3 files changed, 41 insertions, 5 deletions
diff --git a/library/core/src/char/decode.rs b/library/core/src/char/decode.rs
index 71297acd1..dc8ea66cc 100644
--- a/library/core/src/char/decode.rs
+++ b/library/core/src/char/decode.rs
@@ -1,5 +1,7 @@
 //! UTF-8 and UTF-16 decoding iterators
 
+#[cfg(not(bootstrap))]
+use crate::error::Error;
 use crate::fmt;
 
 use super::from_u32_unchecked;
@@ -121,3 +123,12 @@ impl fmt::Display for DecodeUtf16Error {
         write!(f, "unpaired surrogate found: {:x}", self.code)
     }
 }
+
+#[cfg(not(bootstrap))]
+#[stable(feature = "decode_utf16", since = "1.9.0")]
+impl Error for DecodeUtf16Error {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "unpaired surrogate found"
+    }
+}
diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index eae567cad..b7a63b7c6 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -746,10 +746,19 @@ impl char {
     /// assert!(!'中'.is_lowercase());
     /// assert!(!' '.is_lowercase());
     /// ```
+    ///
+    /// In a const context:
+    ///
+    /// ```
+    /// #![feature(const_unicode_case_lookup)]
+    /// const CAPITAL_DELTA_IS_LOWERCASE: bool = 'Δ'.is_lowercase();
+    /// assert!(!CAPITAL_DELTA_IS_LOWERCASE);
+    /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_unicode_case_lookup", issue = "101400")]
     #[inline]
-    pub fn is_lowercase(self) -> bool {
+    pub const fn is_lowercase(self) -> bool {
         match self {
             'a'..='z' => true,
             c => c > '\x7f' && unicode::Lowercase(c),
@@ -779,10 +788,19 @@ impl char {
     /// assert!(!'中'.is_uppercase());
     /// assert!(!' '.is_uppercase());
     /// ```
+    ///
+    /// In a const context:
+    ///
+    /// ```
+    /// #![feature(const_unicode_case_lookup)]
+    /// const CAPITAL_DELTA_IS_UPPERCASE: bool = 'Δ'.is_uppercase();
+    /// assert!(CAPITAL_DELTA_IS_UPPERCASE);
+    /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_unicode_case_lookup", issue = "101400")]
     #[inline]
-    pub fn is_uppercase(self) -> bool {
+    pub const fn is_uppercase(self) -> bool {
         match self {
             'A'..='Z' => true,
             c => c > '\x7f' && unicode::Uppercase(c),
@@ -892,8 +910,7 @@ impl char {
     ///
     /// The general categories for numbers (`Nd` for decimal digits, `Nl` for letter-like numeric
     /// characters, and `No` for other numeric characters) are specified in the [Unicode Character
-    /// Database][ucd] [`UnicodeData.txt`]. Note that this means ideographic numbers like '三'
-    /// are considered alphabetic, not numeric. Please consider to use `is_ascii_digit` or `is_digit`.
+    /// Database][ucd] [`UnicodeData.txt`].
     ///
     /// This method doesn't cover everything that could be considered a number, e.g. ideographic numbers like '三'.
     /// If you want everything including characters with overlapping purposes then you might want to use
diff --git a/library/core/src/char/mod.rs b/library/core/src/char/mod.rs
index 0df23e7bb..72d63ac4b 100644
--- a/library/core/src/char/mod.rs
+++ b/library/core/src/char/mod.rs
@@ -1,4 +1,6 @@
-//! A character type.
+//! Utilities for the `char` primitive type.
+//!
+//! *[See also the `char` primitive type](primitive@char).*
 //!
 //! The `char` type represents a single character. More specifically, since
 //! 'character' isn't a well-defined concept in Unicode, `char` is a '[Unicode
@@ -36,6 +38,8 @@ pub use self::methods::encode_utf16_raw;
 #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
 pub use self::methods::encode_utf8_raw;
 
+#[cfg(not(bootstrap))]
+use crate::error::Error;
 use crate::fmt::{self, Write};
 use crate::iter::FusedIterator;
 
@@ -582,3 +586,7 @@ impl fmt::Display for TryFromCharError {
         "unicode code point out of range".fmt(fmt)
     }
 }
+
+#[cfg(not(bootstrap))]
+#[stable(feature = "u8_from_char", since = "1.59.0")]
+impl Error for TryFromCharError {}