traits.rs

//! Trait implementations for `str`.

use super::ParseBoolError;
use crate::cmp::Ordering;
use crate::intrinsics::unchecked_sub;
use crate::slice::SliceIndex;
use crate::ub_checks::assert_unsafe_precondition;
use crate::{ops, ptr, range};

/// Implements ordering of strings.
///
/// Strings are ordered  [lexicographically](Ord#lexicographical-comparison) by their byte values. This orders Unicode code
/// points based on their positions in the code charts. This is not necessarily the same as
/// "alphabetical" order, which varies by language and locale. Sorting strings according to
/// culturally-accepted standards requires locale-specific data that is outside the scope of
/// the `str` type.
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const Ord for str {
    #[inline]
    fn cmp(&self, other: &str) -> Ordering {
        self.as_bytes().cmp(other.as_bytes())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const PartialEq for str {
    #[inline]
    fn eq(&self, other: &str) -> bool {
        self.as_bytes() == other.as_bytes()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const Eq for str {}

/// Implements comparison operations on strings.
///
/// Strings are compared [lexicographically](Ord#lexicographical-comparison) by their byte values. This compares Unicode code
/// points based on their positions in the code charts. This is not necessarily the same as
/// "alphabetical" order, which varies by language and locale. Comparing strings according to
/// culturally-accepted standards requires locale-specific data that is outside the scope of
/// the `str` type.
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const PartialOrd for str {
    #[inline]
    fn partial_cmp(&self, other: &str) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
impl<I> const ops::Index<I> for str
where
    I: [const] SliceIndex<str>,
{
    type Output = I::Output;

    #[inline]
    fn index(&self, index: I) -> &I::Output {
        index.index(self)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
impl<I> const ops::IndexMut<I> for str
where
    I: [const] SliceIndex<str>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut I::Output {
        index.index_mut(self)
    }
}

#[inline(never)]
#[cold]
#[track_caller]
const fn str_index_overflow_fail() -> ! {
    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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::RangeFull {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        Some(slice)
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        Some(slice)
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        slice
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        slice
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        slice
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        slice
    }
}

/// 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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::Range<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            // We also checked char boundaries, so this is valid UTF-8.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary.
            // We know the pointer is unique because we got it from `slice`.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    #[track_caller]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let slice = slice as *const [u8];

        assert_unsafe_precondition!(
            // We'd like to check that the bounds are on char boundaries,
            // but there's not really a way to do so without reading
            // behind the pointer, which has aliasing implications.
            // It's also not possible to move this check up to
            // `str::get_unchecked` without adding a special function
            // to `SliceIndex` just for this.
            check_library_ub,
            "str::get_unchecked requires that the range is within the string slice",
            (
                start: usize = self.start,
                end: usize = self.end,
                len: usize = slice.len()
            ) => end >= start && end <= len,
        );

        // SAFETY: the caller guarantees that `self` is in bounds of `slice`
        // which satisfies all the conditions for `add`.
        unsafe {
            let new_len = unchecked_sub(self.end, self.start);
            ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), new_len) as *const str
        }
    }
    #[inline]
    #[track_caller]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let slice = slice as *mut [u8];

        assert_unsafe_precondition!(
            check_library_ub,
            "str::get_unchecked_mut requires that the range is within the string slice",
            (
                start: usize = self.start,
                end: usize = self.end,
                len: usize = slice.len()
            ) => end >= start && end <= len,
        );

        // SAFETY: see comments for `get_unchecked`.
        unsafe {
            let new_len = unchecked_sub(self.end, self.start);
            ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), new_len) as *mut str
        }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, self.end);
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        // is_char_boundary checks that the index is in [0, .len()]
        // cannot reuse `get` as above, because of NLL trouble
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, self.end)
        }
    }
}

#[unstable(feature = "new_range_api", issue = "125687")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for range::Range<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            // We also checked char boundaries, so this is valid UTF-8.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary.
            // We know the pointer is unique because we got it from `slice`.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    #[track_caller]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let slice = slice as *const [u8];

        assert_unsafe_precondition!(
            // We'd like to check that the bounds are on char boundaries,
            // but there's not really a way to do so without reading
            // behind the pointer, which has aliasing implications.
            // It's also not possible to move this check up to
            // `str::get_unchecked` without adding a special function
            // to `SliceIndex` just for this.
            check_library_ub,
            "str::get_unchecked requires that the range is within the string slice",
            (
                start: usize = self.start,
                end: usize = self.end,
                len: usize = slice.len()
            ) => end >= start && end <= len,
        );

        // SAFETY: the caller guarantees that `self` is in bounds of `slice`
        // which satisfies all the conditions for `add`.
        unsafe {
            let new_len = unchecked_sub(self.end, self.start);
            ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), new_len) as *const str
        }
    }
    #[inline]
    #[track_caller]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let slice = slice as *mut [u8];

        assert_unsafe_precondition!(
            check_library_ub,
            "str::get_unchecked_mut requires that the range is within the string slice",
            (
                start: usize = self.start,
                end: usize = self.end,
                len: usize = slice.len()
            ) => end >= start && end <= len,
        );

        // SAFETY: see comments for `get_unchecked`.
        unsafe {
            let new_len = unchecked_sub(self.end, self.start);
            ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), new_len) as *mut str
        }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, self.end);
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        // is_char_boundary checks that the index is in [0, .len()]
        // cannot reuse `get` as above, because of NLL trouble
        if self.start <= self.end
            && slice.is_char_boundary(self.start)
            && slice.is_char_boundary(self.end)
        {
            // SAFETY: just checked that `start` and `end` are on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, self.end)
        }
    }
}

/// Implements substring slicing for arbitrary bounds.
///
/// Returns a slice of the given string bounded by the byte indices
/// provided by each bound.
///
/// This operation is *O*(1).
///
/// # Panics
///
/// Panics if `begin` or `end` (if it exists and once adjusted for
/// inclusion/exclusion) does not point to the starting byte offset of
/// a character (as defined by `is_char_boundary`), if `begin > end`, or if
/// `end > len`.
#[stable(feature = "slice_index_str_with_ops_bound_pair", since = "1.73.0")]
unsafe impl SliceIndex<str> for (ops::Bound<usize>, ops::Bound<usize>) {
    type Output = str;

    #[inline]
    fn get(self, slice: &str) -> Option<&str> {
        crate::slice::index::into_range(slice.len(), self)?.get(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut str> {
        crate::slice::index::into_range(slice.len(), self)?.get_mut(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const str {
        let len = (slice as *const [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { crate::slice::index::into_range_unchecked(len, self).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut str {
        let len = (slice as *mut [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { crate::slice::index::into_range_unchecked(len, self).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &str) -> &str {
        crate::slice::index::into_slice_range(slice.len(), self).index(slice)
    }

    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut str {
        crate::slice::index::into_slice_range(slice.len(), self).index_mut(slice)
    }
}

/// 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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::RangeTo<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (0..self.end).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..self.end).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let end = self.end;
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, 0, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if slice.is_char_boundary(self.end) {
            // SAFETY: just checked that `end` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, 0, self.end)
        }
    }
}

/// 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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::RangeFrom<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let len = (slice as *const [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (self.start..len).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let len = (slice as *mut [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (self.start..len).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, slice.len());
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, slice.len())
        }
    }
}

#[unstable(feature = "new_range_api", issue = "125687")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for range::RangeFrom<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &*self.get_unchecked(slice) })
        } else {
            None
        }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            Some(unsafe { &mut *self.get_unchecked_mut(slice) })
        } else {
            None
        }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        let len = (slice as *const [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (self.start..len).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        let len = (slice as *mut [u8]).len();
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (self.start..len).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        let (start, end) = (self.start, slice.len());
        match self.get(slice) {
            Some(s) => s,
            None => super::slice_error_fail(slice, start, end),
        }
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if slice.is_char_boundary(self.start) {
            // SAFETY: just checked that `start` is on a char boundary,
            // and we are passing in a safe reference, so the return value will also be one.
            unsafe { &mut *self.get_unchecked_mut(slice) }
        } else {
            super::slice_error_fail(slice, self.start, slice.len())
        }
    }
}

/// 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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::RangeInclusive<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if *self.end() == usize::MAX { None } else { self.into_slice_range().get(slice) }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if *self.end() == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
        unsafe { self.into_slice_range().get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
        unsafe { self.into_slice_range().get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        if *self.end() == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index(slice)
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if *self.end() == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index_mut(slice)
    }
}

#[unstable(feature = "new_range_api", issue = "125687")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for range::RangeInclusive<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        if self.last == usize::MAX { None } else { self.into_slice_range().get(slice) }
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        if self.last == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
        unsafe { self.into_slice_range().get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
        unsafe { self.into_slice_range().get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        if self.last == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index(slice)
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        if self.last == usize::MAX {
            str_index_overflow_fail();
        }
        self.into_slice_range().index_mut(slice)
    }
}

/// 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")]
#[rustc_const_unstable(feature = "const_index", issue = "143775")]
unsafe impl const SliceIndex<str> for ops::RangeToInclusive<usize> {
    type Output = str;
    #[inline]
    fn get(self, slice: &str) -> Option<&Self::Output> {
        (0..=self.end).get(slice)
    }
    #[inline]
    fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
        (0..=self.end).get_mut(slice)
    }
    #[inline]
    unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
        unsafe { (0..=self.end).get_unchecked(slice) }
    }
    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
        // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..=self.end).get_unchecked_mut(slice) }
    }
    #[inline]
    fn index(self, slice: &str) -> &Self::Output {
        (0..=self.end).index(slice)
    }
    #[inline]
    fn index_mut(self, slice: &mut str) -> &mut Self::Output {
        (0..=self.end).index_mut(slice)
    }
}

/// Parse a value from a string
///
/// `FromStr`'s [`from_str`] method is often used implicitly, through
/// [`str`]'s [`parse`] method. See [`parse`]'s documentation for examples.
///
/// [`from_str`]: FromStr::from_str
/// [`parse`]: str::parse
///
/// `FromStr` does not have a lifetime parameter, and so you can only parse types
/// that do not contain a lifetime parameter themselves. In other words, you can
/// parse an `i32` with `FromStr`, but not a `&i32`. You can parse a struct that
/// contains an `i32`, but not one that contains an `&i32`.
///
/// # Input format and round-tripping
///
/// The input format expected by a type's `FromStr` implementation depends on the type. Check the
/// type's documentation for the input formats it knows how to parse. Note that the input format of
/// a type's `FromStr` implementation might not necessarily accept the output format of its
/// `Display` implementation, and even if it does, the `Display` implementation may not be lossless
/// so the round-trip may lose information.
///
/// However, if a type has a lossless `Display` implementation whose output is meant to be
/// conveniently machine-parseable and not just meant for human consumption, then the type may wish
/// to accept the same format in `FromStr`, and document that usage. Having both `Display` and
/// `FromStr` implementations where the result of `Display` cannot be parsed with `FromStr` may
/// surprise users.
///
/// # Examples
///
/// Basic implementation of `FromStr` on an example `Point` type:
///
/// ```
/// use std::str::FromStr;
///
/// #[derive(Debug, PartialEq)]
/// struct Point {
///     x: i32,
///     y: i32
/// }
///
/// #[derive(Debug, PartialEq, Eq)]
/// struct ParsePointError;
///
/// impl FromStr for Point {
///     type Err = ParsePointError;
///
///     fn from_str(s: &str) -> Result<Self, Self::Err> {
///         let (x, y) = s
///             .strip_prefix('(')
///             .and_then(|s| s.strip_suffix(')'))
///             .and_then(|s| s.split_once(','))
///             .ok_or(ParsePointError)?;
///
///         let x_fromstr = x.parse::<i32>().map_err(|_| ParsePointError)?;
///         let y_fromstr = y.parse::<i32>().map_err(|_| ParsePointError)?;
///
///         Ok(Point { x: x_fromstr, y: y_fromstr })
///     }
/// }
///
/// let expected = Ok(Point { x: 1, y: 2 });
/// // Explicit call
/// assert_eq!(Point::from_str("(1,2)"), expected);
/// // Implicit calls, through parse
/// assert_eq!("(1,2)".parse(), expected);
/// assert_eq!("(1,2)".parse::<Point>(), expected);
/// // Invalid input string
/// assert!(Point::from_str("(1 2)").is_err());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const trait FromStr: Sized {
    /// The associated error which can be returned from parsing.
    #[stable(feature = "rust1", since = "1.0.0")]
    type Err;

    /// Parses a string `s` to return a value of this type.
    ///
    /// If parsing succeeds, return the value inside [`Ok`], otherwise
    /// when the string is ill-formatted return an error specific to the
    /// inside [`Err`]. The error type is specific to the implementation of the trait.
    ///
    /// # Examples
    ///
    /// Basic usage with [`i32`], a type that implements `FromStr`:
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// let s = "5";
    /// let x = i32::from_str(s).unwrap();
    ///
    /// assert_eq!(5, x);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_diagnostic_item = "from_str_method"]
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl FromStr for bool {
    type Err = ParseBoolError;

    /// Parse a `bool` from a string.
    ///
    /// The only accepted values are `"true"` and `"false"`. Any other input
    /// will return an error.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::str::FromStr;
    ///
    /// assert_eq!(FromStr::from_str("true"), Ok(true));
    /// assert_eq!(FromStr::from_str("false"), Ok(false));
    /// assert!(<bool as FromStr>::from_str("not even a boolean").is_err());
    /// ```
    ///
    /// Note, in many cases, the `.parse()` method on `str` is more proper.
    ///
    /// ```
    /// assert_eq!("true".parse(), Ok(true));
    /// assert_eq!("false".parse(), Ok(false));
    /// assert!("not even a boolean".parse::<bool>().is_err());
    /// ```
    #[inline]
    fn from_str(s: &str) -> Result<bool, ParseBoolError> {
        match s {
            "true" => Ok(true),
            "false" => Ok(false),
            _ => Err(ParseBoolError),
        }
    }
}