Rollup merge of #57604 - alercah:str-index, r=sfackler

Make `str` indexing generic on `SliceIndex`.

Fixes #55603

3 files changed, 132 insertions, 195 deletions

diff --git a/src/libcore/ops/index.rs b/src/libcore/ops/index.rs
index 6cfa36741d0..d4ed8614276 100644
--- a/src/libcore/ops/index.rs
+++ b/src/libcore/ops/index.rs
@@ -51,21 +51,6 @@
 /// ```
 #[lang = "index"]
 #[rustc_on_unimplemented(
-    on(
-        _Self="&str",
-        note="you can use `.chars().nth()` or `.bytes().nth()`
-see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
-    ),
-    on(
-        _Self="str",
-        note="you can use `.chars().nth()` or `.bytes().nth()`
-see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
-    ),
-    on(
-        _Self="std::string::String",
-        note="you can use `.chars().nth()` or `.bytes().nth()`
-see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
-    ),
     message="the type `{Self}` cannot be indexed by `{Idx}`",
     label="`{Self}` cannot be indexed by `{Idx}`",
 )]
diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index df4d97ee6a4..9f9515e6d9b 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -2383,7 +2383,6 @@ impl [u8] {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
 impl<T, I> ops::Index<I> for [T]
     where I: SliceIndex<[T]>
 {
@@ -2396,7 +2395,6 @@ impl<T, I> ops::Index<I> for [T]
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
 impl<T, I> ops::IndexMut<I> for [T]
     where I: SliceIndex<[T]>
 {
@@ -2447,7 +2445,19 @@ mod private_slice_index {
 
 /// A helper trait used for indexing operations.
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
-#[rustc_on_unimplemented = "slice indices are of type `usize` or ranges of `usize`"]
+#[rustc_on_unimplemented(
+    on(
+        T = "str",
+        label = "string indices are ranges of `usize`",
+    ),
+    on(
+        all(any(T = "str", T = "&str", T = "std::string::String"), _Self="{integer}"),
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
+    message = "the type `{T}` cannot be indexed by `{Self}`",
+    label = "slice indices are of type `usize` or ranges of `usize`",
+)]
 pub trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
     /// The output type returned by methods.
     #[stable(feature = "slice_get_slice", since = "1.28.0")]
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index bdde187d931..1ee8b7735c1 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1621,190 +1621,26 @@ mod traits {
         }
     }
 
-    /// Implements substring slicing with syntax `&self[begin .. end]`.
-    ///
-    /// Returns a slice of the given string from the byte range
-    /// [`begin`..`end`).
-    ///
-    /// This operation is `O(1)`.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `begin` or `end` does not point to the starting
-    /// byte offset of a character (as defined by `is_char_boundary`).
-    /// Requires that `begin <= end` and `end <= len` where `len` is the
-    /// length of the string.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// let s = "Löwe 老虎 Léopard";
-    /// assert_eq!(&s[0 .. 1], "L");
-    ///
-    /// assert_eq!(&s[1 .. 9], "öwe 老");
-    ///
-    /// // these will panic:
-    /// // byte 2 lies within `ö`:
-    /// // &s[2 ..3];
-    ///
-    /// // byte 8 lies within `老`
-    /// // &s[1 .. 8];
-    ///
-    /// // byte 100 is outside the string
-    /// // &s[3 .. 100];
-    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl ops::Index<ops::Range<usize>> for str {
-        type Output = str;
-        #[inline]
-        fn index(&self, index: ops::Range<usize>) -> &str {
-            index.index(self)
-        }
-    }
-
-    /// Implements mutable substring slicing with syntax
-    /// `&mut self[begin .. end]`.
-    ///
-    /// Returns a mutable slice of the given string from the byte range
-    /// [`begin`..`end`).
-    ///
-    /// This operation is `O(1)`.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `begin` or `end` does not point to the starting
-    /// byte offset of a character (as defined by `is_char_boundary`).
-    /// Requires that `begin <= end` and `end <= len` where `len` is the
-    /// length of the string.
-    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
-    impl ops::IndexMut<ops::Range<usize>> for str {
-        #[inline]
-        fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
-            index.index_mut(self)
-        }
-    }
-
-    /// Implements substring slicing with syntax `&self[.. end]`.
-    ///
-    /// Returns a slice of the string from the beginning to byte offset
-    /// `end`.
-    ///
-    /// Equivalent to `&self[0 .. end]`.
-    #[stable(feature = "rust1", since = "1.0.0")]
-    impl ops::Index<ops::RangeTo<usize>> for str {
-        type Output = str;
-
-        #[inline]
-        fn index(&self, index: ops::RangeTo<usize>) -> &str {
-            index.index(self)
-        }
-    }
-
-    /// Implements mutable substring slicing with syntax `&mut self[.. end]`.
-    ///
-    /// Returns a mutable slice of the string from the beginning to byte offset
-    /// `end`.
-    ///
-    /// Equivalent to `&mut self[0 .. end]`.
-    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
-    impl ops::IndexMut<ops::RangeTo<usize>> for str {
-        #[inline]
-        fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
-            index.index_mut(self)
-        }
-    }
-
-    /// Implements substring slicing with syntax `&self[begin ..]`.
-    ///
-    /// Returns a slice of the string from byte offset `begin`
-    /// to the end of the string.
-    ///
-    /// Equivalent to `&self[begin .. len]`.
-    #[stable(feature = "rust1", since = "1.0.0")]
-    impl ops::Index<ops::RangeFrom<usize>> for str {
-        type Output = str;
+    impl<I> ops::Index<I> for str
+    where
+        I: SliceIndex<str>,
+    {
+        type Output = I::Output;
 
         #[inline]
-        fn index(&self, index: ops::RangeFrom<usize>) -> &str {
+        fn index(&self, index: I) -> &I::Output {
             index.index(self)
         }
     }
 
-    /// Implements mutable substring slicing with syntax `&mut self[begin ..]`.
-    ///
-    /// Returns a mutable slice of the string from byte offset `begin`
-    /// to the end of the string.
-    ///
-    /// Equivalent to `&mut self[begin .. len]`.
-    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
-    impl ops::IndexMut<ops::RangeFrom<usize>> for str {
-        #[inline]
-        fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
-            index.index_mut(self)
-        }
-    }
-
-    /// Implements substring slicing with syntax `&self[..]`.
-    ///
-    /// Returns a slice of the whole string. This operation can
-    /// never panic.
-    ///
-    /// Equivalent to `&self[0 .. len]`.
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl ops::Index<ops::RangeFull> for str {
-        type Output = str;
-
-        #[inline]
-        fn index(&self, _index: ops::RangeFull) -> &str {
-            self
-        }
-    }
-
-    /// Implements mutable substring slicing with syntax `&mut self[..]`.
-    ///
-    /// Returns a mutable slice of the whole string. This operation can
-    /// never panic.
-    ///
-    /// Equivalent to `&mut self[0 .. len]`.
-    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
-    impl ops::IndexMut<ops::RangeFull> for str {
-        #[inline]
-        fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
-            self
-        }
-    }
-
-    #[stable(feature = "inclusive_range", since = "1.26.0")]
-    impl ops::Index<ops::RangeInclusive<usize>> for str {
-        type Output = str;
-
-        #[inline]
-        fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
-            index.index(self)
-        }
-    }
-
-    #[stable(feature = "inclusive_range", since = "1.26.0")]
-    impl ops::Index<ops::RangeToInclusive<usize>> for str {
-        type Output = str;
-
-        #[inline]
-        fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
-            index.index(self)
-        }
-    }
-
-    #[stable(feature = "inclusive_range", since = "1.26.0")]
-    impl ops::IndexMut<ops::RangeInclusive<usize>> for str {
-        #[inline]
-        fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
-            index.index_mut(self)
-        }
-    }
-    #[stable(feature = "inclusive_range", since = "1.26.0")]
-    impl ops::IndexMut<ops::RangeToInclusive<usize>> for str {
+    impl<I> ops::IndexMut<I> for str
+    where
+        I: SliceIndex<str>,
+    {
         #[inline]
-        fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
+        fn index_mut(&mut self, index: I) -> &mut I::Output {
             index.index_mut(self)
         }
     }
@@ -1815,6 +1651,18 @@ mod traits {
         panic!("attempted to index str up to maximum usize");
     }
 
+    /// Implements substring slicing with syntax `&self[..]` or `&mut self[..]`.
+    ///
+    /// Returns a slice of the whole string, i.e., returns `&self` or `&mut
+    /// self`. Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`. Unlike
+    /// other indexing operations, this can never panic.
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// Prior to 1.20.0, these indexing operations were still supported by
+    /// direct implementation of `Index` and `IndexMut`.
+    ///
+    /// Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`.
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
     impl SliceIndex<str> for ops::RangeFull {
         type Output = str;
@@ -1844,6 +1692,41 @@ mod traits {
         }
     }
 
+    /// Implements substring slicing with syntax `&self[begin .. end]` or `&mut
+    /// self[begin .. end]`.
+    ///
+    /// Returns a slice of the given string from the byte range
+    /// [`begin`, `end`).
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// Prior to 1.20.0, these indexing operations were still supported by
+    /// direct implementation of `Index` and `IndexMut`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `begin` or `end` does not point to the starting byte offset of
+    /// a character (as defined by `is_char_boundary`), if `begin > end`, or if
+    /// `end > len`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let s = "Löwe 老虎 Léopard";
+    /// assert_eq!(&s[0 .. 1], "L");
+    ///
+    /// assert_eq!(&s[1 .. 9], "öwe 老");
+    ///
+    /// // these will panic:
+    /// // byte 2 lies within `ö`:
+    /// // &s[2 ..3];
+    ///
+    /// // byte 8 lies within `老`
+    /// // &s[1 .. 8];
+    ///
+    /// // byte 100 is outside the string
+    /// // &s[3 .. 100];
+    /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
     impl SliceIndex<str> for ops::Range<usize> {
         type Output = str;
@@ -1898,6 +1781,21 @@ mod traits {
         }
     }
 
+    /// Implements substring slicing with syntax `&self[.. end]` or `&mut
+    /// self[.. end]`.
+    ///
+    /// Returns a slice of the given string from the byte range [`0`, `end`).
+    /// Equivalent to `&self[0 .. end]` or `&mut self[0 .. end]`.
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// Prior to 1.20.0, these indexing operations were still supported by
+    /// direct implementation of `Index` and `IndexMut`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `end` does not point to the starting byte offset of a
+    /// character (as defined by `is_char_boundary`), or if `end > len`.
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
     impl SliceIndex<str> for ops::RangeTo<usize> {
         type Output = str;
@@ -1943,6 +1841,22 @@ mod traits {
         }
     }
 
+    /// Implements substring slicing with syntax `&self[begin ..]` or `&mut
+    /// self[begin ..]`.
+    ///
+    /// Returns a slice of the given string from the byte range [`begin`,
+    /// `len`). Equivalent to `&self[begin .. len]` or `&mut self[begin ..
+    /// len]`.
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// Prior to 1.20.0, these indexing operations were still supported by
+    /// direct implementation of `Index` and `IndexMut`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `begin` does not point to the starting byte offset of
+    /// a character (as defined by `is_char_boundary`), or if `begin >= len`.
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
     impl SliceIndex<str> for ops::RangeFrom<usize> {
         type Output = str;
@@ -1990,6 +1904,22 @@ mod traits {
         }
     }
 
+    /// Implements substring slicing with syntax `&self[begin ..= end]` or `&mut
+    /// self[begin ..= end]`.
+    ///
+    /// Returns a slice of the given string from the byte range
+    /// [`begin`, `end`]. Equivalent to `&self [begin .. end + 1]` or `&mut
+    /// self[begin .. end + 1]`, except if `end` has the maximum value for
+    /// `usize`.
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `begin` does not point to the starting byte offset of
+    /// a character (as defined by `is_char_boundary`), if `end` does not point
+    /// to the ending byte offset of a character (`end + 1` is either a starting
+    /// byte offset or equal to `len`), if `begin > end`, or if `end >= len`.
     #[stable(feature = "inclusive_range", since = "1.26.0")]
     impl SliceIndex<str> for ops::RangeInclusive<usize> {
         type Output = str;
@@ -2023,8 +1953,20 @@ mod traits {
         }
     }
 
-
-
+    /// Implements substring slicing with syntax `&self[..= end]` or `&mut
+    /// self[..= end]`.
+    ///
+    /// Returns a slice of the given string from the byte range [0, `end`].
+    /// Equivalent to `&self [0 .. end + 1]`, except if `end` has the maximum
+    /// value for `usize`.
+    ///
+    /// This operation is `O(1)`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `end` does not point to the ending byte offset of a character
+    /// (`end + 1` is either a starting byte offset as defined by
+    /// `is_char_boundary`, or equal to `len`), or if `end >= len`.
     #[stable(feature = "inclusive_range", since = "1.26.0")]
     impl SliceIndex<str> for ops::RangeToInclusive<usize> {
         type Output = str;