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Rollup merge of #102961 - reitermarkus:const-cstr-from-ptr, r=oli-obk

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Make `CStr::from_ptr` `const`.

Should be included in https://github.com/rust-lang/rust/issues/101719.

cc ``@WaffleLapkin``
This commit is contained in:
Matthias Krüger 2022-10-29 08:57:34 +02:00 committed by GitHub
commit b3ca68f9e9
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1 changed files with 37 additions and 11 deletions
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48
library/core/src/ffi/c_str.rs
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@ -221,9 +221,7 @@ impl CStr {
/// # Examples
///
/// ```ignore (extern-declaration)
/// # fn main() {
/// use std::ffi::CStr;
/// use std::os::raw::c_char;
/// use std::ffi::{c_char, CStr};
///
/// extern "C" {
/// fn my_string() -> *const c_char;
@ -233,14 +231,26 @@ impl CStr {
/// let slice = CStr::from_ptr(my_string());
/// println!("string returned: {}", slice.to_str().unwrap());
/// }
/// # }
/// ```
///
/// ```
/// #![feature(const_cstr_methods)]
///
/// use std::ffi::{c_char, CStr};
///
/// const HELLO_PTR: *const c_char = {
/// const BYTES: &[u8] = b"Hello, world!\0";
/// BYTES.as_ptr().cast()
/// };
/// const HELLO: &CStr = unsafe { CStr::from_ptr(HELLO_PTR) };
/// ```
///
/// [valid]: core::ptr#safety
#[inline]
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
#[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
pub const unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
// SAFETY: The caller has provided a pointer that points to a valid C
// string with a NUL terminator of size less than `isize::MAX`, whose
// content remain valid and doesn't change for the lifetime of the
@ -252,13 +262,29 @@ pub unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
//
// The cast from c_char to u8 is ok because a c_char is always one byte.
unsafe {
extern "C" {
/// Provided by libc or compiler_builtins.
fn strlen(s: *const c_char) -> usize;
const fn strlen_ct(s: *const c_char) -> usize {
let mut len = 0;
// SAFETY: Outer caller has provided a pointer to a valid C string.
while unsafe { *s.add(len) } != 0 {
len += 1;
}
len
}
let len = strlen(ptr);
let ptr = ptr as *const u8;
CStr::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr, len as usize + 1))
fn strlen_rt(s: *const c_char) -> usize {
extern "C" {
/// Provided by libc or compiler_builtins.
fn strlen(s: *const c_char) -> usize;
}
// SAFETY: Outer caller has provided a pointer to a valid C string.
unsafe { strlen(s) }
}
let len = intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt);
Self::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr.cast(), len + 1))
}
}