Lossily coerce invalid UTF-8 in sqlite collation callback (#4219)

The safe Fn(&str, &str) -> Ordering signature exposed by
SqliteConnectOptions::collation() and LockedSqliteHandle::create_collation()
was backed by from_utf8_unchecked, so a database containing invalid UTF-8
text could reach the user callback and materialize &str values that violate
Rust's UTF-8 invariant inside a safe API.

SQLite explicitly documents that invalid UTF-8 may be passed into
application-defined collating sequences, so the FFI shim must not assume
well-formed bytes. Replace from_utf8_unchecked with String::from_utf8_lossy,
which matches the sqlite3_create_collation_v2 SQLITE_UTF8 flag and keeps
the safe signature sound without changing correct-UTF-8 behavior.

Fixes #4194

Co-authored-by: Joaquin Hui Gomez <joaquinhuigomez@users.noreply.github.com>

Author: joaquinhuigomez

sqlx-sqlite/src/connection/collation.rs

@@ -3,7 +3,6 @@ use std::ffi::CString;
 use std::fmt::{self, Debug, Formatter};
 use std::os::raw::{c_int, c_void};
 use std::slice;
-use std::str::from_utf8_unchecked;
 use std::sync::Arc;
 
 use libsqlite3_sys::{sqlite3_create_collation_v2, SQLITE_OK, SQLITE_UTF8};
@@ -137,15 +136,19 @@ where
     let right_len = usize::try_from(right_len)
         .unwrap_or_else(|_| panic!("right_len out of range: {right_len}"));
 
+    // SQLite explicitly documents that invalid UTF-8 may be passed into
+    // application-defined collating sequences. The safe `Fn(&str, &str)`
+    // signature exposed to users must never observe invalid UTF-8, so
+    // lossily coerce the raw bytes here.
     let s1 = {
         let c_slice = slice::from_raw_parts(left_ptr as *const u8, left_len);
-        from_utf8_unchecked(c_slice)
+        String::from_utf8_lossy(c_slice)
     };
     let s2 = {
         let c_slice = slice::from_raw_parts(right_ptr as *const u8, right_len);
-        from_utf8_unchecked(c_slice)
+        String::from_utf8_lossy(c_slice)
     };
-    let t = (*boxed_f)(s1, s2);
+    let t = (*boxed_f)(&s1, &s2);
 
     match t {
         Ordering::Less => -1,