View memory layout gives incorrect layout for unions

[antonilol]

rust-analyzer version: rust-analyzer version: 0.4.1830-standalone

rustc version: rustc 1.77.0 (aedd173a2 2024-03-17)

editor or extension: Code OSS (on arch linux, installed with pacman, code 1.87.2-1)

relevant settings: None

code snippet to reproduce:

union Union {
    a: u16,
    b: u32,
}

result when hitting Ctrl+Shift+P and going to 'view memory layout':

when hovering on the b: u32 field, the tooltip reports an offset of 0, although it is not placed at an offset of 0 in here:

this is a cool feature, but it just needs some more polishing:

for example, it knows PhantomData<T> always has zero size, although in the tooltip it will say A<{unknown}>, but [T; 0] also has zero size and for that it says 'Not Available'

struct Struct<T> {
    a: usize,
    b: PhantomData<T>,
}

it does not account for fat pointers:

struct Struct<'a, T: ?Sized> {
    a: usize,
    b: &'a T,
}

this will report the &T being 8 bytes wide (&{unknown} here too). this is only correct if T: Sized

and for enums just size and align is displayed, nothing is done with the members

[roife]

rust-analyzer/crates/ide/src/view_memory_layout.rs

Lines 119 to 133 in 46702ff

	let mut fields = ty
	.fields(db)
	.into_iter()
	.map(|(f, ty)| (FieldOrTupleIdx::Field(f), ty))
	.chain(
	ty.tuple_fields(db)
	.into_iter()
	.enumerate()
	.map(|(i, ty)| (FieldOrTupleIdx::TupleIdx(i), ty)),
	)
	.collect::<Vec<_>>();
	if fields.is_empty() {
	return;
	}

Here we're simply using .fields() to fetch all the fields, which works for both struct and union (but does not work for enum). Do we need to add viewing-memory-layout for unions? Or simply add a check to bail out if it's a union. 👀

[Veykril]

Yes we need to check what kind of ADT we are dealing with here, ideally we'd render things differently for union showing that they overlap.

Tbf, this feature needs a design overhaul, it is very hard to parse imo :) Iirc Visual Studio (not Code) has a very nice looking one, and there is an extension for vscode that has a really nice one for C++ that I can't recall the name of (might've been builtin into the clang extension)

[roife]

there is an extension for vscode

Might be https://marketplace.visualstudio.com/items?itemName=RamonViladomat.StructLayout. Really cool.