 #![unstable(issue = "none", feature = "windows_stdio")]
 
-use crate::char::decode_utf16;
+use crate::char::{decode_utf16, MAX_UTF8_LEN};
 use crate::cmp;
 use crate::io;
 use crate::os::windows::io::{FromRawHandle, IntoRawHandle};
@@ -27,7 +27,7 @@ pub struct Stderr {
 }
 
 struct IncompleteUtf8 {
-    bytes: [u8; 4],
+    bytes: [u8; MAX_UTF8_LEN],
     len: u8,
 }
 
@@ -377,7 +377,7 @@ fn utf16_to_utf8(utf16: &[u16], utf8: &mut [u8]) -> io::Result<usize> {
 
 impl IncompleteUtf8 {
     pub const fn new() -> IncompleteUtf8 {
-        IncompleteUtf8 { bytes: [0; 4], len: 0 }
+        IncompleteUtf8 { bytes: [0; MAX_UTF8_LEN], len: 0 }
     }
 }
 

library/std/src/sys_common/wtf8.rs

@@ -205,7 +205,7 @@ impl Wtf8Buf {
     /// Copied from String::push
     /// This does **not** include the WTF-8 concatenation check.
     fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
-        let mut bytes = [0; 4];
+        let mut bytes = [0; char::MAX_UTF8_LEN];
         let bytes = char::encode_utf8_raw(code_point.value, &mut bytes);
         self.bytes.extend_from_slice(bytes)
     }
@@ -878,7 +878,7 @@ impl<'a> Iterator for EncodeWide<'a> {
             return Some(tmp);
         }
 
-        let mut buf = [0; 2];
+        let mut buf = [0; char::MAX_UTF16_LEN];
         self.code_points.next().map(|code_point| {
             let n = char::encode_utf16_raw(code_point.value, &mut buf).len();
             if n == 2 {