1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
//! Ways to create a `str` from bytes slice.

use crate::mem;

use super::validations::run_utf8_validation;
use super::Utf8Error;

/// Converts a slice of bytes to a string slice.
///
/// A string slice ([`&str`]) is made of bytes ([`u8`]), and a byte slice
/// ([`&[u8]`][byteslice]) is made of bytes, so this function converts between
/// the two. Not all byte slices are valid string slices, however: [`&str`] requires
/// that it is valid UTF-8. `from_utf8()` checks to ensure that the bytes are valid
/// UTF-8, and then does the conversion.
///
/// [`&str`]: str
/// [byteslice]: slice
///
/// If you are sure that the byte slice is valid UTF-8, and you don't want to
/// incur the overhead of the validity check, there is an unsafe version of
/// this function, [`from_utf8_unchecked`], which has the same
/// behavior but skips the check.
///
/// If you need a `String` instead of a `&str`, consider
/// [`String::from_utf8`][string].
///
/// [string]: ../../std/string/struct.String.html#method.from_utf8
///
/// Because you can stack-allocate a `[u8; N]`, and you can take a
/// [`&[u8]`][byteslice] of it, this function is one way to have a
/// stack-allocated string. There is an example of this in the
/// examples section below.
///
/// [byteslice]: slice
///
/// # Errors
///
/// Returns `Err` if the slice is not UTF-8 with a description as to why the
/// provided slice is not UTF-8.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // some bytes, in a vector
/// let sparkle_heart = vec![240, 159, 146, 150];
///
/// // We know these bytes are valid, so just use `unwrap()`.
/// let sparkle_heart = str::from_utf8(&sparkle_heart).unwrap();
///
/// assert_eq!("💖", sparkle_heart);
/// ```
///
/// Incorrect bytes:
///
/// ```
/// use std::str;
///
/// // some invalid bytes, in a vector
/// let sparkle_heart = vec![0, 159, 146, 150];
///
/// assert!(str::from_utf8(&sparkle_heart).is_err());
/// ```
///
/// See the docs for [`Utf8Error`] for more details on the kinds of
/// errors that can be returned.
///
/// A "stack allocated string":
///
/// ```
/// use std::str;
///
/// // some bytes, in a stack-allocated array
/// let sparkle_heart = [240, 159, 146, 150];
///
/// // We know these bytes are valid, so just use `unwrap()`.
/// let sparkle_heart = str::from_utf8(&sparkle_heart).unwrap();
///
/// assert_eq!("💖", sparkle_heart);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
    run_utf8_validation(v)?;
    // SAFETY: Just ran validation.
    Ok(unsafe { from_utf8_unchecked(v) })
}

/// Converts a mutable slice of bytes to a mutable string slice.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // "Hello, Rust!" as a mutable vector
/// let mut hellorust = vec![72, 101, 108, 108, 111, 44, 32, 82, 117, 115, 116, 33];
///
/// // As we know these bytes are valid, we can use `unwrap()`
/// let outstr = str::from_utf8_mut(&mut hellorust).unwrap();
///
/// assert_eq!("Hello, Rust!", outstr);
/// ```
///
/// Incorrect bytes:
///
/// ```
/// use std::str;
///
/// // Some invalid bytes in a mutable vector
/// let mut invalid = vec![128, 223];
///
/// assert!(str::from_utf8_mut(&mut invalid).is_err());
/// ```
/// See the docs for [`Utf8Error`] for more details on the kinds of
/// errors that can be returned.
#[stable(feature = "str_mut_extras", since = "1.20.0")]
pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
    run_utf8_validation(v)?;
    // SAFETY: Just ran validation.
    Ok(unsafe { from_utf8_unchecked_mut(v) })
}

/// Converts a slice of bytes to a string slice without checking
/// that the string contains valid UTF-8.
///
/// See the safe version, [`from_utf8`], for more information.
///
/// # Safety
///
/// This function is unsafe because it does not check that the bytes passed to
/// it are valid UTF-8. If this constraint is violated, undefined behavior
/// results, as the rest of Rust assumes that [`&str`]s are valid UTF-8.
///
/// [`&str`]: str
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// // some bytes, in a vector
/// let sparkle_heart = vec![240, 159, 146, 150];
///
/// let sparkle_heart = unsafe {
///     str::from_utf8_unchecked(&sparkle_heart)
/// };
///
/// assert_eq!("💖", sparkle_heart);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_str_from_utf8_unchecked", since = "1.55.0")]
#[rustc_allow_const_fn_unstable(const_fn_transmute)]
pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
    // SAFETY: the caller must guarantee that the bytes `v` are valid UTF-8.
    // Also relies on `&str` and `&[u8]` having the same layout.
    unsafe { mem::transmute(v) }
}

/// Converts a slice of bytes to a string slice without checking
/// that the string contains valid UTF-8; mutable version.
///
/// See the immutable version, [`from_utf8_unchecked()`] for more information.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::str;
///
/// let mut heart = vec![240, 159, 146, 150];
/// let heart = unsafe { str::from_utf8_unchecked_mut(&mut heart) };
///
/// assert_eq!("💖", heart);
/// ```
#[inline]
#[stable(feature = "str_mut_extras", since = "1.20.0")]
pub unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
    // SAFETY: the caller must guarantee that the bytes `v`
    // are valid UTF-8, thus the cast to `*mut str` is safe.
    // Also, the pointer dereference is safe because that pointer
    // comes from a reference which is guaranteed to be valid for writes.
    unsafe { &mut *(v as *mut [u8] as *mut str) }
}