From c6377f8e17cd7f3cf906397f3f48b522b0521471 Mon Sep 17 00:00:00 2001 From: Ryan Cumming Date: Tue, 23 Jan 2018 19:08:16 +1100 Subject: [PATCH] Avoid underflow in render_source_line While testing rust-lang/rust#47655 I was able to make the compiler panic when it's compiled with debug assertions: ```shell > rustc /dev/null --crate-type proc-macro 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 1.25.0-dev running on x86_64-apple-darwin note: run with `RUST_BACKTRACE=1` for a backtrace thread 'rustc' panicked at 'attempt to subtract with overflow', librustc_errors/emitter.rs:287:49 ``` Without debug assertions the following warning is emitted: ```shell > rustc /dev/null --crate-type proc-macro warning: unused variable: `registrar` --> /dev/null:0:1 | | = note: #[warn(unused_variables)] on by default = note: to avoid this warning, consider using `_registrar` instead ``` The panic is due to the unused variable warning being spanned to `/dev/null:0:1`. When `render_source_line` subtracts 1 from the line number to look up the source line it panics due to underflow. Without debug assertions this would wrap and cause us to return a blank string instead. Fix by explicitly testing for 0 and exiting early. I'm unsure how to automatically test this now that rust-lang/rust#46655 has been approved. --- src/librustc_errors/emitter.rs | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/src/librustc_errors/emitter.rs b/src/librustc_errors/emitter.rs index 58f851aea3817..62ae8d855897e 100644 --- a/src/librustc_errors/emitter.rs +++ b/src/librustc_errors/emitter.rs @@ -284,6 +284,10 @@ impl EmitterWriter { line: &Line, width_offset: usize, code_offset: usize) -> Vec<(usize, Style)> { + if line.line_index == 0 { + return Vec::new(); + } + let source_string = match file.get_line(line.line_index - 1) { Some(s) => s, None => return Vec::new(),