"Reached the type-length limit" error while compiling zebrad with nightly rust

[teor2345]

When I build zebrad with rust nightly 2020-08-18, I see the following error:

Compiling zebrad v3.0.0-alpha.0 (/home/dev/zebra/zebrad)
error: reached the type-length limit while instantiating std::thread::LocalKey::<std::cel...Result<(), color_eyre: :Report>>>
|
= note: consider adding a #![type_length_limit="1466135"] attribute to your crate

I see similar errors for other types, it seems like the maximum size is around 2MB.

The first bad Zebra commit is 6aeefce, which adds extra tracing and metrics.

The first bad rustc commit is rust-lang/rust@0a49057 which changes how types are normalised. It seems to interact badly with a known issue with exponential blowouts in nested types.

As far as rustc goes, I get this error on:

• nightly-2020-08-18
• nightly-2020-08-17
• nightly-2020-08-16
• nightly-2020-08-15
• nightly-2020-08-14

But not on:

• nightly-2020-08-13
• stable

Some related rustc tickets are:

• Exponential compile-time and type_length_limit blowup when nesting closure wrappers rust-lang/rust#54540
• Nightly 2019-12-29 causes reached the type-length limit error in previously working code rust-lang/rust#67757 (fixed)

It seems to be caused by futures, tracing has also had similar issues:

• tracing::instrument: "Reached the type-length limit while instantiating ..." tokio-rs/tracing#616

[teor2345]

We didn't catch this bug in our CI, because we don't run a nightly compile job.

I opened #924 to see if that's worthwhile.

[teor2345]

I'm going to need help to:

• write a rustc bug report
• find a minimal test case

@dconnolly or @yaahc, can you help me report a bug in rustc?

[teor2345]

I've created a rust zulip thread to get help.

[teor2345]

I bisected Zebra, the first bad commit is 6aeefce, which adds extra tracing and metrics.

[teor2345]

Looks like the instrument proc macro is responsible for this issue.