Provide feedback to users when selecting declarations

[dschrempf]

This is an intermediate step between #1013 and #1028. (Things have changed since then).

In the Select pass, we report delayed parse messages. After PR #1013, we report traces for declarations that we have failed to parse.

We also want to report a trace when we select a declaration we did not parse. (This is epecially useful when program slicing is enabled). We may also want to distinguish if we failed to parse, or if we did not attempt to parse the missng declarations.

[dschrempf]

On Friday we discussed implementing this next. However, after pondering on this ticket for some time, I think we should first decide and implement #1028. The reason is that even for skipped but selected messages, we need to resolve binding specs etc. So we can not directly look through the non-parsed declarations.

I think we have to discuss the three of us. @TravisCardwell @edsko

[edsko]

The purpose of this ticket is to ensure that when users enable program slicing, but some transitive dependency is not available to bindgen because it was not parsed, that we can give the user a more informative error message explaining what has gone wrong and what they can do to address it.

[dschrempf]

See TODO items in Select.hs. These are leftovers after #1192.

Solution

1. Start from a list of unavailable declarations.
2. Traver decl-use graph to use sites.
3. Remove declarations that have unmet dependencies from list of handled declarations.
4. Emit trace messages for all declarations that we can not handle due to missing dependencies.

Golden test!

struct Foo {
  long double x;
  struct Baz y;
};

struct Bar {
  struct Foo *x;
};

struct Baz {
  int x;
};

• We can not generate bindings for Bar, because we fail to parse Foo.
• We are not aware that Foo depends on Baz, but that is not a problem, because we can not generate bindings for Foo anyways.
• We can generate bindings for Baz.