libstd build without backtrace feature broken

[bjorn3]

Likely because of #64154.

error[E0432]: unresolved import `crate::sys_common::backtrace`
  --> sysroot_src/src/libstd/backtrace.rs:98:24
   |
98 | use crate::sys_common::backtrace::{output_filename, lock};
   |                        ^^^^^^^^^ could not find `backtrace` in `sys_common`

error[E0432]: unresolved import `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:100:5
    |
100 | use backtrace::BytesOrWideString;
    |     ^^^^^^^^^ help: a similar path exists: `crate::backtrace`
    |
    = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:146:12
    |
146 |     frame: backtrace::Frame,
    |            ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:231:13
    |
231 |             backtrace::trace_unsynchronized(|frame| {
    |             ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:285:35
    |
285 |             (&capture.frames[..], backtrace::PrintFmt::Full)
    |                                   ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:287:55
    |
287 |             (&capture.frames[capture.actual_start..], backtrace::PrintFmt::Short)
    |                                                       ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:299:21
    |
299 |         let mut f = backtrace::BacktraceFmt::new(fmt, style, &mut print_path);
    |                     ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:309:54
    |
309 |                         symbol.name.as_ref().map(|b| backtrace::SymbolName::new(b)),
    |                                                      ^^^^^^^^^ use of undeclared type or module `backtrace`

error[E0433]: failed to resolve: use of undeclared type or module `backtrace`
   --> sysroot_src/src/libstd/backtrace.rs:339:17
    |
339 |                 backtrace::resolve_frame_unsynchronized(&frame.frame, |symbol| {
    |                 ^^^^^^^^^ use of undeclared type or module `backtrace`

[RalfJung]

@alexcrichton is it expected that building libstd without explicitly also enabling the backtrace feature is broken?
Seems odd to have a feature that must be enabled for things to build.

[alexcrichton]

Nah I think this is just a bug. We don't happen to test it on CI so I wasn't able to fix it myself :(

I'd be more than happy to review a PR to fix this support though!

[RalfJung]

Well, given that backtrace functionality is now publicly exposed, what should it do when the backtrace feature is disabled? Would it be ok to just print "backtraces not supported" or so?

[sfackler]

Yep, there's already handling for that case: https://github.com/rust-lang/rust/blob/master/src/libstd/backtrace.rs#L119

[RalfJung]

I am working on a fix at #64444.