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mir: adjust conditional in recursion limit check #72540

Merged
merged 2 commits into from
May 30, 2020
Merged

mir: adjust conditional in recursion limit check #72540

merged 2 commits into from
May 30, 2020

Commits on May 28, 2020

  1. mir: adjust conditional in recursion limit check 

    This commit adjusts the condition used in the recursion limit check of
the monomorphization collector, from `>` to `>=`.

In rust-lang#67552, the test case had infinite indirect recursion, repeating a
handful of functions (from the perspective of the monomorphization
collector): `rec` -> `identity` -> `Iterator::count` -> `Iterator::fold`
-> `Iterator::next` -> `rec`.

During this process, `resolve_associated_item` was invoked for
`Iterator::fold` (during the construction of an `Instance`), and
ICE'd due to substitutions needing inference. However, previous
iterations of this recursion would have called this function for
`Iterator::fold` - and did! - and succeeded in doing so (trivially
checkable from debug logging, `()` is present where `_` is in the substs
of the failing execution).

The expected outcome of this test case would be a recursion limit error
(which is present when the `identity` fn indirection is removed), and
the recursion depth of `rec` is increasing (other functions finish
collecting their neighbours and thus have their recursion depths reset).

When the ICE occurs, the recursion depth of `rec` is 256 (which matches
the recursion limit), which suggests perhaps that a different part of
the compiler is using a `>=` comparison and returning a different result
on this recursion rather than what it returned in every previous
recursion, thus stopping the monomorphization collector from reporting
an error on the next recursion, where `recursion_depth_of_rec > 256`
would have been true.

With grep and some educated guesses, we can determine that
the recursion limit check at line 818 in
`src/librustc_trait_selection/traits/project.rs` is the other check that
is using a different comparison. Modifying either comparison to be `>` or
`>=` respectively will fix the error, but changing the monomorphization
collector produces the nicer error.

Signed-off-by: David Wood <david@davidtw.co>
    @davidtwco
    davidtwco committed May 28, 2020
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  2. standardize limit comparisons with Limit type 

    This commit introduces a `Limit` type which is used to ensure that all
comparisons against limits within the compiler are consistent (which can
result in ICEs if they aren't).

Signed-off-by: David Wood <david@davidtw.co>
    @davidtwco
    davidtwco committed May 28, 2020
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