I was hoping to add nested datatypes into the language, but it turns out
doing so is quite tricky and not all that useful with Roc's current
compilation model. Basically every implementation strategy I could think
of ended up requiring a uniform representation for the data layout
(or some ugly workaround). Furhermore it increased the complexity of the
checker/mono IR generator a little bit - basically, we must always pass
around the alias definitions of nested datatypes and instantiate them
at usage sites, rather than being able to unroll aliases as we currently
do during canonicalization.
So, especially because we don't support polymorphic recursion anyway, I
think it may be better to simply disallow any kind of nested datatypes
in the language. In any case, Stephanie Weirich [seems to think nested
datatypes are not needed](https://www.cis.upenn.edu/~plclub/blog/2020-12-04-nested-datatypes/).
Closes#2293
By emitting a runtime error rather than panicking when we can't layout
a record, we help programs like
```
main =
get = \{a} -> a
get {b: "hello world"}
```
execute as
```
Mismatch in compiler/unify/src/unify.rs Line 1071 Column 13
Trying to unify two flat types that are incompatible: EmptyRecord ~ { 'a' : Demanded(122), }<130>
🔨 Rebuilding host...
── TYPE MISMATCH ───────────────────────────────────────────────────────────────
The 1st argument to get is not what I expect:
8│ get {b: "hello world"}
^^^^^^^^^^^^^^^^^^
This argument is a record of type:
{ b : Str }
But get needs the 1st argument to be:
{ a : a }b
Tip: Seems like a record field typo. Maybe a should be b?
Tip: Can more type annotations be added? Type annotations always help
me give more specific messages, and I think they could help a lot in
this case
────────────────────────────────────────────────────────────────────────────────
'+fast-variable-shuffle' is not a recognized feature for this target (ignoring feature)
'+fast-variable-shuffle' is not a recognized feature for this target (ignoring feature)
Done!
Application crashed with message
Can't create record with improper layout
Shutting down
```
rather than the hanging
```
Mismatch in compiler/unify/src/unify.rs Line 1071 Column 13
Trying to unify two flat types that are incompatible: EmptyRecord ~ { 'a' : Demanded(122), }<130>
thread '<unnamed>' panicked at 'invalid layout from var: UnresolvedTypeVar(104)', compiler/mono/s
rc/layout.rs:1510:52
```
that was previously produced.
Part of #2227
