Auto merge of rust-lang#114299 - clarfonthey:char-min, r=dtolnay,BurntSushi

Add char::MIN

ACP: rust-lang/libs-team#252
Tracking issue: rust-lang#114298

r? `@rust-lang/libs-api`

Author: bors

library/core/src/char/methods.rs

@@ -9,16 +9,66 @@ use crate::unicode::{self, conversions};
 use super::*;
 
 impl char {
+    /// The lowest valid code point a `char` can have, `'\0'`.
+    ///
+    /// Unlike integer types, `char` actually has a gap in the middle,
+    /// meaning that the range of possible `char`s is smaller than you
+    /// might expect. Ranges of `char` will automatically hop this gap
+    /// for you:
+    ///
+    /// ```
+    /// #![feature(char_min)]
+    /// let dist = u32::from(char::MAX) - u32::from(char::MIN);
+    /// let size = (char::MIN..=char::MAX).count() as u32;
+    /// assert!(size < dist);
+    /// ```
+    ///
+    /// Despite this gap, the `MIN` and [`MAX`] values can be used as bounds for
+    /// all `char` values.
+    ///
+    /// [`MAX`]: char::MAX
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(char_min)]
+    /// # fn something_which_returns_char() -> char { 'a' }
+    /// let c: char = something_which_returns_char();
+    /// assert!(char::MIN <= c);
+    ///
+    /// let value_at_min = u32::from(char::MIN);
+    /// assert_eq!(char::from_u32(value_at_min), Some('\0'));
+    /// ```
+    #[unstable(feature = "char_min", issue = "114298")]
+    pub const MIN: char = '\0';
+
     /// The highest valid code point a `char` can have, `'\u{10FFFF}'`.
     ///
+    /// Unlike integer types, `char` actually has a gap in the middle,
+    /// meaning that the range of possible `char`s is smaller than you
+    /// might expect. Ranges of `char` will automatically hop this gap
+    /// for you:
+    ///
+    /// ```
+    /// #![feature(char_min)]
+    /// let dist = u32::from(char::MAX) - u32::from(char::MIN);
+    /// let size = (char::MIN..=char::MAX).count() as u32;
+    /// assert!(size < dist);
+    /// ```
+    ///
+    /// Despite this gap, the [`MIN`] and `MAX` values can be used as bounds for
+    /// all `char` values.
+    ///
+    /// [`MIN`]: char::MIN
+    ///
     /// # Examples
     ///
     /// ```
     /// # fn something_which_returns_char() -> char { 'a' }
     /// let c: char = something_which_returns_char();
     /// assert!(c <= char::MAX);
     ///
-    /// let value_at_max = char::MAX as u32;
+    /// let value_at_max = u32::from(char::MAX);
     /// assert_eq!(char::from_u32(value_at_max), Some('\u{10FFFF}'));
     /// assert_eq!(char::from_u32(value_at_max + 1), None);
     /// ```