add MAX_LEN_UTF8 and MAX_LEN_UTF16 constants

library/core/src/char/methods.rs

@@ -71,6 +71,16 @@ impl char {
     #[stable(feature = "assoc_char_consts", since = "1.52.0")]
     pub const MAX: char = '\u{10FFFF}';
 
+    /// The maximum number of bytes required to [encode](char::encode_utf8) a `char` to
+    /// UTF-8 encoding.
+    #[unstable(feature = "char_max_len", issue = "121714")]
+    pub const MAX_LEN_UTF8: usize = 4;
+
+    /// The maximum number of two-byte units required to [encode](char::encode_utf16) a `char`
+    /// to UTF-16 encoding.
+    #[unstable(feature = "char_max_len", issue = "121714")]
+    pub const MAX_LEN_UTF16: usize = 2;
+
     /// `U+FFFD REPLACEMENT CHARACTER` (<28>) is used in Unicode to represent a
     /// decoding error.
     ///

library/core/src/char/mod.rs

@@ -95,6 +95,16 @@ const MAX_THREE_B: u32 = 0x10000;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MAX: char = char::MAX;
 
+/// The maximum number of bytes required to [encode](char::encode_utf8) a `char` to
+/// UTF-8 encoding.
+#[unstable(feature = "char_max_len", issue = "121714")]
+pub const MAX_LEN_UTF8: usize = char::MAX_LEN_UTF8;
+
+/// The maximum number of two-byte units required to [encode](char::encode_utf16) a `char`
+/// to UTF-16 encoding.
+#[unstable(feature = "char_max_len", issue = "121714")]
+pub const MAX_LEN_UTF16: usize = char::MAX_LEN_UTF16;
+
 /// `U+FFFD REPLACEMENT CHARACTER` (<28>) is used in Unicode to represent a
 /// decoding error. Use [`char::REPLACEMENT_CHARACTER`] 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_LEN_UTF8};
 use crate::marker::PhantomData;
 use crate::num::fmt as numfmt;
 use crate::ops::Deref;
@@ -187,7 +187,7 @@ 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_LEN_UTF8]))
     }
 
     /// Glue for usage of the [`write!`] macro with implementors of this trait.
@@ -2768,7 +2768,7 @@ impl Display for char {
         if f.options.width.is_none() && f.options.precision.is_none() {
             f.write_char(*self)
         } else {
-            f.pad(self.encode_utf8(&mut [0; 4]))
+            f.pad(self.encode_utf8(&mut [0; MAX_LEN_UTF8]))
         }
     }
 }

library/core/src/str/pattern.rs

@@ -38,6 +38,7 @@
     issue = "27721"
 )]
 
+use crate::char::MAX_LEN_UTF8;
 use crate::cmp::Ordering;
 use crate::convert::TryInto as _;
 use crate::slice::memchr;
@@ -561,8 +562,8 @@ impl Pattern for char {
     type Searcher<'a> = CharSearcher<'a>;
 
     #[inline]
-    fn into_searcher(self, haystack: &str) -> Self::Searcher<'_> {
-        let mut utf8_encoded = [0; 4];
+    fn into_searcher<'a>(self, haystack: &'a str) -> Self::Searcher<'a> {
+        let mut utf8_encoded = [0; MAX_LEN_UTF8];
         let utf8_size = self
             .encode_utf8(&mut utf8_encoded)
             .len()