Address review comments

Author: Jules-Bertholet

library/alloc/src/str.rs

@@ -510,7 +510,8 @@ impl str {
     /// Since some characters can expand into multiple characters when case folding,
     /// this function returns a [`String`] instead of modifying the parameter in-place.
     ///
-    /// This function does not perform any normalization (e.g. NFC).
+    /// This function does not perform any [normalization] (e.g. NFC),
+    /// so semantically and visually identical strings may compare unequal.
     ///
     /// Like [`char::to_casefold()`] this method does not handle language-specific
     /// casing, like Turkish and Azeri I/ı/İ/i. See that method's documentation
@@ -551,7 +552,7 @@ impl str {
     /// assert_eq!(s0.to_casefold(), "tschüss");
     /// ```
     ///
-    /// No NFC normalization is performed:
+    /// No NFC [normalization] is performed:
     ///
     /// ```rust
     /// #![feature(casefold)]
@@ -569,6 +570,8 @@ impl str {
     /// assert_eq!(comp.to_casefold(), "\u{E1}");
     /// assert_eq!(decomp.to_casefold(), "a\u{0301}");
     /// ```
+    ///
+    /// [normalization]: https://www.unicode.org/faq/normalization
     #[cfg(not(no_global_oom_handling))]
     #[rustc_allow_incoherent_impl]
     #[must_use = "this returns the case-folded string as a new String, \

library/core/src/char/methods.rs

@@ -1075,16 +1075,17 @@ impl char {
     }
 
     /// Returns `true` if this `char` has the `Case_Ignorable` property. This narrow-use property
-    /// is used to implement context-dependent casing for the Greek letter sigma (uppercase Σ),
+    /// is used to implement context-dependent casing for the Greek letter sigma (uppercase 'Σ'),
     /// which has two lowercase forms.
     ///
     /// `Case_Ignorable` is [described][D136] in Chapter 3 (Conformance) of the Unicode Core Specification,
-    /// and specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`];
-    /// see those resources for more information.
+    /// and specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`].
+    /// See those resources, as well as [`to_lowercase()`]'s documentation, for more information.
     ///
     /// [D136]: https://www.unicode.org/versions/latest/core-spec/chapter-3/#G63116
     /// [ucd]: https://www.unicode.org/reports/tr44/
     /// [`DerivedCoreProperties.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt
+    /// [`to_lowercase()`]: Self::to_lowercase()
     #[must_use]
     #[inline]
     #[unstable(feature = "case_ignorable", issue = "154848")]
@@ -1154,8 +1155,6 @@ impl char {
     /// If this `char` expands to multiple `char`s, the iterator yields the `char`s given by
     /// [`SpecialCasing.txt`]. The maximum number of `char`s in a case mapping is 3.
     ///
-    /// [`SpecialCasing.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt
-    ///
     /// This operation performs an unconditional mapping without tailoring. That is, the conversion
     /// is independent of context and language. See [below](#notes-on-context-and-locale)
     /// for more information.
@@ -1210,28 +1209,39 @@ impl char {
     ///
     /// ## Greek sigma
     ///
-    /// In Greek, the letter simga (uppercase Σ) has two lowercase forms:
-    /// ς which is used only at the end of a word, and σ which is used everywhere else.
-    /// `to_lowercase()` always uses the second form:
+    /// In Greek, the letter simga (uppercase 'Σ') has two lowercase forms:
+    /// 'σ' which is used in most situations, and 'ς' which appears only
+    /// at the end of a word. [`char::to_lowercase()`] always uses the first form:
     ///
     /// ```
     /// assert_eq!('Σ'.to_lowercase().to_string(), "σ");
     /// ```
     ///
+    /// `str::to_lowercase()` (only available with the `alloc` crate)
+    /// *does* properly handle this contextual mapping,
+    /// so prefer using that method if you can. Alternatively, you can use
+    /// [`is_cased()`] and [`is_case_ignorable()`] to implement it yourself.
+    /// See `Final_Sigma` in [Table 3.17] of the Unicode Standard,
+    /// along with [`SpecialCasing.txt`], for more details.
+    ///
+    /// [`is_cased()`]: Self::is_cased()
+    /// [`is_case_ignorable()`]: Self::is_case_ignorable()
+    /// [Table 3.17]: https://www.unicode.org/versions/latest/core-spec/chapter-3/#G54277
+    ///
     /// ## Turkish and Azeri I/ı/İ/i
     ///
     /// In Turkish and Azeri, the equivalent of 'i' in Latin has five forms instead of two:
     ///
     /// * 'Dotless': I / ı, sometimes written ï
     /// * 'Dotted': İ / i
     ///
-    /// Note that the uppercase undotted 'I' is the same as the Latin. Therefore:
+    /// Note that the uppercase undotted 'I' is the same codepoint as the Latin. Therefore:
     ///
     /// ```
     /// let lower_i = 'I'.to_lowercase().to_string();
     /// ```
     ///
-    /// The value of `lower_i` here relies on the language of the text: if we're
+    /// `'I'`'s correct lowercase relies on the language of the text: if we're
     /// in `en-US`, it should be `"i"`, but if we're in `tr-TR` or `az-AZ`, it should
     /// be `"ı"`. `to_lowercase()` does not take this into account, and so:
     ///
@@ -1242,6 +1252,8 @@ impl char {
     /// ```
     ///
     /// holds across languages.
+    ///
+    /// [`SpecialCasing.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt
     #[must_use = "this returns the lowercased character as a new iterator, \
                   without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1392,22 +1404,22 @@ impl char {
     /// As stated above, this method is locale-insensitive.
     /// If you need locale support, consider using an external crate,
     /// like [`icu_casemap`](https://crates.io/crates/icu_casemap)
-    /// which is developed by Unicode. A description of a common
-    /// locale-dependent casing issue follows:
+    /// which is developed by Unicode. A description of one common
+    /// locale-dependent casing issue follows (there are others):
     ///
     /// In Turkish and Azeri, the equivalent of 'i' in Latin has five forms instead of two:
     ///
     /// * 'Dotless': I / ı, sometimes written ï
     /// * 'Dotted': İ / i
     ///
-    /// Note that the lowercase dotted 'i' is the same as the Latin. Therefore:
+    /// Note that the lowercase dotted 'i' is the same codepoint as the Latin. Therefore:
     ///
     /// ```
     /// #![feature(titlecase)]
     /// let upper_i = 'i'.to_titlecase().to_string();
     /// ```
     ///
-    /// The value of `upper_i` here relies on the language of the text: if we're
+    /// `'i'`'s correct titlecase relies on the language of the text: if we're
     /// in `en-US`, it should be `"I"`, but if we're in `tr-TR` or `az-AZ`, it should
     /// be `"İ"`. `to_titlecase()` does not take this into account, and so:
     ///
@@ -1504,21 +1516,21 @@ impl char {
     /// As stated above, this method is locale-insensitive.
     /// If you need locale support, consider using an external crate,
     /// like [`icu_casemap`](https://crates.io/crates/icu_casemap)
-    /// which is developed by Unicode. A description of a common
-    /// locale-dependent casing issue follows:
+    /// which is developed by Unicode. A description of one common
+    /// locale-dependent casing issue follows (there are others):
     ///
     /// In Turkish and Azeri, the equivalent of 'i' in Latin has five forms instead of two:
     ///
     /// * 'Dotless': I / ı, sometimes written ï
     /// * 'Dotted': İ / i
     ///
-    /// Note that the lowercase dotted 'i' is the same as the Latin. Therefore:
+    /// Note that the lowercase dotted 'i' is the same codepoint as the Latin. Therefore:
     ///
     /// ```
     /// let upper_i = 'i'.to_uppercase().to_string();
     /// ```
     ///
-    /// The value of `upper_i` here relies on the language of the text: if we're
+    /// `'i'`'s correct uppercase relies on the language of the text: if we're
     /// in `en-US`, it should be `"I"`, but if we're in `tr-TR` or `az-AZ`, it should
     /// be `"İ"`. `to_uppercase()` does not take this into account, and so:
     ///
@@ -1542,20 +1554,23 @@ impl char {
     /// Returns an iterator that yields the case folding of this `char` as one or more
     /// `char`s.
     ///
-    /// Case folding is meant to be used when performing case-insensitive string comparisons,
-    /// but case-folded strings should not generally be exposed directly to users. For most,
+    /// Case folding is meant to be used when performing case-insensitive string comparisons.
+    /// Case-folded strings should not usually be exposed directly to users. For most,
     /// but not all, characters, the casefold mapping is identical to the lowercase one.
     ///
     /// This iterator yields the `char`(s) in the common or full case folding for this `char`,
     /// as given by the [Unicode Character Database][ucd] [`CaseFolding.txt`].
+    /// The maximum number of `char`s in a case folding is 3.
     ///
     /// [ucd]: https://www.unicode.org/reports/tr44/
     /// [`CaseFolding.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt
     ///
     /// This operation performs an unconditional mapping without tailoring. That is, the conversion
     /// is independent of context and language.
     ///
-    /// It also does not perform any normalization (e.g. NFC).
+    /// It also does not perform any [normalization] (e.g. NFC).
+    ///
+    /// [normalization]: https://www.unicode.org/faq/normalization
     ///
     /// In the [Unicode Standard], Chapter 4 (Character Properties) discusses case folding in
     /// general and Chapter 3 (Conformance) discusses the default algorithm for case folding.
@@ -1591,14 +1606,14 @@ impl char {
     /// * 'Dotless': I / ı, sometimes written ï
     /// * 'Dotted': İ / i
     ///
-    /// Note that the uppercase undotted 'I' is the same as the Latin. Therefore:
+    /// Note that the uppercase undotted 'I' is the same codepoint as the Latin. Therefore:
     ///
     /// ```
     /// #![feature(casefold)]
     /// let casefold_i = 'I'.to_casefold().to_string();
     /// ```
     ///
-    /// The value of `casefold_i` here relies on the language of the text: if we're
+    /// `'I'`'s correct case folding relies on the language of the text: if we're
     /// in `en-US`, it should be `"i"`, but if we're in `tr-TR` or `az-AZ`, it should
     /// be `"ı"`. `to_casefold()` does not take this into account, and so:
     ///