Feature request: expose type sizes

[mqudsi]

When working on trying to improve the performance of a rust codebase, it's important to be able to introspect the nature of types. While there are a number of aspects that one might want to explore (alignment, size, etc.) I think the most important of these is probably the size of a type to help decide between Option<T> and Box<T> or to figure out if pigeonholing is being used to make Option<T> the same size as T, etc.

Since these are all awesomely available as const functions that take a type but no value, it would be wonderful if the size of types could be visible (preferably directly in the hover info, or alternatively via a code action).

Since #10933 was merged I think it should be possible although it might also depend on #10825 (going by the title but not by the description) in order to evaluate constant expressions.

e.g. a step in the right direction would be to make it possible to do this myself in the IDE, by typing out the following:

const X: usize = core::mem::size_of::<i32>();

then hovering over X and seeing the result of the constant evaluation. Currently hovering over X just shows const X: usize = core::mem::size_of::<CxxWString>, but it would be beyond amazing if it could show (instead or in addition) the result of the evaluation (e.g. const X: usize = core::mem::size_of::<i32>(); // 4).

Once that works, rust-analyzer could basically "write and evaluate" in the background an expression to get the size of any type T when T is hovered over to retrieve the size (either in advance or on-the-fly).

Taking a step back and looking at what I'm asking: would you like me to open a second PR to formally request "evaluate and display the result of compile-time constant expressions evaluating to primitive types" or is this PR sufficient?

[HKalbasi]

Currently it shows size of structs and enums in hover, so if you want to see the type of i32, you can hover on struct Foo(i32). About const evaluation, your example will work after #14040

[mqudsi]

Hmm I see it now but only for some types. Is it only for types native to the crate being developed? Actually no because I see it with some of my dependencies...

Any reason why it wouldn't show the size of some types, for example libc::termios with the following definition?

    pub struct termios {
        pub c_iflag: ::tcflag_t,
        pub c_oflag: ::tcflag_t,
        pub c_cflag: ::tcflag_t,
        pub c_lflag: ::tcflag_t,
        pub c_line: ::cc_t,
        pub c_cc: [::cc_t; ::NCCS],
        #[cfg(not(any(
            target_arch = "sparc",
            target_arch = "sparc64",
            target_arch = "mips",
            target_arch = "mips64")))]
        pub c_ispeed: ::speed_t,
        #[cfg(not(any(
            target_arch = "sparc",
            target_arch = "sparc64",
            target_arch = "mips",
            target_arch = "mips64")))]
        pub c_ospeed: ::speed_t,
    }

I just get

libc::unix::linux_like::linux::gnu
──────────────────────────────────
pub struct termios

[HKalbasi]

If it fails to find the correct data layout, it will show nothing. In your example, my guess is that it can't evaluate the NCCS constant.

[mqudsi]

Ok, thanks.

Is there anything new in this request or should I close it out?

[HKalbasi]

I think we can close it in favor of #4091