Type inference regression and ICE in nightly 2020-10-06

[mattico]

I tried this code:

FluenTech/embedded-time@521569d

I expected to see this happen:

Successful compilation. This works with stable, and nightly 2020-10-05.

Instead, this happened:

https://github.com/FluenTech/embedded-time/pull/77/checks?check_run_id=1218050695

error[E0284]: type annotations needed: cannot satisfy `<<Clock as Clock>::T as Div<_>>::Output == <Clock as Clock>::T`
Error:    --> src/instant.rs:183:66
    |
183 |         if add_ticks <= (<Clock::T as num::Bounded>::max_value() / 2.into()) {
    |                                                                  ^ cannot satisfy `<<Clock as Clock>::T as Div<_>>::Output == <Clock as Clock>::T`

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (98edd1fbf 2020-10-06)
binary: rustc
commit-hash: 98edd1fbf8a68977a2a7c1312eb1ebff80515a92
commit-date: 2020-10-06
host: x86_64-pc-windows-msvc
release: 1.49.0-nightly
LLVM version: 11.0

[mattico]

#73905 looks suspicious?

[mattico]

Minimal repro: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=55f8795c91867fec5550107d789a60f8

(at time of writing the playground does not have the affected nightly yet)

[tesuji]

Yeah, bisecting it's indeed 08e2d46 or #73905 .
However, that PR is a significant improvement that we may don't want to revert.
But I will cc @matthewjasper for their information.

[mstallmo]

Assigning P-critical as discussed as part of the Prioritization Working Group procedure and removing I-prioritize

[gnunicorn]

(at time of writing the playground does not have the affected nightly yet)

that playground link now fails with the error described above.

[bkchr]

Hey, so I tried porting our code to compile with latest nightly and I almost got it working but ultimately it ended with an internal compiler error:

error: internal compiler error: compiler/rustc_trait_selection/src/traits/codegen/mod.rs:121:9: Encountered errors `[FulfillmentError(Obligation(predicate=TraitPredicate(<dyn sc_client_api::PrunableStateChangesTrieStorage<sp_runtime::generic::Block<sp_runtime::generic::Header<u32, sp_runtime::traits::BlakeTwo256>, sp_runtime::OpaqueExtrinsic>> as sp_state_machine::changes_trie::Storage<sp_runtime::traits::BlakeTwo256, u32>>), depth=0),Unimplemented)]` resolving bounds after type-checking

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:945:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (98edd1fbf 2020-10-06) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: aborting due to previous error

error: could not compile `node-testing`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

I created a branch that fails with this or a similar ICE when you run cargo test --all. You can find it here: https://github.com/paritytech/substrate/tree/bkchr-fix-for-latest-nightly