Enforce const evaluation in path! macro

By using a temporary const value, we can enforce that the path! macro
always causes a compiler error for illegal values even if it is not used
in a const context.  This makes it easier to write correct code.

CHANGELOG.md

@@ -18,6 +18,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed build error that would occur on Windows systems.
 - Added path iteration utilities ([#47][])
 
+## Changed
+
+- Enforced const evaluation for `path!`.
+
 [#47]: https://github.com/trussed-dev/littlefs2/pull/47
 [#57]: https://github.com/trussed-dev/littlefs2/pull/57
 

src/lib.rs

@@ -165,14 +165,15 @@ pub struct Version {
 
 /// Creates a path from a string without a trailing null.
 ///
-/// Panics if the string contains null bytes or non-ascii characters.
+/// Panics and causes a compiler error if the string contains null bytes or non-ascii characters.
 ///
 /// # Examples
 ///
 /// ```
 /// use littlefs2::{path, path::Path};
 ///
 /// const HOME: &Path = path!("/home");
+/// let root = path!("/");
 /// ```
 ///
 /// Illegal values:
@@ -186,11 +187,21 @@ pub struct Version {
 /// # use littlefs2::{path, path::Path};
 /// const WITH_UTF8: &Path = path!("/höme");  // does not compile
 /// ```
+///
+/// The macro enforces const evaluation so that compilation fails for illegal values even if the
+/// macro is not used in a const context:
+///
+/// ```compile_fail
+/// # use littlefs2::path;
+/// let path = path!("te\0st");  // does not compile
+/// ```
 #[macro_export]
 macro_rules! path {
-    ($path:literal) => {
-        $crate::path::Path::from_str_with_nul(::core::concat!($path, "\0"))
-    };
+    ($path:literal) => {{
+        const _PATH: &$crate::path::Path =
+            $crate::path::Path::from_str_with_nul(::core::concat!($path, "\0"));
+        _PATH
+    }};
 }
 
 #[cfg(test)]