Code that used to error on Rust 1.77 causes a compiler hang on 1.78 and later

[zachs18]

Code

I tried this code:

From @Buzzec via discord. Somewhat minimized (removed anyhow dependancy), but seems fragile to changes, so I wasn't able to quickly minimize further.

use std::borrow::Cow;
use std::fmt::Debug;
use std::iter::once;

pub trait SqlType: Sized {
    fn from_sql() -> Result<Self, ()>;
    fn to_sql(&self) -> Result<String, ()>;
}

pub trait Table {
    type Columns;
}

pub trait TopLevelTable: Table {
    const TABLE_NAME: &'static str;
}

pub trait ColumnType: Into<<Self::Table as Table>::Columns> + 'static + Copy + Debug {
    type Table: Table;
    type ColumnType;

    fn column_sql() -> Cow<'static, str>;
}

pub trait ProvidesColumns<C> {
    type ColumnIds;
    type Output;

    fn column_ids(&self, columns: &C) -> Self::ColumnIds;

    fn result_columns(
        &self,
        columns: &C,
        column_ids: &Self::ColumnIds,
    ) -> impl Iterator<Item = Cow<'static, str>>;

    fn output_from_row(
        &self,
        column: &C,
        column_ids: &Self::ColumnIds,
    ) -> Result<Self::Output, ()>;
}

impl<T, C> ProvidesColumns<C> for T
where
    T: TopLevelTable,
    C: ColumnType<Table = T>,
    C::ColumnType: SqlType,
{
    type ColumnIds = usize;
    type Output = C::ColumnType;

    fn column_ids(&self, _columns: &C) -> Self::ColumnIds {
        todo!()
    }

    fn result_columns(
        &self,
        columns: &C,
        column_ids: &Self::ColumnIds,
    ) -> impl Iterator<Item = Cow<'static, str>> {
        once(format!("t{}.{} AS c{column_ids}", "cool", columns.column_sql()).into())
    }

    fn output_from_row(
        &self,
        _column: &C,
        column_ids: &Self::ColumnIds,
    ) -> Result<Self::Output, ()> {
        todo!()
    }
}

impl<C1, C2, T> ProvidesColumns<(C1, C2)> for T
where
    T: ProvidesColumns<C1>,
    T: ProvidesColumns<C2>,
{
    type ColumnIds = (
        <T as ProvidesColumns<C1>>::ColumnIds,
        <T as ProvidesColumns<C2>>::ColumnIds,
    );
    type Output = (
        <T as ProvidesColumns<C1>>::Output,
        <T as ProvidesColumns<C2>>::Output,
    );

    fn column_ids(
        &self,
        columns: &(C1, C2),
    ) -> Self::ColumnIds {
        (
            <T as ProvidesColumns<C1>>::column_ids(self, &columns.0),
            <T as ProvidesColumns<C2>>::column_ids(self, &columns.1),
        )
    }

    fn result_columns(
        &self,
        columns: &(C1, C2),
        column_ids: &Self::ColumnIds,
    ) -> impl Iterator<Item = Cow<'static, str>> {
        let c1_sql =
            <T as ProvidesColumns<C1>>::result_columns(self, &columns.0, &column_ids.0);
        let c2_sql =
            <T as ProvidesColumns<C2>>::result_columns(self, &columns.1, &column_ids.1);
        c1_sql.chain(c2_sql)
    }

    fn output_from_row(
        &self,
        column: &(C1, C2),
        column_ids: &Self::ColumnIds,
    ) -> Result<Self::Output, ()> {
        let c1_output = <T as ProvidesColumns<C1>>::output_from_row(
            self,
            &column.0,
            &column_ids.0,
        )?;
        let c2_output = <T as ProvidesColumns<C2>>::output_from_row(
            self,
            &column.1,
            &column_ids.1,
        )?;
        Ok((c1_output, c2_output))
    }
}

I expected to see this happen: An error is emitted and compilation fails (in less than a second) on 1.77.

$ cargo +1.77 build
   Compiling bisect-thing v0.1.0 (/tmp/bisect-thing)
error[E0599]: no method named `column_sql` found for reference `&C` in the current scope
  --> src/lib.rs:62:65
   |
62 |         once(format!("t{}.{} AS c{column_ids}", "cool", columns.column_sql()).into())
   |                                                         --------^^^^^^^^^^--
   |                                                         |       |
   |                                                         |       this is an associated function, not a method
   |                                                         help: use associated function syntax instead: `C::column_sql()`
   |
   = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
note: the candidate is defined in the trait `ColumnType`
  --> src/lib.rs:22:5
   |
22 |     fn column_sql() -> Cow<'static, str>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0599`.
error: could not compile `bisect-thing` (lib) due to 1 previous error

Instead, this happened: cargo +1.78 build seems to hang forever.

Version it worked on

It most recently worked on: Rust 1.77 and nightly-2024-02-23

(nightly-2024-02-23)

rustc 1.78.0-nightly (397937d81 2024-02-22)
binary: rustc
commit-hash: 397937d812852f9bbeb671005cb399dbcb357cde
commit-date: 2024-02-22
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

(stable 1.77)

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: x86_64-unknown-linux-gnu
release: 1.77.2
LLVM version: 17.0.6

Version with regression

rustc --version --verbose: (nightly-2024-02-24)

rustc 1.78.0-nightly (8f359beca 2024-02-23)
binary: rustc
commit-hash: 8f359beca4e58bc3ae795a666301a8f47023044c
commit-date: 2024-02-23
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

rustc --version --verbose: (stable 1.78)

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

Bisection with `cargo-bisect-rustc`

Bisection script:

!/bin/bash
timeout 3 cargo build
if [[ "$?" = 124 ]]; then
    # timeout; failure
    exit 1
else
    # error emitted and exited; success
    exit 0
fi

Bisection result:

searched toolchains nightly-2024-02-01 through nightly-2024-03-16


********************************************************************************
Regression in nightly-2024-02-24
********************************************************************************

fetching https://static.rust-lang.org/dist/2024-02-23/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-02-23: 40 B / 40 B [================================================================================================================================================================================================================] 100.00 % 1.36 MB/s converted 2024-02-23 to 397937d812852f9bbeb671005cb399dbcb357cde
fetching https://static.rust-lang.org/dist/2024-02-24/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-02-24: 40 B / 40 B [==============================================================================================================================================================================================================] 100.00 % 205.23 KB/s converted 2024-02-24 to 8f359beca4e58bc3ae795a666301a8f47023044c
looking for regression commit between 2024-02-23 and 2024-02-24
fetching (via remote github) commits from max(397937d812852f9bbeb671005cb399dbcb357cde, 2024-02-21) to 8f359beca4e58bc3ae795a666301a8f47023044c
ending github query because we found starting sha: 397937d812852f9bbeb671005cb399dbcb357cde
get_commits_between returning commits, len: 11
  commit[0] 2024-02-22: Auto merge of #119989 - lcnr:sub_relations-bye-bye, r=compiler-errors
  commit[1] 2024-02-23: Auto merge of #120730 - estebank:confusable-api, r=oli-obk
  commit[2] 2024-02-23: Auto merge of #121341 - GrigorenkoPV:bootstrap-rustup-cargo, r=onur-ozkan
  commit[3] 2024-02-23: Auto merge of #121432 - mj10021:issue-119851-fix, r=nnethercote
  commit[4] 2024-02-23: Auto merge of #121448 - klensy:bump-22-02-24, r=clubby789
  commit[5] 2024-02-23: Auto merge of #121442 - lcnr:region-var-universe-uwu, r=compiler-errors
  commit[6] 2024-02-23: Auto merge of #121491 - matthiaskrgr:rollup-wkzqawy, r=matthiaskrgr
  commit[7] 2024-02-23: Auto merge of #121454 - reitermarkus:generic-nonzero-library, r=dtolnay
  commit[8] 2024-02-23: Auto merge of #121514 - matthiaskrgr:rollup-5f0vhv7, r=matthiaskrgr
  commit[9] 2024-02-23: Auto merge of #121303 - GrigorenkoPV:static_mut_refs, r=oli-obk,RalfJung
  commit[10] 2024-02-23: Auto merge of #119536 - Jules-Bertholet:const-barrier, r=dtolnay
ERROR: no CI builds available between 397937d812852f9bbeb671005cb399dbcb357cde and 8f359beca4e58bc3ae795a666301a8f47023044c within last 167 days

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[zachs18]

With this compiler patch

diff --git a/compiler/rustc_data_structures/src/obligation_forest/mod.rs b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
index 2c62034c6e8..eca2f833dc0 100644
--- a/compiler/rustc_data_structures/src/obligation_forest/mod.rs
+++ b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
@@ -478,6 +478,9 @@ pub fn process_obligations<P>(&mut self, processor: &mut P) -> P::OUT
                         node.state.set(NodeState::Success);
 
                         for child in children {
+                            if std::env::var_os("AAA").is_some() {
+                                tracing::warn!("changed: {child:?} at {index:?}");
+                            }
                             let st = self.register_obligation_at(child, Some(index));
                             if let Err(()) = st {
                                 // Error already reported - propagate it

it seems that the compiler gets stuck in a loop trying to prove a few obligations from each other? (C as ProvidesColumns<_>, _ as std::marker::Sized, C as std::marker::Sized)

$ AAA=1 cargo +dev build
// snip
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as ProvidesColumns<_>>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56645
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<_ as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56648
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<_ as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56648
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56648
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as ProvidesColumns<_>>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56648
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as ProvidesColumns<_>>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56648
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<_ as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56649
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<_ as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56649
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56649
 WARN rustc_data_structures::obligation_forest changed: PendingPredicateObligation { obligation: Obligation(predicate=Binder { value: TraitPredicate(<C as ProvidesColumns<_>>, polarity:Positive), bound_vars: [] }, depth=15), stalled_on: [] } at 56649
// snip

[5225225]

pub trait Table {}

pub trait ProvidesColumns<C> {
    fn column_ids(&self, columns: &u32);
}

impl<T, C> ProvidesColumns<C> for T
where
    T: Table,
{
    fn column_ids(&self, columns: &u32) {
        // weirdly, the NAME here matters.
        // if it's too dissimilar from column_ids it fails
        // (is the bug in suggestion code?)
        columns.column_idss();
    }
}

impl<C1, C2, T> ProvidesColumns<(C1, C2)> for T
where
    // removing either of these gets error[E0275]: overflow evaluating the requirement `_: Sized`
    T: ProvidesColumns<C1>,
    T: ProvidesColumns<C2>,
{}

fn main() {}

minimized further

[lqd]

// (is the bug in suggestion code?)

sounds possible, to compute

help: there is a method `column_ids` with a similar name, but with different arguments
  --> <source>:4:5
   |
4  |     fn column_ids(&self, columns: &u32);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[compiler-errors]

Due to #122317 most likely.