compiler: analyze type and value of global declaration separately

This commit separates semantic analysis of the annotated type vs value
of a global declaration, therefore allowing recursive and mutually
recursive values to be declared.

Every `Nav` which undergoes analysis now has *two* corresponding
`AnalUnit`s: `.{ .nav_val = n }` and `.{ .nav_ty = n }`. The `nav_val`
unit is responsible for *fully resolving* the `Nav`: determining its
value, linksection, addrspace, etc. The `nav_ty` unit, on the other
hand, resolves only the information necessary to construct a *pointer*
to the `Nav`: its type, addrspace, etc. (It does also analyze its
linksection, but that could be moved to `nav_val` I think; it doesn't
make any difference).

Analyzing a `nav_ty` for a declaration with no type annotation will just
mark a dependency on the `nav_val`, analyze it, and finish. Conversely,
analyzing a `nav_val` for a declaration *with* a type annotation will
first mark a dependency on the `nav_ty` and analyze it, using this as
the result type when evaluating the value body.

The `nav_val` and `nav_ty` units always have references to one another:
so, if a `Nav`'s type is referenced, its value implicitly is too, and
vice versa. However, these dependencies are trivial, so, to save memory,
are only known implicitly by logic in `resolveReferences`.

In general, analyzing ZIR `decl_val` will only analyze `nav_ty` of the
corresponding `Nav`. There are two exceptions to this. If the
declaration is an `extern` declaration, then we immediately ensure the
`Nav` value is resolved (which doesn't actually require any more
analysis, since such a declaration has no value body anyway).
Additionally, if the resolved type has type tag `.@"fn"`, we again
immediately resolve the `Nav` value. The latter restriction is in place
for two reasons:

* Functions are special, in that their externs are allowed to trivially
  alias; i.e. with a declaration `extern fn foo(...)`, you can write
  `const bar = foo;`. This is not allowed for non-function externs, and
  it means that function types are the only place where it is possible
  for a declaration `Nav` to have a `.@"extern"` value without actually
  being declared `extern`. We need to identify this situation
  immediately so that the `decl_ref` can create a pointer to the *real*
  extern `Nav`, not this alias.
* In certain situations, such as taking a pointer to a `Nav`, Sema needs
  to queue analysis of a runtime function if the value is a function. To
  do this, the function value needs to be known, so we need to resolve
  the value immediately upon `&foo` where `foo` is a function.

This restriction is simple to codify into the eventual language
specification, and doesn't limit the utility of this feature in
practice.

A consequence of this commit is that codegen and linking logic needs to
be more careful when looking at `Nav`s. In general:

* When `updateNav` or `updateFunc` is called, it is safe to assume that
  the `Nav` being updated (the owner `Nav` for `updateFunc`) is fully
  resolved.
* Any `Nav` whose value is/will be an `@"extern"` or a function is fully
  resolved; see `Nav.getExtern` for a helper for a common case here.
* Any other `Nav` may only have its type resolved.

This didn't seem to be too tricky to satisfy in any of the existing
codegen/linker backends.

Resolves: #131

1 files changed, 1 insertions, 1 deletions

diff --git a/src/link.zig b/src/link.zig
index f0f6e9b01d..58c5cf35af 100644
--- a/src/link.zig
+++ b/src/link.zig
@@ -692,7 +692,7 @@ pub const File = struct {
     /// May be called before or after updateExports for any given Nav.
     pub fn updateNav(base: *File, pt: Zcu.PerThread, nav_index: InternPool.Nav.Index) UpdateNavError!void {
         const nav = pt.zcu.intern_pool.getNav(nav_index);
-        assert(nav.status == .resolved);
+        assert(nav.status == .fully_resolved);
         switch (base.tag) {
             inline else => |tag| {
                 dev.check(tag.devFeature());