Remove duplicated constraints from CondTree

[leana8959]

CondTree is defined as follows:

cabal/Cabal-syntax/src/Distribution/Types/CondTree.hs

Lines 57 to 61 in c5c2072

	data CondTree v c a = CondNode
	{ condTreeData :: a
	, condTreeConstraints :: c
	, condTreeComponents :: [CondBranch v c a]
	}

CondTree is often instantiated with a being a component that has BuildInfo and c as [Dependency]. The [Dependency] is derived from the BuildInfo during construction (see the last line).

cabal/Cabal-syntax/src/Distribution/PackageDescription/Parsec.hs

Lines 485 to 496 in c5c2072

	parseCondTree v hasElif grammar commonStanzas fromBuildInfo cond = go
	where
	go fields0 = do
	(fields, endo) <-
	if v >= CabalSpecV3_0
	then processImports v fromBuildInfo commonStanzas fields0
	else traverse (warnImport v) fields0 >>= \fields1 -> return (catMaybes fields1, id)
	let (fs, ss) = partitionFields fields
	x <- parseFieldGrammar v fs grammar
	branches <- concat <$> traverse parseIfs ss
	return $ endo $ CondNode x (cond x) branches

The accessors are exposed, this duplication of [Dependency] can cause the data to be inconsistent.

Cabal exact print aims to allow modifications to the GPD. Not having a single source of truth can also confuse programmers using GPD as an API to exact print.

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[leana8959]

cabal-install:mem-use-tests module's "basic space leak test" fails due to the dummy packages having bounds with trailing zeros.

Removing these trailing zeros causes the test "duplicate dependencies" and "duplicate flagged dependencies" to fail.

Maybe I uncovered a bug in the test suite with this PR?

When mkSimpleVersion n = C.mkVersion [n]:

space leak test works, duplicated flag tests don't.

$ cabal test cabal-install:mem-use-tests
Test suite mem-use-tests: RUNNING...
Memory Usage
  UnitTests.Distribution.Solver.Modular.MemoryUsage
    basic space leak test:          OK (6.66s)
    package with many flags:        OK (2.03s)
    issue #2899:                    OK (1.39s)
    duplicate dependencies:         FAIL
      Unexpected error:
      Could not resolve dependencies:
      [__0] trying: A-1.0.0 (user goal)
      [__1] next goal: B (dependency of A +/-flag-1 +/-flag-10 +/-flag-11 +/-flag-12 +/-flag-13 +/-flag-14 +/-flag-15 +/-flag-16 +/-flag-17 +/-flag-18 +/-flag-19 +/-flag-2 +/-
      ... elided
    duplicate flagged dependencies: FAIL (0.05s)

When mkSimpleVersion n = C.mkVersion [n, 0, 0]

space leak test trips gpd check due to trailing zeros. duplicated flag tests work.

$ cabal test cabal-install:mem-use-tests
Test suite mem-use-tests: RUNNING...
Memory Usage
  UnitTests.Distribution.Solver.Modular.MemoryUsage
    basic space leak test:          FAIL
      Exception: invalid GenericPackageDescription for package pkg-1-2.0.0: [[tz-upper-bounds] On library, these packages have upper bounds with trailing zeros:
        - pkg-2
      Please avoid trailing zeros for upper bounds.]
      Use -p '/basic space leak test/' to rerun this test only.
    package with many flags:        OK (2.01s)
    issue #2899:                    OK (1.39s)
    duplicate dependencies:         OK (0.01s)
    duplicate flagged dependencies: OK (0.08s)

[Bodigrim]

@leana8959 I suggest you squash commits and mark as ready for review, if you want someone's feedback. People often perceive "drafts" as private experiments, not asking for immediate feedback.

---

Essentially the PR removes condTreeConstraints from CondTree, which was apparently pretty much unused. I tried to trace back why it was introduced in the first place. I arrived back to 2007:

cabal/Distribution/Configuration.hs

Lines 221 to 224 in 0dbb6d4

	-- Each path is annotated with a number of constraints, which must be
	-- fulfilled and a modifier, describing which information to add to the final
	-- value. We record constraints and data separately, to allow possible
	-- optimizations later on.

Almost twenty years after the "possible optimizations later on" never materialized :) So high time to clean it up.

[leana8959]

@Bodigrim You're right, I rebased and fixed the formatting. Glad to know that this refactor is heading in the right direction!