fmt::Display compiler panic: len is 0 but the index is 0

[andrewmorton]

Hi there! This is the second time I've compiled code with Rust ever, and my first time filing a bug report, so I may need some guidance if the info here isn't quite enough to work with :).

Learning about println! in Rust by Example, I incorrectly entered the below code. I looked at the documentation and realized I had made a mistake, but the compiler still panicked and recommended I open a bug report. I really believe in Rust and want to see this project succeed, hopefully this information is helpful!

• I tried this code:

println!("Pi is roughly: {:.*}, 3, 3.141592")

• Expected to see this happen:

"Pi is roughly 3.142"

• Instead, this happened:

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /rustc/625451e376bb2e5283fc4741caa0a3e8a2ca4d54/src/libcore/slice/mod.rs:2715:10
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.38.0 (625451e 2019-09-23) running on x86_64-unknown-linux-gnu

Meta

rustc --version --verbose:rustc 1.38.0 (625451e 2019-09-23)
binary: rustc
commit-hash: 625451e
commit-date: 2019-09-23
host: x86_64-unknown-linux-gnu
release: 1.38.0
LLVM version: 9.0

Backtrace:

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /rustc/625451e376bb2e5283fc4741caa0a3e8a2ca4d54/src/libcore/slice/mod.rs:2715:10
stack backtrace:
0: backtrace::backtrace::libunwind::trace
at /cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
1: backtrace::backtrace::trace_unsynchronized
at /cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
2: std::sys_common::backtrace::_print
at src/libstd/sys_common/backtrace.rs:47
3: std::sys_common::backtrace::print
at src/libstd/sys_common/backtrace.rs:36
4: std::panicking::default_hook::{{closure}}
at src/libstd/panicking.rs:200
5: std::panicking::default_hook
at src/libstd/panicking.rs:214
6: rustc::util::common::panic_hook
7: std::panicking::rust_panic_with_hook
at src/libstd/panicking.rs:481
8: std::panicking::continue_panic_fmt
at src/libstd/panicking.rs:384
9: rust_begin_unwind
at src/libstd/panicking.rs:311
10: core::panicking::panic_fmt
at src/libcore/panicking.rs:85
11: core::panicking::panic_bounds_check
at src/libcore/panicking.rs:61
12: syntax_ext::format::expand_preparsed_format_args
13: syntax_ext::format::expand_format_args_impl
14: ::expand
15: syntax::ext::expand::MacroExpander::expand_fragment
16: syntax::ext::expand::MacroExpander::expand_crate
17: rustc_interface::passes::configure_and_expand_inner::{{closure}}
18: rustc::util::common::time
19: rustc_interface::passes::configure_and_expand_inner
20: rustc_interface::passes::configure_and_expand::{{closure}}
21: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
22: rustc_interface::passes::configure_and_expand
23: rustc_interface::queries::Query::compute
24: rustc_interface::queries::::expansion
25: rustc_interface::interface::run_compiler_in_existing_thread_pool
26: std::thread::local::LocalKey::with
27: syntax::with_globals
note: Some details are omitted, run with RUST_BACKTRACE=full for a verbose backtrace.
query stack during panic:
end of query stack

[hman523]

This code fails on nightly, beta, and stable. Here's a link to the minimal failing code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=e36af7e5086c952448b4138411c4b2a1

[rodrimati1992]

The entirety of the println is a string:
println!("Pi is roughly: {:.*}, 3, 3.141592")

So it shouldn't compile,instead it should print this error:

error: 2 positional arguments in format string, but no arguments were given
 --> src/main.rs:3:29
  |
3 |     println!("Pi is roughly: {:.*}, 3, 3.141592")
  |                              ^^^^^

[estebank]

I believe the error is caused in the following line:

rust/src/libsyntax_ext/format.rs

Line 1108 in 8467cef

(cx.args[i].span, msg)

@andrewmorton the correct code would be println!("Pi is roughly: {:.*}", 3, 3.141592);, but you have encountered a bug in the compiler's diagnostic machinery. The output you should have seen is

error: 2 positional arguments in format string, but no arguments were given
 --> src/test/ui/issues/issue-66065.rs:2:15
  |
2 |     println!("{:.*}");
  |               ^^--^
  |                 |
  |                 this precision flag adds an extra required argument at position 0, which is why there are 2 arguments expected
  |
  = note: positional arguments are zero-based
  = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html