Test case that can validate fix for #4968.

* With hard_tabs, rustfmt preserves tabs and counts them as multiple
  characters/columns (based on configuration).
* The annotated_snippet dependency, used to display errors, always
  counts tabs as 1 character.
* If rustfmt tries to report an error on a line containing tabs,
  the indices are mismatched.
* annotated_snippet will display the wrong range of the source code
  slice; in the extreme case, it can panic with out-of-bounds access.
* The test case added in this commit is expected to currently fail,
  since this commit doesn't include the fix.

src/test/parser.rs

@@ -3,6 +3,7 @@ use std::path::PathBuf;
 
 use super::read_config;
 
+use crate::FormatReportFormatterBuilder;
 use crate::modules::{ModuleResolutionError, ModuleResolutionErrorKind};
 use crate::{ErrorKind, Input, Session};
 
@@ -69,3 +70,21 @@ fn crate_parsing_stashed_diag2() {
     let filename = "tests/parser/stashed-diag2.rs";
     assert_parser_error(filename);
 }
+
+#[test]
+fn indexing_mismatch_with_annotated_snippet() {
+    // See also https://github.com/rust-lang/rustfmt/issues/4968
+    let filename = "tests/parser/issue_4968.rs";
+    let file = PathBuf::from(filename);
+    let config = read_config(&file);
+    let mut session = Session::<io::Stdout>::new(config, None);
+    let report = session.format(Input::File(filename.into())).unwrap();
+    let report = FormatReportFormatterBuilder::new(&report).build();
+    {
+        // Panic can only be triggered if we actually try to write the report
+        // and call into the annotated_snippet dependency.
+        use std::io::Write;
+
+        write!(&mut Vec::new(), "{report}").unwrap();
+    }
+}

tests/parser/issue_4968.rs

@@ -0,0 +1,12 @@
+// rustfmt-hard_tabs: true
+// rustfmt-max_width: 40
+// rustfmt-error_on_unformatted: true
+// rustfmt-error_on_line_overflow: true
+
+fn foo(x: u32) {
+	if x > 10 {
+		if x > 20 {
+			println!("0123456789abcdefghijklmnopqrstuvwxyz");
+		}
+	}
+}