Add 'must_use' attribute to 'char::len_utf8' and 'char::len_utf16';

Author: bjoernager

library/core/src/char/methods.rs

@@ -606,6 +606,7 @@ impl char {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_char_len_utf", since = "1.52.0")]
     #[inline]
+    #[must_use]
     pub const fn len_utf8(self) -> usize {
         len_utf8(self as u32)
     }
@@ -637,6 +638,7 @@ impl char {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_char_len_utf", since = "1.52.0")]
     #[inline]
+    #[must_use]
     pub const fn len_utf16(self) -> usize {
         len_utf16(self as u32)
     }
@@ -1738,6 +1740,7 @@ impl EscapeDebugExtArgs {
 }
 
 #[inline]
+#[must_use]
 const fn len_utf8(code: u32) -> usize {
     match code {
         ..MAX_ONE_B => 1,
@@ -1748,6 +1751,7 @@ const fn len_utf8(code: u32) -> usize {
 }
 
 #[inline]
+#[must_use]
 const fn len_utf16(code: u32) -> usize {
     if (code & 0xFFFF) == code { 1 } else { 2 }
 }