[m-rev.] diff: Delete unused imports.

[Peter Wang]

On Thu, 31 Mar 2022 11:19:38 +1100 Peter Wang <novalazy at gmail.com> wrote:
> 
> I think "use" is about resolving names. If a source file mentions a name
> "foo" which refers to a "foo" in another module, that's a use.
> The other "use" I can think of is importing a module for its typeclass
> instances. An instance declaration may be used without being explicitly
> named.

I think I'm on the right track with this. Then what about this?
The compiler could maintain a table of

  - explicitly imported modules
  - the locations where those imports occur, for error reporting
  - two flags indicating whether an imported module was used
    in the interface section or the implementation section
    of the current module for:
     1. resolving a name mentioned in the current module
        to an entity exported by the imported module; or
     2. satisfying a typeclass constraint using a typeclass instance
        exported by that module.

Maintaining the table at each point that a "use" can arise is
conceptually straightforward compared to the unused_imports.m analysis,
which has to work backwards.

There's one more reason a module may need to be imported: the language
requires that if a submodule is imported then all its ancestor modules
are imported as well. Bug #490 proposes to remove that requirement.

Peter