schema: add const-fn helper to compare strings

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-schema/src/const_test_utils.rs

@@ -0,0 +1,25 @@
+/// Note: this only compares *bytes* and is not strictly speaking equivalent to str::cmp!
+pub const fn byte_string_cmp(a: &[u8], b: &[u8]) -> std::cmp::Ordering {
+    use std::cmp::Ordering::*;
+
+    // const-version of `min(a.len(), b.len())` while simultaneously remembering
+    // `cmp(a.len(), b.len())`.
+    let (end, len_result) = if a.len() < b.len() {
+        (a.len(), Less)
+    } else if a.len() > b.len() {
+        (b.len(), Greater)
+    } else {
+        (a.len(), Equal)
+    };
+
+    let mut i = 0;
+    while i != end {
+        if a[i] < b[i] {
+            return Less;
+        } else if a[i] > b[i] {
+            return Greater;
+        }
+        i += 1;
+    }
+    len_result
+}

proxmox-schema/src/lib.rs

@@ -31,3 +31,5 @@ pub mod upid;
 
 #[cfg(feature = "api-types")]
 pub mod api_types;
+
+pub(crate) mod const_test_utils;