Do not warn on unused newtype constructors when used with FFI

One of the things I love about GHC are the accurate warnings about unused code. This makes refactoring a joy and prevents dead code that is so common in large Java/C++/etc. projects.

It's quite rare that it gets in your way, but one situation where this can happen is with FFI code. Here is an example:

module SomeModule (Context, mkContext) where
import Foreign

newtype Context = Context (Ptr ())
foreign import ccall mkContext :: IO Context

Here GHC warns about the unused newtype consructor.

There are a couple of workarounds that you could use to make this code -Wall sane. But I think they either leak more information about the Context type, or they require an internal "Types" module (which adds maintenance costs and decreases code locality).

I think from a users perspective it would be desirable to omit warnings about unused newtype constructors if the newtype is used in a foreign import declaration.

Trac metadata

Trac field	Value
Version	7.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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