Do not use CString in the examples of CStr.

Fixes #83999.

Author: hkBst

library/core/src/ffi/c_str.rs

@@ -55,18 +55,15 @@ use crate::{fmt, ops, slice, str};
 /// Passing a Rust-originating C string:
 ///
 /// ```
-/// use std::ffi::{CString, CStr};
+/// use std::ffi::CStr;
 /// use std::os::raw::c_char;
 ///
 /// fn work(data: &CStr) {
-/// #   /* Extern functions are awkward in doc comments - fake it instead
-///     extern "C" { fn work_with(data: *const c_char); }
-/// #   */ unsafe extern "C" fn work_with(s: *const c_char) {}
-///
+///     unsafe extern "C" fn work_with(s: *const c_char) {}
 ///     unsafe { work_with(data.as_ptr()) }
 /// }
 ///
-/// let s = CString::new("data data data data").expect("CString::new failed");
+/// let s = c"Hello world!";
 /// work(&s);
 /// ```
 ///
@@ -384,13 +381,12 @@ impl CStr {
     /// # Examples
     ///
     /// ```
-    /// use std::ffi::{CStr, CString};
+    /// use std::ffi::CStr;
     ///
-    /// unsafe {
-    ///     let cstring = CString::new("hello").expect("CString::new failed");
-    ///     let cstr = CStr::from_bytes_with_nul_unchecked(cstring.to_bytes_with_nul());
-    ///     assert_eq!(cstr, &*cstring);
-    /// }
+    /// let bytes = b"Hello world!\0";
+    ///
+    /// let cstr = unsafe { CStr::from_bytes_with_nul_unchecked(bytes) };
+    /// assert_eq!(cstr.to_bytes_with_nul(), bytes);
     /// ```
     #[inline]
     #[must_use]
@@ -449,38 +445,60 @@ impl CStr {
     /// behavior when `ptr` is used inside the `unsafe` block:
     ///
     /// ```no_run
-    /// # #![allow(unused_must_use)]
     /// # #![expect(dangling_pointers_from_temporaries)]
-    /// use std::ffi::CString;
-    ///
     /// // Do not do this:
-    /// let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();
-    /// unsafe {
-    ///     // `ptr` is dangling
-    ///     *ptr;
-    /// }
+    /// let ptr = std::ffi::CStr::from_bytes_with_nul(
+    ///     &"Hello world!"
+    ///         .to_uppercase()
+    ///         .chars()
+    ///         .map(|c| c.try_into().unwrap())
+    ///         .chain(std::iter::once(0))
+    ///         .collect::<Vec<u8>>(),
+    /// )
+    /// .unwrap()
+    /// .as_ptr();
+    /// // above CStr temporary was dropped and `ptr` is now dangling!
+    ///
+    /// // shows ptr is now garbage
+    /// assert_eq!(
+    ///     (0..)
+    ///         .map_while(|i| unsafe {
+    ///             let p = ptr.add(i);
+    ///             (*p != 0).then_some(*p as u8 as char)
+    ///         })
+    ///         .collect::<String>(),
+    ///     "Hello world!".to_uppercase()
+    /// );
     /// ```
     ///
     /// This happens because the pointer returned by `as_ptr` does not carry any
-    /// lifetime information and the `CString` is deallocated immediately after
-    /// the `CString::new("Hello").expect("CString::new failed").as_ptr()`
-    /// expression is evaluated.
-    /// To fix the problem, bind the `CString` to a local variable:
+    /// lifetime information and the `CStr` is deallocated immediately after
+    /// the expression it is part of is evaluated.
+    /// To fix the problem, bind the `CStr` to a local variable:
     ///
     /// ```no_run
-    /// # #![allow(unused_must_use)]
-    /// use std::ffi::CString;
-    ///
-    /// let hello = CString::new("Hello").expect("CString::new failed");
-    /// let ptr = hello.as_ptr();
-    /// unsafe {
-    ///     // `ptr` is valid because `hello` is in scope
-    ///     *ptr;
-    /// }
+    /// let c_str = std::ffi::CStr::from_bytes_with_nul(
+    ///     &"Hello world!"
+    ///         .to_uppercase()
+    ///         .chars()
+    ///         .map(|c| c.try_into().unwrap())
+    ///         .chain(std::iter::once(0))
+    ///         .collect::<Vec<u8>>(),
+    /// )
+    /// .unwrap();
+    /// // above CStr is now bound and will not be dropped immediately
+    /// let ptr = c_str.as_ptr();
+    ///
+    /// assert_eq!(
+    ///     (0..)
+    ///         .map_while(|i| unsafe {
+    ///             let p = ptr.add(i);
+    ///             (*p != 0).then_some(*p as u8 as char)
+    ///         })
+    ///         .collect::<String>(),
+    ///     "Hello world!".to_uppercase()
+    /// );
     /// ```
-    ///
-    /// This way, the lifetime of the `CString` in `hello` encompasses
-    /// the lifetime of `ptr` and the `unsafe` block.
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]