1//! Ways to create a `str` from bytes slice.
2
3use super::Utf8Error;
4use super::validations::run_utf8_validation;
5use crate::{mem, ptr};
6
7/// Converts a slice of bytes to a string slice.
8///
9/// This is an alias to [`str::from_utf8`].
10///
11/// A string slice ([`&str`]) is made of bytes ([`u8`]), and a byte slice
12/// ([`&[u8]`][byteslice]) is made of bytes, so this function converts between
13/// the two. Not all byte slices are valid string slices, however: [`&str`] requires
14/// that it is valid UTF-8. `from_utf8()` checks to ensure that the bytes are valid
15/// UTF-8, and then does the conversion.
16///
17/// [`&str`]: str
18/// [byteslice]: slice
19///
20/// If you are sure that the byte slice is valid UTF-8, and you don't want to
21/// incur the overhead of the validity check, there is an unsafe version of
22/// this function, [`from_utf8_unchecked`], which has the same
23/// behavior but skips the check.
24///
25/// If you need a `String` instead of a `&str`, consider
26/// [`String::from_utf8`][string].
27///
28/// [string]: ../../std/string/struct.String.html#method.from_utf8
29///
30/// Because you can stack-allocate a `[u8; N]`, and you can take a
31/// [`&[u8]`][byteslice] of it, this function is one way to have a
32/// stack-allocated string. There is an example of this in the
33/// examples section below.
34///
35/// [byteslice]: slice
36///
37/// # Errors
38///
39/// Returns `Err` if the slice is not UTF-8 with a description as to why the
40/// provided slice is not UTF-8.
41///
42/// # Examples
43///
44/// Basic usage:
45///
46/// ```
47/// use std::str;
48///
49/// // some bytes, in a vector
50/// let sparkle_heart = vec![240, 159, 146, 150];
51///
52/// // We can use the ? (try) operator to check if the bytes are valid
53/// let sparkle_heart = str::from_utf8(&sparkle_heart)?;
54///
55/// assert_eq!("💖", sparkle_heart);
56/// # Ok::<_, str::Utf8Error>(())
57/// ```
58///
59/// Incorrect bytes:
60///
61/// ```
62/// use std::str;
63///
64/// // some invalid bytes, in a vector
65/// let sparkle_heart = vec![0, 159, 146, 150];
66///
67/// assert!(str::from_utf8(&sparkle_heart).is_err());
68/// ```
69///
70/// See the docs for [`Utf8Error`] for more details on the kinds of
71/// errors that can be returned.
72///
73/// A "stack allocated string":
74///
75/// ```
76/// use std::str;
77///
78/// // some bytes, in a stack-allocated array
79/// let sparkle_heart = [240, 159, 146, 150];
80///
81/// // We know these bytes are valid, so just use `unwrap()`.
82/// let sparkle_heart: &str = str::from_utf8(&sparkle_heart).unwrap();
83///
84/// assert_eq!("💖", sparkle_heart);
85/// ```
86#[stable(feature = "rust1", since = "1.0.0")]
87#[rustc_const_stable(feature = "const_str_from_utf8_shared", since = "1.63.0")]
88#[rustc_diagnostic_item = "str_from_utf8"]
89pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
90 // FIXME(const-hack): This should use `?` again, once it's `const`
91 match run_utf8_validation(v) {
92 Ok(_) => {
93 // SAFETY: validation succeeded.
94 Ok(unsafe { from_utf8_unchecked(v) })
95 }
96 Err(err: Utf8Error) => Err(err),
97 }
98}
99
100/// Converts a mutable slice of bytes to a mutable string slice.
101///
102/// This is an alias to [`str::from_utf8_mut`].
103///
104/// # Examples
105///
106/// Basic usage:
107///
108/// ```
109/// use std::str;
110///
111/// // "Hello, Rust!" as a mutable vector
112/// let mut hellorust = vec![72, 101, 108, 108, 111, 44, 32, 82, 117, 115, 116, 33];
113///
114/// // As we know these bytes are valid, we can use `unwrap()`
115/// let outstr = str::from_utf8_mut(&mut hellorust).unwrap();
116///
117/// assert_eq!("Hello, Rust!", outstr);
118/// ```
119///
120/// Incorrect bytes:
121///
122/// ```
123/// use std::str;
124///
125/// // Some invalid bytes in a mutable vector
126/// let mut invalid = vec![128, 223];
127///
128/// assert!(str::from_utf8_mut(&mut invalid).is_err());
129/// ```
130/// See the docs for [`Utf8Error`] for more details on the kinds of
131/// errors that can be returned.
132#[stable(feature = "str_mut_extras", since = "1.20.0")]
133#[rustc_const_stable(feature = "const_str_from_utf8", since = "1.87.0")]
134#[rustc_diagnostic_item = "str_from_utf8_mut"]
135pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
136 // FIXME(const-hack): This should use `?` again, once it's `const`
137 match run_utf8_validation(v) {
138 Ok(_) => {
139 // SAFETY: validation succeeded.
140 Ok(unsafe { from_utf8_unchecked_mut(v) })
141 }
142 Err(err: Utf8Error) => Err(err),
143 }
144}
145
146/// Converts a slice of bytes to a string slice without checking
147/// that the string contains valid UTF-8.
148///
149/// This is an alias to [`str::from_utf8_unchecked`].
150///
151/// See the safe version, [`from_utf8`], for more information.
152///
153/// # Safety
154///
155/// The bytes passed in must be valid UTF-8.
156///
157/// # Examples
158///
159/// Basic usage:
160///
161/// ```
162/// use std::str;
163///
164/// // some bytes, in a vector
165/// let sparkle_heart = vec![240, 159, 146, 150];
166///
167/// let sparkle_heart = unsafe {
168/// str::from_utf8_unchecked(&sparkle_heart)
169/// };
170///
171/// assert_eq!("💖", sparkle_heart);
172/// ```
173#[inline]
174#[must_use]
175#[stable(feature = "rust1", since = "1.0.0")]
176#[rustc_const_stable(feature = "const_str_from_utf8_unchecked", since = "1.55.0")]
177#[rustc_diagnostic_item = "str_from_utf8_unchecked"]
178pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
179 // SAFETY: the caller must guarantee that the bytes `v` are valid UTF-8.
180 // Also relies on `&str` and `&[u8]` having the same layout.
181 unsafe { mem::transmute(src:v) }
182}
183
184/// Converts a slice of bytes to a string slice without checking
185/// that the string contains valid UTF-8; mutable version.
186///
187/// This is an alias to [`str::from_utf8_unchecked_mut`].
188///
189/// See the immutable version, [`from_utf8_unchecked()`] for documentation and safety requirements.
190///
191/// # Examples
192///
193/// Basic usage:
194///
195/// ```
196/// use std::str;
197///
198/// let mut heart = vec![240, 159, 146, 150];
199/// let heart = unsafe { str::from_utf8_unchecked_mut(&mut heart) };
200///
201/// assert_eq!("💖", heart);
202/// ```
203#[inline]
204#[must_use]
205#[stable(feature = "str_mut_extras", since = "1.20.0")]
206#[rustc_const_stable(feature = "const_str_from_utf8_unchecked_mut", since = "1.83.0")]
207#[rustc_diagnostic_item = "str_from_utf8_unchecked_mut"]
208pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
209 // SAFETY: the caller must guarantee that the bytes `v`
210 // are valid UTF-8, thus the cast to `*mut str` is safe.
211 // Also, the pointer dereference is safe because that pointer
212 // comes from a reference which is guaranteed to be valid for writes.
213 unsafe { &mut *(v as *mut [u8] as *mut str) }
214}
215
216/// Creates a `&str` from a pointer and a length.
217///
218/// The pointed-to bytes must be valid UTF-8.
219/// If this might not be the case, use `str::from_utf8(slice::from_raw_parts(ptr, len))`,
220/// which will return an `Err` if the data isn't valid UTF-8.
221///
222/// This function is the `str` equivalent of [`slice::from_raw_parts`](crate::slice::from_raw_parts).
223/// See that function's documentation for safety concerns and examples.
224///
225/// The mutable version of this function is [`from_raw_parts_mut`].
226#[inline]
227#[must_use]
228#[unstable(feature = "str_from_raw_parts", issue = "119206")]
229pub const unsafe fn from_raw_parts<'a>(ptr: *const u8, len: usize) -> &'a str {
230 // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
231 unsafe { &*ptr::from_raw_parts(data_pointer:ptr, metadata:len) }
232}
233
234/// Creates a `&mut str` from a pointer and a length.
235///
236/// The pointed-to bytes must be valid UTF-8.
237/// If this might not be the case, use `str::from_utf8_mut(slice::from_raw_parts_mut(ptr, len))`,
238/// which will return an `Err` if the data isn't valid UTF-8.
239///
240/// This function is the `str` equivalent of [`slice::from_raw_parts_mut`](crate::slice::from_raw_parts_mut).
241/// See that function's documentation for safety concerns and examples.
242///
243/// The immutable version of this function is [`from_raw_parts`].
244#[inline]
245#[must_use]
246#[unstable(feature = "str_from_raw_parts", issue = "119206")]
247pub const unsafe fn from_raw_parts_mut<'a>(ptr: *mut u8, len: usize) -> &'a mut str {
248 // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
249 unsafe { &mut *ptr::from_raw_parts_mut(data_pointer:ptr, metadata:len) }
250}
251