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compiler: split Decl into Nav and Cau #20964

Merged
merged 2 commits into from
Aug 11, 2024
Merged

compiler: split Decl into Nav and Cau #20964

merged 2 commits into from
Aug 11, 2024

Commits on Aug 11, 2024

  1. compiler: split Decl into Nav and Cau 

    The type `Zcu.Decl` in the compiler is problematic: over time it has
gained many responsibilities. Every source declaration, container type,
generic instantiation, and `@extern` has a `Decl`. The functions of
these `Decl`s are in some cases entirely disjoint.

After careful analysis, I determined that the two main responsibilities
of `Decl` are as follows:
* A `Decl` acts as the "subject" of semantic analysis at comptime. A
  single unit of analysis is either a runtime function body, or a
  `Decl`. It registers incremental dependencies, tracks analysis errors,
  etc.
* A `Decl` acts as a "global variable": a pointer to it is consistent,
  and it may be lowered to a specific symbol by the codegen backend.

This commit eliminates `Decl` and introduces new types to model these
responsibilities: `Cau` (Comptime Analysis Unit) and `Nav` (Named
Addressable Value).

Every source declaration, and every container type requiring resolution
(so *not* including `opaque`), has a `Cau`. For a source declaration,
this `Cau` performs the resolution of its value. (When ziglang#131 is
implemented, it is unsolved whether type and value resolution will share
a `Cau` or have two distinct `Cau`s.) For a type, this `Cau` is the
context in which type resolution occurs.

Every non-`comptime` source declaration, every generic instantiation,
and every distinct `extern` has a `Nav`. These are sent to codegen/link:
the backends by definition do not care about `Cau`s.

This commit has some minor technically-breaking changes surrounding
`usingnamespace`. I don't think they'll impact anyone, since the changes
are fixes around semantics which were previously inconsistent (the
behavior changed depending on hashmap iteration order!).

Aside from that, this changeset has no significant user-facing changes.
Instead, it is an internal refactor which makes it easier to correctly
model the responsibilities of different objects, particularly regarding
incremental compilation. The performance impact should be negligible,
but I will take measurements before merging this work into `master`.

Co-authored-by: Jacob Young <jacobly0@users.noreply.github.com>
Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>
    @mlugg @jacobly0 @kubkon
    3 people committed Aug 11, 2024
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  2. frontend: give all container types namespaces 

    Eliding the namespace when a container type has no decls was an
experiment in saving memory, but it ended up causing more trouble than
it was worth in various places. So, take the small memory hit for
reified types, and just give every container type a namespace.
    @mlugg
    mlugg committed Aug 11, 2024
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