This was referenced in the `List` documentation and in the
[tutorial](./TUTORIAL.md), but wasn't actually implemented prior to this
commit!
Part of #2227
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
It contradicts build_tag in compiler/gen_llvm/src/llvm/build.rs,
where we create a NonNullableUnwrapped by calling
reserve_with_refcount_union_as_block_of_memory
