cycle error building rustc with debug logging

[ehuss]

When using RUSTC_LOG to debug building rustc itself, I ran into this error:

error[E0391]: cycle detected when processing `hir::def::<impl at src/librustc/hir/def.rs:255:1: 270:2>::present_items::{{opaque}}#0`
   --> src/librustc/hir/def.rs:262:35
    |
262 |     pub fn present_items(self) -> impl Iterator<Item=T> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^
    |                                   |
    |                                   in this expansion of `desugaring of `existential type``
    |                                   in this macro invocation
    |
note: ...which requires processing `hir::def::<impl at src/librustc/hir/def.rs:255:1: 270:2>::present_items::{{opaque}}#0`...
   --> src/librustc/hir/def.rs:262:35
    |
262 |     pub fn present_items(self) -> impl Iterator<Item=T> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^
    |                                   |
    |                                   in this expansion of `desugaring of `existential type``
    |                                   in this macro invocation
note: ...which requires processing `hir::def::<impl at src/librustc/hir/def.rs:255:1: 270:2>::present_items::{{opaque}}#0`...
   --> src/librustc/hir/def.rs:262:35
    |
262 |     pub fn present_items(self) -> impl Iterator<Item=T> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^
    |                                   |
    |                                   in this expansion of `desugaring of `existential type``
    |                                   in this macro invocation
    = note: ...which again requires processing `hir::def::<impl at src/librustc/hir/def.rs:255:1: 270:2>::present_items::{{opaque}}#0`, completing the cycle
note: cycle used when processing `hir::def::<impl at src/librustc/hir/def.rs:255:1: 270:2>::present_items`
   --> src/librustc/hir/def.rs:262:5
    |
262 |     pub fn present_items(self) -> impl Iterator<Item=T> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Repro:

1. Set debug = true in config.toml.
2. ./x.py build to get a debug compiler (and go through several steps in stage 1 we want to skip).
3. touch src/librustc/lib.rs
4. RUSTC_LOG=rustc::ty=debug ./x.py build --stage=1 --keep-stage=0 > dlog 2>&1

The last step should rebuild librustc using a compiler with debugging enabled. I imagine there is a faster way to do this, but these are the simplest instructions I could think of.

Is this known/expected? I have no idea what the error means, but it makes debugging more difficult.

Currently on master (7cdaffd).

[jonas-schievink]

Ran into the same problem while investigating #60414 (comment), very annoying indeed!

[Aaron1011]

I ran into a similar issue when using RUSTC_LOG=rustc::ty=debug - however, I'm not sure if this is exactly the same cause.

I tracked down the cycle error I saw to the pretty-printing code for opaque types. When not printing in 'verbose mode', it attempts to call tcx.predicates_of on the opaque type. If we're already trying to get the predicates of an opaque type, this results in a cycle error.

As a hacky workaround, I changed if self.tcx().sess.verbose() to if self.tcx().sess.verbose() || true. However, the real fix is to avoid running queries during type pretty printing.