Describe and use CStr literals in CStr and CString docs

Author: GKFX

library/alloc/src/ffi/c_str.rs

@@ -41,6 +41,7 @@ use crate::sync::Arc;
 /// or anything that implements <code>[Into]<[Vec]<[u8]>></code> (for
 /// example, you can build a `CString` straight out of a [`String`] or
 /// a <code>&[str]</code>, since both implement that trait).
+/// You can create a `CString` from a literal with `CString::from(c"Text")`.
 ///
 /// The [`CString::new`] method will actually check that the provided <code>&[[u8]]</code>
 /// does not have 0 bytes in the middle, and return an error if it
@@ -1069,27 +1070,22 @@ impl CStr {
     ///
     /// # Examples
     ///
-    /// Calling `to_string_lossy` on a `CStr` containing valid UTF-8:
+    /// Calling `to_string_lossy` on a `CStr` containing valid UTF-8. The leading
+    /// `c` on the string literal denotes a `CStr`.
     ///
     /// ```
     /// use std::borrow::Cow;
-    /// use std::ffi::CStr;
     ///
-    /// let cstr = CStr::from_bytes_with_nul(b"Hello World\0")
-    ///                  .expect("CStr::from_bytes_with_nul failed");
-    /// assert_eq!(cstr.to_string_lossy(), Cow::Borrowed("Hello World"));
+    /// assert_eq!(c"Hello World".to_string_lossy(), Cow::Borrowed("Hello World"));
     /// ```
     ///
     /// Calling `to_string_lossy` on a `CStr` containing invalid UTF-8:
     ///
     /// ```
     /// use std::borrow::Cow;
-    /// use std::ffi::CStr;
     ///
-    /// let cstr = CStr::from_bytes_with_nul(b"Hello \xF0\x90\x80World\0")
-    ///                  .expect("CStr::from_bytes_with_nul failed");
     /// assert_eq!(
-    ///     cstr.to_string_lossy(),
+    ///     c"Hello \xF0\x90\x80World",
     ///     Cow::Owned(String::from("Hello �World")) as Cow<'_, str>
     /// );
     /// ```

library/core/src/ffi/c_str.rs

@@ -23,28 +23,32 @@ use crate::str;
 ///
 /// This type represents a borrowed reference to a nul-terminated
 /// array of bytes. It can be constructed safely from a <code>&[[u8]]</code>
-/// slice, or unsafely from a raw `*const c_char`. It can then be
-/// converted to a Rust <code>&[str]</code> by performing UTF-8 validation, or
-/// into an owned `CString`.
+/// slice, or unsafely from a raw `*const c_char`. It can be expressed as a
+/// literal in the form `c"Hello world"`.
+///
+/// The `CStr` can then be converted to a Rust <code>&[str]</code> by performing
+/// UTF-8 validation, or into an owned `CString`.
 ///
 /// `&CStr` is to `CString` as <code>&[str]</code> is to `String`: the former
 /// in each pair are borrowed references; the latter are owned
 /// strings.
 ///
 /// Note that this structure does **not** have a guaranteed layout (the `repr(transparent)`
-/// notwithstanding) and is not recommended to be placed in the signatures of FFI functions.
-/// Instead, safe wrappers of FFI functions may leverage the unsafe [`CStr::from_ptr`] constructor
-/// to provide a safe interface to other consumers.
+/// notwithstanding) and should not be placed in the signatures of FFI functions.
+/// Instead, safe wrappers of FFI functions may leverage [`CStr::as_ptr`] and the unsafe
+/// [`CStr::from_ptr`] constructor to provide a safe interface to other consumers.
 ///
 /// # Examples
 ///
 /// Inspecting a foreign C string:
 ///
-/// ```ignore (extern-declaration)
+/// ```
 /// use std::ffi::CStr;
 /// use std::os::raw::c_char;
 ///
+/// # #[cfg(any())] // Extern functions are awkward in doc comments - fake it instead
 /// extern "C" { fn my_string() -> *const c_char; }
+/// # unsafe extern "C" fn my_string() -> *const c_char { c"hello".as_ptr() }
 ///
 /// unsafe {
 ///     let slice = CStr::from_ptr(my_string());
@@ -54,12 +58,14 @@ use crate::str;
 ///
 /// Passing a Rust-originating C string:
 ///
-/// ```ignore (extern-declaration)
+/// ```
 /// use std::ffi::{CString, CStr};
 /// use std::os::raw::c_char;
 ///
 /// fn work(data: &CStr) {
+/// #   #[cfg(any())] // 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 { work_with(data.as_ptr()) }
 /// }
@@ -70,11 +76,13 @@ use crate::str;
 ///
 /// Converting a foreign C string into a Rust `String`:
 ///
-/// ```ignore (extern-declaration)
+/// ```
 /// use std::ffi::CStr;
 /// use std::os::raw::c_char;
 ///
+/// # #[cfg(any())] // Extern functions are awkward in doc comments - fake it instead
 /// extern "C" { fn my_string() -> *const c_char; }
+/// # unsafe extern "C" fn my_string() -> *const c_char { c"hello".as_ptr() }
 ///
 /// fn my_string_safe() -> String {
 ///     let cstr = unsafe { CStr::from_ptr(my_string()) };
@@ -241,11 +249,11 @@ impl CStr {
     ///
     /// # Examples
     ///
-    /// ```ignore (extern-declaration)
+    /// ```
     /// use std::ffi::{c_char, CStr};
     ///
-    /// extern "C" {
-    ///     fn my_string() -> *const c_char;
+    /// fn my_string() -> *const c_char {
+    ///     c"hello".as_ptr()
     /// }
     ///
     /// unsafe {
@@ -264,6 +272,8 @@ impl CStr {
     ///     BYTES.as_ptr().cast()
     /// };
     /// const HELLO: &CStr = unsafe { CStr::from_ptr(HELLO_PTR) };
+    ///
+    /// assert_eq!(c"Hello, world!", HELLO);
     /// ```
     ///
     /// [valid]: core::ptr#safety
@@ -549,6 +559,7 @@ impl CStr {
     ///
     /// let empty_cstr = CStr::from_bytes_with_nul(b"\0")?;
     /// assert!(empty_cstr.is_empty());
+    /// assert!(c"".is_empty());
     /// # Ok(())
     /// # }
     /// ```