This commit fixes a long-standing bug wherein bindings to polymorphic,
non-function expressions would be lowered at binding site, rather than
being specialized at the call site.
Concretely, consider the program
```
main =
n = 1
idU8 : U8 -> U8
idU8 = \m -> m
idU8 n
```
Prior to this commit, we would lower `n = 1` as part of the IR, and the
`n` at the call site `idU8 n` would reference the lowered definition.
However, at the definition site, `1` has the polymorphic type `Num *` -
it is not until the the call site that we are able to refine the type
bound by `n`, but at that point it's too late. Since the default layout
for `Num *` is a signed 64-bit int, we would generate IR like
```
procedure main():
let App.n : Builtin(Int(I64)) = 1i64;
...
let App.5 : Builtin(Int(U8)) = CallByName Add.idU8 App.n;
ret App.5;
```
But we know `idU8` expects a `u8`; giving it an `i64` is nonsense.
Indeed this would trigger LLVM miscompilations later on.
To remedy this, we now keep a sidecar table that maps symbols to the
polymorphic expression they reference, when they do so. We then
specialize references to symbols on the fly at usage sites, similar to
how we specialize function usages.
Looking at our example, the definition `n = 1` is now never lowered to
the IR directly. We only generate code for `1` at each place `n` is
referenced. As a larger example, you can imagine that
```
main =
n = 1
asU8 : U8 -> U8
asU32 : U32 -> U8
asU8 n + asU32 n
```
is lowered to the moral equivalent of
```
main =
asU8 : U8 -> U8
asU32 : U32 -> U8
asU8 1 + asU32 1
```
Moreover, transient usages of polymorphic expressions are lowered
successfully with this approach. See for example the
`monomorphized_tag_with_polymorphic_arg_and_monomorphic_arg` test in
this commit, which checks that
```
main =
mono : U8
mono = 15
poly = A
wrap = Wrapped poly mono
useWrap1 : [Wrapped [A] U8, Other] -> U8
useWrap1 =
\w -> when w is
Wrapped A n -> n
Other -> 0
useWrap2 : [Wrapped [A, B] U8] -> U8
useWrap2 =
\w -> when w is
Wrapped A n -> n
Wrapped B _ -> 0
useWrap1 wrap * useWrap2 wrap
```
has proper code generated for it, in the presence of the polymorphic
`wrap` which references the polymorphic `poly`.
https://github.com/rtfeldman/roc/pull/2347 had a different approach to
this - polymorphic expressions would be converted to (possibly capturing) thunks.
This has the benefit of reducing code size if there are many polymorphic
usages, but may make the generated code slower and makes integration
with the existing IR implementation harder. In practice I think the
average number of polymorphic usages of an expression will be very
small.
Closes https://github.com/rtfeldman/roc/issues/2336
Closes https://github.com/rtfeldman/roc/issues/2254
Closes https://github.com/rtfeldman/roc/issues/2344
Figuring out what this module was doing, and why, took me a bit less
than half an hour. We should document what's happening for others in the
future so they don't need to follow up on Zulip necessarily.
