thread 'main' panicked at 'bad span: `.`: ``', src/tools/rustfmt/src/source_map.rs:52:13

#[derive(Debug)]
struct S(String);

#[derive(Debug)]
struct T(S);


fn no_ref_nested() {
    let mut t = T(S());
    let c = || {
        println!("{:?}", t.0);
        let mut c = move || {
            t.0.10i32 = "new S".into();
            println!("{:?}", t.0.0);
        };
        c();
    };
    c();
}


fn main() {
    no_ref_nested();
}

warning: suffixes on a tuple index are invalid
  --> /tmp/im/2F2289E79BEA4CD52EB15181FD6F33470C0267578597C72C401DEA25348F0F1B.rs:13:15
   |
13 |             t.0.10i32 = "new S".into();
   |               ^^^^^^^ invalid suffix `i32`
   |
   = help: `i32` is *temporarily* accepted on tuple index fields as it was incorrectly accepted on stable for a few releases
   = help: on proc macros, you'll want to use `syn::Index::from` or `proc_macro::Literal::*_unsuffixed` for code that will desugar to tuple field access
   = help: see issue #60210 <https://github.com/rust-lang/rust/issues/60210> for more information

thread 'main' panicked at 'bad span: `.`: ``', src/tools/rustfmt/src/source_map.rs:52:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/ab654863c3d50482f260cf862647f1fe0ff5e010/library/std/src/panicking.rs:579:5
   1: core::panicking::panic_fmt
             at /rustc/ab654863c3d50482f260cf862647f1fe0ff5e010/library/core/src/panicking.rs:64:14
   2: <rustfmt_nightly::visitor::SnippetProvider as rustfmt_nightly::source_map::SpanUtils>::span_before
   3: rustfmt_nightly::chains::rewrite_chain
   4: rustfmt_nightly::expr::format_expr
   5: rustfmt_nightly::expr::rewrite_assignment
   6: rustfmt_nightly::expr::format_expr
   7: rustfmt_nightly::stmt::format_stmt
   8: <rustfmt_nightly::visitor::FmtVisitor>::walk_stmts
   9: <rustfmt_nightly::visitor::FmtVisitor>::visit_block
  10: rustfmt_nightly::expr::rewrite_block_with_visitor
  11: rustfmt_nightly::expr::rewrite_block
  12: rustfmt_nightly::expr::format_expr
  13: rustfmt_nightly::closures::rewrite_closure_expr
  14: rustfmt_nightly::closures::rewrite_closure
  15: rustfmt_nightly::expr::format_expr
  16: rustfmt_nightly::expr::rewrite_assign_rhs_expr::<rustc_ast::ast::Expr>
  17: rustfmt_nightly::expr::rewrite_assign_rhs_with::<alloc::string::String, rustc_ast::ast::Expr>
  18: <rustc_ast::ast::Local as rustfmt_nightly::rewrite::Rewrite>::rewrite
  19: rustfmt_nightly::stmt::format_stmt
  20: <rustfmt_nightly::visitor::FmtVisitor>::walk_stmts
  21: <rustfmt_nightly::visitor::FmtVisitor>::walk_stmts
  22: <rustfmt_nightly::visitor::FmtVisitor>::visit_block
  23: rustfmt_nightly::expr::rewrite_block_with_visitor
  24: rustfmt_nightly::expr::rewrite_block
  25: rustfmt_nightly::expr::format_expr
  26: rustfmt_nightly::closures::rewrite_closure_expr
  27: rustfmt_nightly::closures::rewrite_closure
  28: rustfmt_nightly::expr::format_expr
  29: rustfmt_nightly::expr::rewrite_assign_rhs_expr::<rustc_ast::ast::Expr>
  30: rustfmt_nightly::expr::rewrite_assign_rhs_with::<alloc::string::String, rustc_ast::ast::Expr>
  31: <rustc_ast::ast::Local as rustfmt_nightly::rewrite::Rewrite>::rewrite
  32: rustfmt_nightly::stmt::format_stmt
  33: <rustfmt_nightly::visitor::FmtVisitor>::walk_stmts
  34: <rustfmt_nightly::visitor::FmtVisitor>::walk_stmts
  35: <rustfmt_nightly::visitor::FmtVisitor>::visit_block
  36: <rustfmt_nightly::visitor::FmtVisitor>::visit_fn
  37: <rustfmt_nightly::visitor::FmtVisitor>::visit_item
  38: <rustfmt_nightly::visitor::FmtVisitor>::visit_items_with_reordering
  39: <rustfmt_nightly::visitor::FmtVisitor>::walk_mod_items
  40: <rustfmt_nightly::visitor::FmtVisitor>::format_separate_mod
  41: rustfmt_nightly::formatting::format_project::<rustfmt_nightly::Session<std::io::stdio::Stdout>>
  42: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::with::<<rustfmt_nightly::Session<std::io::stdio::Stdout>>::format_input_inner::{closure#0}, core::result::Result<rustfmt_nightly::FormatReport, rustfmt_nightly::ErrorKind>>
  43: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_span::create_session_if_not_set_then<core::result::Result<rustfmt_nightly::FormatReport, rustfmt_nightly::ErrorKind>, <rustfmt_nightly::Session<std::io::stdio::Stdout>>::format_input_inner::{closure#0}>::{closure#0}, core::result::Result<rustfmt_nightly::FormatReport, rustfmt_nightly::ErrorKind>>
  44: <rustfmt_nightly::Session<std::io::stdio::Stdout>>::format
  45: rustfmt::format_and_emit_report::<std::io::stdio::Stdout>
  46: <rustfmt_nightly::Session<std::io::stdio::Stdout>>::override_config::<rustfmt::format::{closure#0}, ()>
  47: rustfmt::execute
  48: rustfmt::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

rustfmt 1.5.2-nightly (ab65486 2023-03-15)
bad_span.zip

Reproduce the panic with the Rust snippet or bad_span.zip, then start at src/tools/rustfmt/src/source_map.rs:52 and follow the stack into chains::rewrite_chain. Done means rustfmt no longer panics on this input and completes formatting.