avoid iterating from LenT::MAX down for truncation

Author: DPlayer234

src/string.rs

@@ -1,7 +1,7 @@
 use alloc::{
     borrow::{Cow, ToOwned},
     boxed::Box,
-    string::{String},
+    string::String,
     sync::Arc,
 };
 use core::{borrow::Borrow, hash::Hash, str::FromStr};
@@ -29,13 +29,17 @@ fn truncate_string(err: InvalidStrLength, max_len: usize) -> String {
 
 #[cold]
 fn truncate_str(string: &str, max_len: usize) -> &str {
+    if string.len() <= max_len {
+        return string;
+    }
+
     for len in (0..=max_len).rev() {
         if string.is_char_boundary(len) {
             return &string[..len];
         }
     }
 
-    unreachable!("Len 0 is a char boundary")
+    unreachable!("Len 0 is a char boundary");
 }
 
 /// A fixed size String with length provided at creation denoted in [`ValidLength`], by default [`u32`].
@@ -93,7 +97,7 @@ impl<LenT: ValidLength> FixedString<LenT> {
     /// For lossless fallible conversion, convert to [`Box<str>`] using [`String::into_boxed_str`] and use [`TryFrom`].
     #[must_use]
     pub fn from_string_trunc(str: String) -> Self {
-        match str.into_boxed_str().try_into() {
+        match str.try_into() {
             Ok(val) => val,
             Err(err) => Self::from_string_trunc(truncate_string(err, LenT::MAX.to_usize())),
         }
@@ -438,6 +442,21 @@ mod test {
         }
     }
 
+    // primarily intended to ensure no hangs occur
+    fn check_u32_partial_roundtrip_generic(to_fixed: fn(String) -> FixedString<u32>) {
+        for i in 0..=400u32 {
+            let original = "a".repeat(i as usize);
+            let fixed = to_fixed(original);
+
+            assert!(fixed.bytes().all(|c| c == b'a'));
+            assert_eq!(fixed.len(), i);
+
+            if !fixed.is_static() {
+                assert_eq!(fixed.is_inline(), fixed.len() <= 12);
+            }
+        }
+    }
+
     #[test]
     fn test_truncating_behaviour() {
         const STR: &str = "______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________🦀";
@@ -482,6 +501,21 @@ mod test {
         });
     }
 
+    #[test]
+    fn check_u32_partial_roundtrip() {
+        check_u32_partial_roundtrip_generic(|original| {
+            FixedString::<u32>::try_from(original).unwrap()
+        });
+    }
+
+    #[test]
+    fn check_u32_partial_roundtrip_static() {
+        check_u32_partial_roundtrip_generic(|original| {
+            let static_str = Box::leak(original.into_boxed_str());
+            FixedString::from_static_trunc(static_str)
+        });
+    }
+
     #[test]
     #[cfg(feature = "serde")]
     fn check_u8_roundtrip_serde() {