Confusing error when forgetting #[fixed_stack_segment]

[catamorphism]

I think this error message would be slightly confusing for a novice:

/Users/tjc/rust3/src/libstd/os.rs:2091:20: 2091:31 error: invoking non-Rust fn in fn without #[fixed_stack_segment] [-D cstack (default)]
/Users/tjc/rust3/src/libstd/os.rs:2091                     libc::fopen(fromp, modebuf)
                                                           ^~~~~~~~~~~

One way to read the error is that you should literally type #[fixed_stack_segment] [-D cstack (default)] in your code. Maybe there's a way to improve this error message to make it clearer that you have the choice of either using the fixed_stack_segment attribute, or the -D command-line flag. The error doesn't make it totally clear that the second thing-in-brackets is a command-line flag.

[adrientetar]

This warning is issued at src/librustc/middle/stack_check.rs:171:

    cx.tcx.sess.add_lint(lint::cstack,
                         callee.id,
                         callee.span,
                         fmt!("invoking non-Rust fn in fn without \
                              #[fixed_stack_segment]"));

But there's no trace of [-D cstack (default)] there...

[alexcrichton]

The extra words come from being a lint pass. It's attempting to show you why the lint is being emitted (in case you didn't already know. You'll find that code in librustc/middle/lint.rs

[alexcrichton]

Closing in favor of #10155