Add support for tuple-structs/variants in the rust backend?

[Gankra]

In kdl-script you can choose to omit the names of fields, as demonstrated by anons.kdl:

abi-cafe/include/tests/normal/anons.kdl

Lines 1 to 36 in 6ba6865

	// Cases that allow a backend to encounter tuple-structs/tuple-variants
	struct "MyTupleStruct" {
	_ "bool"
	_ "i32"
	_ "u64"
	}
	tagged "MyTagged" {
	TupleVariant {
	_ "bool"
	_ "i32"
	_ "u64"
	}
	HybridVariant {
	x "bool"
	_ "u32"
	}
	StructVariant {
	x "u8"
	z "i64"
	}
	EmptyVariant {
	}
	NoneLike
	}
	fn "anons" {
	inputs {
	_ "u32"
	_ "MyTupleStruct"
	z "i8"
	}
	outputs {
	_ "MyTagged"
	}
	}

While this is partially just a convenience for not writing stuff you don't care about, it is intended that the rust backend could lower these to tuple-structs/variants when applicable. The kdl-script compiler already caches/computes whether this is applicable to a struct/variant with all_fields_were_blank:

abi-cafe/kdl-script/src/types.rs

Line 238 in 6ba6865

pub all_fields_were_blank: bool,

abi-cafe/kdl-script/src/types.rs

Line 284 in 6ba6865

pub all_fields_were_blank: bool,

The main reason I didn't follow through on this is that:

• if the difference affects ABI, you are Should Be Very Sad, so there's no "need" to implement this
  • ...but that assumption should be tested, huh
• just tuple structs adds 3 grody branches in the code:

on struct definition:

abi-cafe/src/abis/rust/declare.rs

Lines 167 to 180 in 6ba6865

	if has_borrows {
	writeln!(f, "struct {}<'a> {{", struct_ty.name)?;
	} else {
	writeln!(f, "#[derive(Copy, Clone)]")?;
	writeln!(f, "struct {} {{", struct_ty.name)?;
	}
	f.add_indent(1);
	for field in &struct_ty.fields {
	let field_name = &field.ident;
	let field_tyname = state
	.borrowed_tynames
	.get(&field.ty)
	.unwrap_or(&state.tynames[&field.ty]);
	writeln!(f, "{field_name}: {field_tyname},")?;

on struct field access:

abi-cafe/src/abis/rust/write.rs

Lines 101 to 106 in 6ba6865

	// recurse into each field
	for field in &struct_ty.fields {
	let field_name = &field.ident;
	let base = format!("{from}.{field_name}");
	self.write_fields(f, state, to, &base, field.ty, vals)?;
	}

on struct init:

abi-cafe/src/abis/rust/init.rs

Lines 120 to 130 in 6ba6865

	write!(f, "{name} {{ ")?;
	for (field_idx, field) in struct_ty.fields.iter().enumerate() {
	if field_idx > 0 {
	write!(f, ", ")?;
	}
	let field_name = &field.ident;
	write!(f, "{field_name}: ")?;
	let ref_temp_name = format!("{ref_temp_name}{field_name}_");
	self.init_value(f, state, field.ty, vals, alias, &ref_temp_name, extra_decls)?;
	}
	write!(f, " }}")?;