From 328f81713cc80861fb79c629e4887de1a8b7bf05 Mon Sep 17 00:00:00 2001 From: Markus Reiter Date: Wed, 12 Oct 2022 12:57:16 +0200 Subject: [PATCH] Make `CStr::from_ptr` `const`. --- library/core/src/ffi/c_str.rs | 32 +++++++++++++++++++++++++------- 1 file changed, 25 insertions(+), 7 deletions(-) diff --git a/library/core/src/ffi/c_str.rs b/library/core/src/ffi/c_str.rs index 55e58c4e0baa5..ae15be793414e 100644 --- a/library/core/src/ffi/c_str.rs +++ b/library/core/src/ffi/c_str.rs @@ -240,7 +240,9 @@ impl CStr { #[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")] + #[rustc_allow_const_fn_unstable(const_eval_select)] + 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 +254,29 @@ impl 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)) } }