implement inherent str constructors

library/core/src/str/mod.rs

@@ -160,6 +160,182 @@ impl str {
         self.len() == 0
     }
 
+    /// 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]: prim@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 can use the ? (try) operator to check if the bytes are valid
+    /// let sparkle_heart = str::from_utf8(&sparkle_heart)?;
+    ///
+    /// assert_eq!("💖", sparkle_heart);
+    /// # Ok::<_, str::Utf8Error>(())
+    /// ```
+    ///
+    /// 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 = str::from_utf8(&sparkle_heart).unwrap();
+    ///
+    /// assert_eq!("💖", sparkle_heart);
+    /// ```
+    #[unstable(feature = "inherent_str_constructors", issue = "131114")]
+    pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error> {
+        converts::from_utf8(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.
+    #[unstable(feature = "inherent_str_constructors", issue = "131114")]
+    #[rustc_const_unstable(feature = "const_str_from_utf8", issue = "91006")]
+    pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
+        converts::from_utf8_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
+    ///
+    /// The bytes passed in must be valid UTF-8.
+    ///
+    /// # 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]
+    #[must_use]
+    #[unstable(feature = "inherent_str_constructors", issue = "131114")]
+    pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
+        // SAFETY: converts::from_utf8_unchecked has the same safety requirements as this function.
+        unsafe { converts::from_utf8_unchecked(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]
+    #[must_use]
+    #[unstable(feature = "inherent_str_constructors", issue = "131114")]
+    pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
+        // SAFETY: converts::from_utf8_unchecked_mut has the same safety requirements as this function.
+        unsafe { converts::from_utf8_unchecked_mut(v) }
+    }
+
     /// Checks that `index`-th byte is the first byte in a UTF-8 code point
     /// sequence or the end of the string.
     ///

tests/ui/suggestions/suggest-std-when-using-type.fixed

@@ -1,8 +1,5 @@
 //@ run-rustfix
 fn main() {
     let pi = std::f32::consts::PI; //~ ERROR ambiguous associated type
-    let bytes = "hello world".as_bytes();
-    let string = std::str::from_utf8(bytes).unwrap();
-    //~^ ERROR no function or associated item named `from_utf8` found
-    println!("{pi} {bytes:?} {string}");
+    println!("{pi}");
 }

tests/ui/suggestions/suggest-std-when-using-type.rs

@@ -1,8 +1,5 @@
 //@ run-rustfix
 fn main() {
     let pi = f32::consts::PI; //~ ERROR ambiguous associated type
-    let bytes = "hello world".as_bytes();
-    let string = str::from_utf8(bytes).unwrap();
-    //~^ ERROR no function or associated item named `from_utf8` found
-    println!("{pi} {bytes:?} {string}");
+    println!("{pi}");
 }

tests/ui/suggestions/suggest-std-when-using-type.stderr

@@ -9,18 +9,6 @@ help: you are looking for the module in `std`, not the primitive type
 LL |     let pi = std::f32::consts::PI;
    |              +++++
 
-error[E0599]: no function or associated item named `from_utf8` found for type `str` in the current scope
-  --> $DIR/suggest-std-when-using-type.rs:5:23
-   |
-LL |     let string = str::from_utf8(bytes).unwrap();
-   |                       ^^^^^^^^^ function or associated item not found in `str`
-   |
-help: you are looking for the module in `std`, not the primitive type
-   |
-LL |     let string = std::str::from_utf8(bytes).unwrap();
-   |                  +++++
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0223, E0599.
-For more information about an error, try `rustc --explain E0223`.
+For more information about this error, try `rustc --explain E0223`.