[Use fewer imports]: combine field imports for same imported type, class, etc. #1133

[Morabbin]

Summary

Combine multiple fields for any imported type, constructor, or class (this is called a "thing" in the GHC API).

Suppose a type, constructor, or class with fields will appear in at most one non-hiding import declaration (as guaranteed by simplify). The "use fewer imports" hint will combine multiple field imports into a single import list element.

Hiding imports are left untouched.

For example:

import A (Foo(A,B), Bar(C,D), Foo(E), Foo(C,D))
import A (Foo(A,F), Baz(D,A))

would first simplify to:

import A (Foo(A,B), Bar(C,D), Foo(E), Foo(C,D), Foo(A,F), Baz(D,A))

and then simplifyFields would yield:

import A (Foo(A,B,C,D,E,F), Bar(C,D), Baz(D,A))

To sort or not to sort?

Note that sorting occurs only when field imports are combined.

The import list itself preserves the order of the types, etc. in the original imports. It also leaves field imports unsorted if there's only one import list element for that type, class, or constructor. The latter is easy (just don't change it), but the former requires a kind of stable nub.

We could use a similar stable nub to preserve the field import order from the original too.

Is that worth doing?

Test Plan

Unit tests

Added several cases to the section in Hint.Import:

~/Work/hlint $ cabal run -- hlint --test
...
Testing (with refactoring)
Source annotations ....................
Input/outputs ..........................................................................................................................................................................
Hint names ........
Hint annotations ........

Tests passed (975)

Running this hlint on hlint src

~/Work/hlint $ cabal run hlint -- src --report
Writing report to report.html ...
No hints