This job isn't quite finished because it caused me to discover bugs
related to reading `ByVal(Undef)` when a `ByValPair` is expected, e.g.
for a fat pointer. This wasn't a problem with the `None` of
`Option<Value>`, but I realized an equivalent bug existed even then,
since you could transmute a `u64` like `ByVal(Bytes(42))` to a fat
pointer type on 32-bit targets.
Likewise, you could transmute a fat pointer to `u64` and get panics
related to expecting `ByVal` but finding `ByValPair`, so the problem
goes both ways.
This is step 1 of a refactoring to fix#95. The `Undefined` variant is
so far unused and the old `bits` and `relocation` fields are emulated
with two new temporary methods. There should be no functional change due
to this commit.
This takes the `AllocId` out of PrimValKind, replacing it with a
`relocation` field on `PrimVal`, which is closer to an earlier design
for `PrimVal` I discussed with @eddyb.
This commit prepares the code for removing the `PrimValKind` from
`PrimVal` and making them more pure bitbags. The only code dealing with
`PrimValKind` will be code making decisions like "what kind of operation
do I need to do on these bits", like operators and casting. Transmutes
of `PrimVal`s will become true no-ops, not even adjusting a `kind`
field.
This commit also removes my horrible `value_to_primval` hack that made
an allocation for every `ByVal` passed in, so it could use `read_value`
to get a `PrimVal` with the right kind. Now I just compute the
`PrimValKind` from the `Ty` and re-tag the `PrimVal`.
The code got slightly messier in some areas here, but I think a _lot_ of
code will simplify in obvious ways once I remove the `kind` field from
`PrimVal`.
Gosh, if my commit messages aren't turning into essays these days.
Now instead of holding a native type based on the tag, all PrimVals
store a u64 (the `bits`), along with a `kind` corresponding to the
variant as it would be in the old PrimVal representation.
This commit makes no major optimizations and attempts to not change any
behaviour. There will be commits to follow that make use of this
representation to eliminate unnecessary allocation hacks like in
`value_to_primval`.
A number of places could be even more cleaned up after this commit,
particularly in `cast.rs`.
