The import-check action now presents problematic import statements
introduced by the PR better.
The idea is roughly:
* Let (V₁, E₁) be the graph of modules as vertices and import statements
as edges before the change
* Let (V₂, E₂) be the graph after the code change, which is typically a small
perturbation of (V₁, E₁).
* X₁ = FAS(V₁, E₁) is the feedback arc set before (a minimal set of edges to
delete to make it acyclic)
* X₂ = FAS(V₂, E₂ ∖ X₁) is the feedback arc set after deletion of the minimal
set of edges that made the old graph acyclic.
* X₃ = FAS(V₂, E₂) is the feedback arc set after
Previously I displayed X₁ and X₃ and users had to diff themselves.
Now, I'm showing X₂, which is a small set, typically directly related to
code changes.
However, it can be that a small code change adding say 2 problematic imports
creates a completely different solution X₃ that only requires deletion of just 1
different import. In that case the user is informed that they can potentially do
less work.
So for PR #48784 the output is now:
> The overall number of problematic import statements increased by 1 from 31 to 32.
> This is likely a direct consequence of the following import statements:
>
> ```
> spack/config imports: spack.spec, spack.util.path, spack.util.remote_file_cache
> ```
>
> However, instead of removing 3 import statements, it is sufficient to remove only 1
> import statement from the following list:
>
> ```
> spack/concretize imports: spack.bootstrap, spack.solver.asp
> spack/environment imports: spack.bootstrap, spack.environment
> spack/fetch_strategy imports: spack.version.git_ref_lookup
> spack/install_test imports: spack.build_environment, spack.package_base
> spack/modules imports: spack.modules
> spack/platforms imports: spack.config
> spack/relocate imports: spack.bootstrap
> spack/repo imports: spack.package_base, spack.patch, spack.tag
> spack/spec imports: spack.binary_distribution, spack.compiler, spack.compilers, spack.concretize, spack.environment, spack.hash_types, spack.provider_index, spack.repo, spack.spec_parser, spack.store, spack.traverse, spack.variant, spack.version.git_ref_lookup
> spack/subprocess_context imports: spack.environment
> spack/util/gpg imports: spack.bootstrap
> spack/util/package_hash imports: spack.package_base
> spack/util/path imports: spack.config, spack.environment
> spack/util/remote_file_cache imports: spack.util.web
> ```
from which the user can figure out that
`spack/util/remote_file_cache imports: spack.util.web` is the "bottleneck" now.
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