350MB-long type name regression in 1.46

[kornelski]

Build fails with:

reached the type-length limit while instantiating std::future::from_generator::<[s...0, ()>}]>, ()}]>>, ()}]>>, ()}]>

To reproduce:

git clone --recursive https://gitlab.com/crates.rs/crates.rs.git/ # f3f0c87e70d57e
cd crates.rs
cargo build

It compiles in 1.45 with the default type length limit.

In 1.46 it won't compile unless I add #![type_length_limit="347827478"] to a couple of libraries in the workspace.

Meta

It affects 1.46 stable, and nightlies since 2020-08-14.

cargo bisect-rustc blames 0a49057 for the regression.

[jyn514]

It sounds like there are two different issues:

1. The diagnostic problem, which is not a regression. This is the issue you're fixing in Show more of the overlong type on type-length error #76777.
2. The type length limit issues, which is a regression (and has several related issues: https://github.com/rust-lang/rust/issues?q=is%3Aissue+is%3Aopen+type+length+limit).

Is that accurate? If so, do you mind if I change this only to track 1, since there are other issues tracking 2?

[kornelski]

Ok, it looks like a dupe of #75992