Auto merge of #91241 - dtolnay:firstchunk, r=oli-obk

Eliminate an unreachable codepath from String::from_utf8_lossy

`Utf8Lossy`'s `Iterator` implementation ensures that only the **final** chunk has an empty slice for `broken`:

/~https://github.com/rust-lang/rust/blob/dd549dcab404ec4c7d07b5a83aca5bdd7171138f/library/core/src/str/lossy.rs#L46-L47

Thus the only way the **first** chunk could have an empty `broken` is if it is the **final** chunk, i.e. there is only one chunk total. And the only way that there could be one chunk total with an empty `broken` is if the whole input is valid utf8 and non-empty.

That condition has already been handled by an early return, so at the point that the first `REPLACEMENT` is being pushed, it's impossible for `first_broken` to be empty.

library/alloc/src/string.rs

@@ -558,13 +558,13 @@ impl String {
     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
         let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
 
-        let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
+        let first_valid = if let Some(chunk) = iter.next() {
             let lossy::Utf8LossyChunk { valid, broken } = chunk;
-            if valid.len() == v.len() {
-                debug_assert!(broken.is_empty());
+            if broken.is_empty() {
+                debug_assert_eq!(valid.len(), v.len());
                 return Cow::Borrowed(valid);
             }
-            (valid, broken)
+            valid
         } else {
             return Cow::Borrowed("");
         };
@@ -573,9 +573,7 @@ impl String {
 
         let mut res = String::with_capacity(v.len());
         res.push_str(first_valid);
-        if !first_broken.is_empty() {
-            res.push_str(REPLACEMENT);
-        }
+        res.push_str(REPLACEMENT);
 
         for lossy::Utf8LossyChunk { valid, broken } in iter {
             res.push_str(valid);