Solver: Enforce dependencies on executables (fixes #4781).

[grayjay]

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
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---

Main commits:

Solver: Use more specific failure reasons for dependency conflicts.

This commit splits the Conflicting constructor of FailReason into five
different constructors, for different types of dependency conflicts. This
change makes it easier to print different error messages for different
conflicts, though it doesn't change any of the error messages yet.

The refactoring caused a small change to the error message for conflicts caused
by self-dependencies.

Solver: Store names of required executables for build-tool-depends dependencies.

This commit changes the field of type IsExe in the Dep data type to
type Maybe UnqualComponentName. It also adds the executable name to error
messages that previously just contained "(exe)".

Solver: Enforce dependencies on executables (fixes #4781).

This commit adds two checks to the validation phase of the solver:

1. It checks that each newly chosen package instance contains all executables
   that are required from that package so far.

2. It checks that each new build tool dependency that refers to a previously
   chosen package can be satisfied by the executables in that package.

This commit also fixes a TODO related to solver log messages. Previously, it was
possible for the log to associate an incorrect executable name with a
dependency.

Add tests for build-tool-depends dependencies using the solver DSL.

Test that the solver ignores build tool dependencies in legacy mode.

The pre-new-build code path sets the solver parameter solveExecutables to
false, which causes the solver to ignore all build-tools and build-tool-depends
dependencies. This commit adds a test for ignoring each of the two types of
dependencies.

---

I compared the performance on this branch (cbf73ac) with master (26426fc) to see the effect of the extra validation steps. I wrote a cabal file with one library build-depends field to test the solver with new-build. (Only new-build handles build tool dependencies.) Then I ran cabal new-build --dry-run --index-state=2017-11-04T18:36:30Z --no-count-conflicts, with two sets of dependencies in the build-depends field. I tried to choose one set of dependencies that caused a long run time and another that created a deep search tree. I turned off conflict counting to prevent the solver from finishing too quickly in the first case. I also compared the -v3 log from master and the branch to ensure that the change didn't have any effect on conflicts, since I only wanted to measure the overhead of checking for executables. Each measurement is the average of three trials.

packages	maximum search tree depth	run time on master	run time on branch	total memory on master, with "+RTS -s"	total memory on branch, with "+RTS -s"
AesonBson	66	18.42 s	18.40 s	244 MB	244 MB
gi-glib, leksah, yi	2349	7.10 s	7.08 s	377 MB	378 MB

It looks like this change didn't have much effect in either case.

/cc @kosmikus

[23Skidoo]

This all sounds absolutely great, but can you please add a short note to the changelog?

[grayjay]

Thanks. I updated the changelog and added a few more tests.

[23Skidoo]

This PR looks good to me, so unless @kosmikus objects I'm going to merge it tomorrow.

[kosmikus]

Looks fantastic. Thanks a lot @grayjay . @23Skidoo Please merge.

[23Skidoo]

Merged. Thanks, @grayjay!

[grayjay]

Thanks for the review!