rust: str: allow dereferencing BStr in a const ctx

impl Deref doesn't work in const context. Add a function
that is similar to implementing `deref` but that can
be used in `const` context.

Signed-off-by: Fabien Parent <[email protected]>

Author: Fabo

rust/kernel/str.rs

@@ -31,6 +31,12 @@ impl BStr {
         // SAFETY: `BStr` is transparent to `[u8]`.
         unsafe { &*(bytes as *const [u8] as *const BStr) }
     }
+
+    /// Returns a reference to the inner [u8].
+    #[inline]
+    pub const fn deref_const(&self) -> &[u8] {
+        &self.0
+    }
 }
 
 impl fmt::Display for BStr {
@@ -102,7 +108,7 @@ impl Deref for BStr {
 
     #[inline]
     fn deref(&self) -> &Self::Target {
-        &self.0
+        self.deref_const()
     }
 }