This commit implements the `@backingInt` and `@fromBackingInt` builtins.
These builtins are meant to replace the now-deprecated `@intFromEnum` and
`@enumFromInt` builtins.
`@backingInt` works with all enums and with bitpacks with *explicit* backing
integer types only. It also works with tagged unions, returning the backing
integer of the active tag value.
An `undefined` enum or bitpack yields an `undefined` backing integer.
The builtin is lowered to `bit_cast`.
`@fromBackingInt` infers its result type, which may be any enum or a bitpack
with an *explicit* backing integer type. It takes a parameter of exactly
that backing integer type.
For enums, passing a backing integer that is either `undefined` or would
yield an invalid tag value results in safety-checked Illegal Behavior.
For bitpacks, passing an `undefined` backing integer yields an `undefined`
bitpack.
The builtin is lowered to `bit_cast` or `bit_cast_safe` (more details below).
`@bitCast` now also performs a safety check for invalid tag values if its
destination type is an `enum`.
This commit also introduces a new AIR instruction, `bit_cast_safe`.
Similarly to `int_cast_safe`, it includes a safety check for invalid enum
tag values. When `@enumFromInt` is eventually removed, `int_cast_safe` won't
need to perform this safety check anymore.
Legalize only implements `expand_bit_cast_safe`, and no additional insts
to scalarize `bit_cast_safe` with vector/array operands. That is because
it can just reuse the existing scalarizations for regular `bit_cast`.
The safety check mandated by `bit_cast_safe` is only desired if the result
type is a scalar enum tag value, so the scalarization process is exactly
equivalent. The only difference is that we have to perform a safety check
on the result of the scalarization.
This means that there's no reason for a backend to want scalarization of
regular `bit_cast`, but not of `bit_cast_safe` (or the other way round).
If a backend is having trouble with the `bit_cast_safe` safety check it
should just use `expand_bit_cast_safe` instead.
Also adds a `std.meta.BackingInt()` function to conveniently get the result
type of `@backingInt` and for use in the langref.
The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.
This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.
Justus Klausecker