Runtime error when passing record with missing default fields to encoder

[jwoudenberg]

I can write a function that takes a record with some default fields and directly encodes it, without providing default values. This compiles, but then results in a runtime failure.

This is a minimal example using roc-json, but I think the error is the same regardless of what encoder you use. Run the file with roc test to get the error.

app [main] {
    cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
    json: "https://github.com/lukewilliamboswell/roc-json/releases/download/0.10.2/FH4N0Sw-JSFXJfG3j54VEDPtXOoN-6I9v_IA8S18IGk.tar.br",
}

import json.Json

main = Task.ok {}

expect
    encode : { hi ? Str } -> List U8
    encode = \arg -> Encode.toBytes arg Json.utf8
    encode {} == ['{', '}']

Here's the error:

thread 'main' panicked at crates/compiler/mono/src/borrow.rs:396:34:
internal error: entered unreachable code: 
	No borrow signature for LambdaName { name: `12.IdentId(26)`, niche: Niche(Captures([])) } layout.

	Tip 1: This can happen when you call a function with less arguments than it expects.
	Like `Arg.list!` instead of `Arg.list! {}`.
	Tip 2: `roc check yourfile.roc` can sometimes give you a helpful error. 
                            
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I don't know what the fix should be. Probably a compile time error rather than allowing the behavior at runtime? The reason I was trying this was to create a cute DSL for writing Html:

Html.ul { class: "list-of-things } [
    Html.li {} [ Html.text "an item!" ],
]

# --- Html.roc
ul : { class ? Str, id ? Str, ............ }, List Html -> Html
li : { class ? Str, id ? Str, ............ }, List Html -> Html

I realize that's not what these default field types are made for though 😅