Extract find and is_cstr const fn to an inner imp module

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "qed"
-version = "1.3.0"
+version = "1.3.1"
 authors = ["Ryan Lopopolo <rjl@hyperbo.la>"]
 license = "MIT"
 edition = "2021"

README.md

@@ -23,7 +23,7 @@ Add this to your `Cargo.toml`:
 
 ```toml
 [dependencies]
-qed = "1.3.0"
+qed = "1.3.1"
 ```
 
 Then make compile time assertions like:

src/imp.rs

@@ -0,0 +1,55 @@
+#[must_use]
+pub const fn find(slice: &[u8], elem: u8) -> Option<usize> {
+    let mut idx = 0;
+    loop {
+        if idx == slice.len() {
+            return None;
+        }
+        if slice[idx] == elem {
+            return Some(idx);
+        }
+        idx += 1;
+    }
+}
+
+#[must_use]
+pub const fn is_cstr(slice: &[u8]) -> bool {
+    matches!(find(slice, 0), Some(nul_pos) if nul_pos + 1 == slice.len())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{find, is_cstr};
+
+    #[test]
+    fn find_nul_byte() {
+        assert_eq!(find(b"", 0), None);
+        assert_eq!(find(b"abc", 0), None);
+        assert_eq!(find(b"abc\xFFxyz", 0), None);
+
+        assert_eq!(find(b"abc\0xyz", 0), Some(3));
+        assert_eq!(find(b"abc\0xyz\0", 0), Some(3));
+        assert_eq!(find(b"abc\xFF\0xyz", 0), Some(4));
+        assert_eq!(find(b"abc\xFF\0xyz\0", 0), Some(4));
+
+        assert_eq!(find(b"\0", 0), Some(0));
+        assert_eq!(find(b"abc\0", 0), Some(3));
+        assert_eq!(find(b"abc\xFFxyz\0", 0), Some(7));
+    }
+
+    #[test]
+    fn check_is_cstr() {
+        assert!(!is_cstr(b""));
+        assert!(!is_cstr(b"abc"));
+        assert!(!is_cstr(b"abc\xFFxyz"));
+
+        assert!(!is_cstr(b"abc\0xyz"));
+        assert!(!is_cstr(b"abc\0xyz\0"));
+        assert!(!is_cstr(b"abc\xFF\0xyz"));
+        assert!(!is_cstr(b"abc\xFF\0xyz\0"));
+
+        assert!(is_cstr(b"\0"));
+        assert!(is_cstr(b"abc\0"));
+        assert!(is_cstr(b"abc\xFFxyz\0"));
+    }
+}

src/lib.rs

@@ -43,7 +43,7 @@
 //! ```
 
 #![no_std]
-#![doc(html_root_url = "https://docs.rs/qed/1.3.0")]
+#![doc(html_root_url = "https://docs.rs/qed/1.3.1")]
 
 #[cfg(any(test, doc))]
 extern crate std;
@@ -53,6 +53,10 @@ extern crate std;
 #[doc = include_str!("../README.md")]
 mod readme {}
 
+#[doc(hidden)]
+#[allow(missing_docs)]
+pub mod imp;
+
 /// Asserts that a boolean expression is true at compile time.
 ///
 /// This will result in a compile time type error if the boolean expression does
@@ -274,25 +278,11 @@ macro_rules! const_assert_size_eq {
 /// ```
 #[macro_export]
 macro_rules! const_assert_bytes_has_no_nul {
-    ($bytes:expr $(,)?) => {
-        $crate::const_assert!({
-            const fn byte_slice_contains(slice: &[u8], elem: u8) -> bool {
-                let mut idx = 0;
-                loop {
-                    if idx >= slice.len() {
-                        return false;
-                    }
-                    if slice[idx] == elem {
-                        return true;
-                    }
-                    idx += 1;
-                }
-            }
-
-            const BYTES: &[u8] = $bytes;
-            !byte_slice_contains(BYTES, 0_u8)
-        });
-    };
+    ($bytes:expr $(,)?) => {{
+        const _: &[u8] = $bytes;
+
+        $crate::const_assert!($crate::imp::find($bytes, 0_u8).is_none());
+    }};
 }
 
 /// Construct a const [`CStr`] from the given bytes at compile time and assert
@@ -345,27 +335,10 @@ macro_rules! const_assert_bytes_has_no_nul {
 #[macro_export]
 macro_rules! const_cstr_from_bytes {
     ($bytes:expr $(,)?) => {{
-        const BYTES: &[u8] = $bytes;
-
-        $crate::const_assert!({
-            const fn byte_slice_is_cstr(slice: &[u8]) -> bool {
-                let mut idx = slice.len() - 1;
-                if slice[idx] != 0 {
-                    return false;
-                }
-                loop {
-                    if idx == 0 {
-                        return true;
-                    }
-                    idx -= 1;
-                    if slice[idx] == 0 {
-                        return false;
-                    }
-                }
-            }
-
-            byte_slice_is_cstr(BYTES)
-        });
+        const _: &[u8] = $bytes;
+
+        $crate::const_assert!($crate::imp::is_cstr($bytes));
+
         // SAFETY
         //
         // The compile time assert above ensures the given bytes:
@@ -376,7 +349,7 @@ macro_rules! const_cstr_from_bytes {
         // which meets the safety criteria for `CStr::from_bytes_with_nul_unchecked`.
         //
         // https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#method.from_bytes_with_nul_unchecked
-        unsafe { ::std::ffi::CStr::from_bytes_with_nul_unchecked(BYTES) }
+        unsafe { ::std::ffi::CStr::from_bytes_with_nul_unchecked($bytes) }
     }};
 }
 
@@ -524,4 +497,13 @@ mod tests {
         assert_eq!(CSTR.to_bytes(), b"Array");
         assert!(EMPTY.to_bytes().is_empty());
     }
+
+    #[test]
+    fn const_assert_bytes_has_no_nul_none_shadow() {
+        #[allow(dead_code)]
+        #[allow(non_upper_case_globals)]
+        const None: () = ();
+
+        crate::const_assert_bytes_has_no_nul!("abcdefg".as_bytes());
+    }
 }