It is possible to get comptime-known values from runtime-known values
for example the length of array. Allowing runtime only instructions to
be emitted outside function bodies allows these operations to happen.
In places where comptime-known values are required we have other methods
to ensure that and they usually result in more specific compile errors too.
Closes#12240
This comment is now deleted because the task is completed in this
commit:
```
// TODO: Update this to behave like `beginComptimePtrLoad` and properly check/use
// `container_ty` and `array_ty`, instead of trusting that the parent decl type
// matches the type used to derive the elem_ptr/field_ptr/etc.
//
// This is needed because the types will not match if the pointer we're mutating
// through is reinterpreting comptime memory.
```
The main strategy is to change the ComptimePtrMutationKit struct so that
instead of `val: *Value` it now returns a tagged union which can be one
of three possibilities:
* The pointer type matches the actual comptime Value so a direct
modification is possible. Before this commit, the implementation
incorrectly assumed this was always the case.
* In the case of needing to write through a reinterpreted pointer, a
mutable base Value pointer is provided along with a byte offset
pointing to the element value in virtual memory.
* Otherwise, it means a compile error must be emitted because one or
both of the types (the owner of the value, or the pointer type being
used to write through) do not have a well-defined memory layout.
After calling beginComptimePtrMutation, the one callsite now switches on
this tagged union and does the appropriate thing. The main new logic is
for the second case, which involves pointer reinterpretation, which now
takes this strategy:
1. write the base value to a memory buffer.
2. perform the pointer store at the proper byte offset, thereby
modifying a subset of the buffer.
3. read the base value from the memory buffer, overwriting the old base
value.
Whenever a `ref` instruction is needed, it is created and saved in
`AstGen.ref_table` instead of being immediately appended to the current
block body. Then, when the referenced instruction is being added to the
parent block (e.g. from setBlockBody), if it has a ref_table entry, then
the ref instruction is added directly after the instruction being referenced.
This makes sure two properties are upheld:
1. All pointers to the same locals return the same address. This is required
to be compliant with the language specification.
2. `ref` instructions will dominate their uses. This is a required property
of ZIR.
A complication arises when a ref instruction refs another ref
instruction. The logic in appendBodyWithFixups must take this into
account, recursively handling ref refs.
Additionally:
* Sema: fix array cat/mul not setting the sentinel value
- This required an LLVM backend enhancement to the handling of the
AIR instruction aggregate_init that likely needs to be
propagated to the other backends.
* Sema: report integer overflow of array concatenation in a proper
compile error instead of crashing.
* Sema: fix not using proper pointer address space for array cat/mul
