Correctly handle string indices in literal_string_with_formatting_arg (#13841)

Fixes #13838.

r? @klensy

changelog: Correctly handle string indices in
`literal_string_with_formatting_arg`

Author: flip1995

clippy_lints/src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(iter_partition_in_place)]
 #![feature(let_chains)]
 #![feature(never_type)]
+#![feature(round_char_boundary)]
 #![feature(rustc_private)]
 #![feature(stmt_expr_attributes)]
 #![feature(unwrap_infallible)]

clippy_lints/src/literal_string_with_formatting_args.rs

@@ -108,7 +108,15 @@ impl LateLintPass<'_> for LiteralStringWithFormattingArg {
                     if error.span.end >= current.len() {
                         break;
                     }
-                    current = &current[error.span.end + 1..];
+                    // We find the closest char to where the error location ends.
+                    let pos = current.floor_char_boundary(error.span.end);
+                    // We get the next character.
+                    current = if let Some((next_char_pos, _)) = current[pos..].char_indices().nth(1) {
+                        // We make the parser start from this new location.
+                        &current[pos + next_char_pos..]
+                    } else {
+                        break;
+                    };
                     diff_len = fmt_str.len() - current.len();
                     parser = Parser::new(current, None, None, false, ParseMode::Format);
                 } else if let Piece::NextArgument(arg) = piece {

tests/ui/literal_string_with_formatting_arg.rs

@@ -30,4 +30,8 @@ fn main() {
     }";
     // Unicode characters escape should not lint either.
     "\u{0052}".to_string();
+
+    // Regression test for <https://github.com/rust-lang/rust-clippy/issues/13838>.
+    let x: Option<usize> = Some(0);
+    x.expect("{…}");
 }