tree 9fe2a8655a617aafdc790405929585492b089d12
parent 47888ad0aeb367a7e1a76a88dd8dbeb120e293a6
author reventlov <dmitri+github.com@xn13.com> 1649186139 -0700
committer reventlov <dmitri+github.com@xn13.com> 1649186139 -0700

Implement the `$next` keyword for convenient packed structures.

This change adds a new keyword, `$next`, which may be used in the offset
expression of a physical field, and which translates to "the offset of
the first byte after the end of the syntactically previous physical
field."

Essentially, it allows `struct`s or `bits` like:

    struct Foo:
      0     [+4]  UInt  a
      $next [+2]  UInt  b
      $next [+1]  UInt  c
      $next [+4]  UInt  d

... where each field is packed directly after the previous one.  See the
updated documentation in `doc/language-reference.md` for details.

This also contains a few incidental changes:

*   A new function in `ir_util`, for scanning subtrees of the IR.  Note
    that a similar function used to exist; this is really a resurrection
    of that functionality.
*   `synthetics.synthesize_fields` has been renamed to
    `synthetics.desugar`, since it now does more than just synthesize
    fields.
*   The "how to use the Emboss compiler" sections of `guide.md` have been
    expanded.
*   The comment in `compiler/front_end/docs_are_up_to_date_test.py` has
    been updated with the correct paths for the open-source Emboss.