Incremental compilation results in linker error when method use is removed

[jonas-schievink]

See: https://github.com/jonas-schievink/rubble/compare/incr-linker-error

Build the first commit (jonas-schievink/rubble@04cf1d1), then the second (jonas-schievink/rubble@cedf7be) using cargo build, and you should get a linker error like this:

  = note: rust-lld: error: undefined symbol: _$LT$rubble..ble..time..Duration$u20$as$u20$core..fmt..Display$GT$::fmt::h343296729f83afc4
          >>> referenced by time.rs:129 (src/ble/time.rs:129)
          >>>               /home/jonas/dev/rubble/target/thumbv7em-none-eabi/debug/deps/rubble-a38354bf4561073f.3qtzuenakrd0qcaf.rcgu.o:(_$LT$$RF$T$u20$as$u20$core..fmt..Debug$GT$::fmt::h26ac05bd6f4f461e)

Note that this needs the thumbv7em-none-eabi target, so you might need to run rustup target add thumbv7em-none-eabi beforehand.

This is on rustc 1.33.0 (2aa4c46cf 2019-02-28).
Also reproduces on rustc 1.35.0-nightly (237bf3244 2019-03-28).

[dtolnay]

I minimized this to ~60 lines in jonas-schievink/rubble#62.

[dtolnay]

#61917 looks like it reproduces this in a non-Cargo environment. Some observations from that issue:

• It only happens with incremental builds. If I either remove the incremental DB or disable incremental builds, it works
• It only happens at opt-level=1; opt-level=0 and opt-level=3 are fine
• It goes away if I make small code changes, but it doesn't seem to matter what (which says incremental)
• Changing codegen-units doesn't affect anything.

@jsgf

[pnkfelix]

pre-triage: P-high; leaving nominated in hopes it increases chance that we discuss it today

[pnkfelix]

discussed at T-compiler meeting. Tentatively assigning to @Zoxc for initial investigation.

@Zoxc, if you do not expect to be able to do much investigation, let me know (and unassign yourself)