Be more explicit about specifications for anonymous declarations

[edsko]

Suppose we have

typedef struct {
  // .. fields
} foo;

struct foo {
  // .. fields
};

and suppose furthermore that we make it possible for binding specs to say that the typedef foo should not be squashed (#799). When this happens, with the existing infrastructure a binding specification for struct foo would become ambiguous. Perhaps @TravisCardwell was right all along and we should have a specific syntax for referring to anonymous declarations. Proposed syntax:

types:
   - header: ..
     cname: ..
    ..
   - header: ..
     anonymous: ..
    ..

[TravisCardwell]

@edsko and I discussed this. We would like to also be able to reference anonymous types when slicing, so we think that it is preferable to use a prefix after all. We would be able to use the same syntax in pretty-printed output.

Note that the prefix must not result in valid C names. Generated names should stand out.

Note that the syntax is strictly for anonymous types. It is not used for renamed types, as binding spec resolution and slicing is done before types are renamed.

We would like to use a $ prefix, mirroring GHC syntax. Selection is done using regular expressions, however, and $ is a special character that could cause problems as well as confusion.

We also considered an anon: prefix, but it does not stand out as much as $. The colon is YAML syntax, so such names would need to be quoted in binding specification files.

[TravisCardwell]

We decided to use an @ prefix. This character is not currently used by YAML, but it is reserved for future use and therefore must be quoted. Example:

cname: '@S1_c'