Serde: What might a slimmer serde look like?

[KodrAus]

Hi! :wave:

I've been experimenting recently with whether it's possible to get a slim dependency on serde that still lets you work generically with its core traits. Basically enough to know that T: Serialize or T: Deserialize<'de>. This usecase is probably pretty niche, so please go ahead and close it if it's not something you're interested in supporting. I've mostly been looking at compile times rather than artifact size, since we'll drop any impls that aren't actually used anyway when compiling serde in a binary.

It looks like about 50% of the compile time is spent on the code generated by these macros:

mod de {
    mod impls {
        array_impls! {
            1 => (0 a)
            2 => (0 a 1 b)
            3 => (0 a 1 b 2 c)
            4 => (0 a 1 b 2 c 3 d)
            5 => (0 a 1 b 2 c 3 d 4 e)
            6 => (0 a 1 b 2 c 3 d 4 e 5 f)
            7 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g)
            8 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h)
            9 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i)
            10 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j)
            11 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k)
            12 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l)
            13 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m)
            14 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n)
            15 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o)
            16 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p)
            17 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q)
            18 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r)
            19 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s)
            20 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t)
            21 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u)
            22 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v)
            23 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w)
            24 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x)
            25 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y)
            26 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z)
            27 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa)
            28 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa 27 ab)
            29 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa 27 ab 28 ac)
            30 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa 27 ab 28 ac 29 ad)
            31 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa 27 ab 28 ac 29 ad 30 ae)
            32 => (0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 i 9 j 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26 aa 27 ab 28 ac 29 ad 30 ae 31 af)
        }

        tuple_impls! {
            1  => (0 T0)
            2  => (0 T0 1 T1)
            3  => (0 T0 1 T1 2 T2)
            4  => (0 T0 1 T1 2 T2 3 T3)
            5  => (0 T0 1 T1 2 T2 3 T3 4 T4)
            6  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5)
            7  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6)
            8  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7)
            9  => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8)
            10 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9)
            11 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10)
            12 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11)
            13 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12)
            14 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13)
            15 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14)
            16 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14 15 T15)
        }
    }
}

mod ser {
    mod impls {
        array_impls!(01 02 03 04 05 06 07 08 09 10
                     11 12 13 14 15 16 17 18 19 20
                     21 22 23 24 25 26 27 28 29 30
                     31 32);

        tuple_impls! {
            1 => (0 T0)
            2 => (0 T0 1 T1)
            3 => (0 T0 1 T1 2 T2)
            4 => (0 T0 1 T1 2 T2 3 T3)
            5 => (0 T0 1 T1 2 T2 3 T3 4 T4)
            6 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5)
            7 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6)
            8 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7)
            9 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8)
            10 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9)
            11 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10)
            12 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11)
            13 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12)
            14 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13)
            15 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14)
            16 => (0 T0 1 T1 2 T2 3 T3 4 T4 5 T5 6 T6 7 T7 8 T8 9 T9 10 T10 11 T11 12 T12 13 T13 14 T14 15 T15)
        }
    }
}

Which seems reasonable since those macros generate a _lot_ of code.

To see how much overhead the impls have over the trait definitions, I've made a few tweaks to the core serde library:

• Move just about all the code from serde into a new crate, serde_core
• Make serde just re-export the serde_core API and forward its existing crate features
• Feature gate the ser and de modules, and the ser::impls and de::impls modules in serde_core, but enable them by default.

The split between serde and serde_core was just so that libraries depending on serde with default-features = false keep the same behaviour.

If a library depends on serde_core instead of serde it can disable crate features to slim down compile times. If serde proper finds its way into the dependency graph then those compile times will return to normal.

The results could look something like this:

| features | compile time | rlib size | what's built |
| -------- | ------------- | -------- | ----------- |
| - | 7.53s | 3.7M | exactly what's in serde today |
| no-default-features + ser | 1.88s | 168K | just the serialization traits without any impls |
| no-default-features + de | 1.44s | 276K | just the deserialization traits without any impls |
| no-default-features + ser + impls | 3.34s | 528K | just the serialization traits and their impls |
| no-default-features + de + impls | 5.69s | 3.2M | just the deserialization traits and their impls |

I'm not necessarily putting this approach forward as a good idea, I haven't thought too hard about it, but was an interesting thought experiment I thought I'd share.

[dtolnay]

In what scenarios would it be useful to depend on a Serde trait that has no impls?

[KodrAus]

The general case would be something like _I only need T: Serialize or T: Deserialize and care a lot about compile times_. I imagine that _I only need T: Serialize or T: Deserialize_ actually covers a fair number of use-cases by pure coincidence, but the number of cases that also care enough about compile times to jump through hoops is probably negligible.

The specific case I'm looking at is structured logging in log, but we don't have consensus that serde is the most appropriate tool for the job there so I wouldn't put too much weight in that.

[dtolnay]

But bounds like that are only useful if some other library in the dependency graph enables all the impls.

If you want a trait with no impls, it should be easy to shim in your own rather than removing impls from Serde's traits:

#[cfg(...)]
use serde::Serialize;

#[cfg(not(...))]
use self::private::Serialize;
#[cfg(not(...))]
mod private {
    pub trait Serialize {}
}

[KodrAus]

Yeh, there's still a fair improvement to compile-times when excluding one of the ser or de modules but retaining its impls but thinking about it a bit more it just doesn't seem genuinely useful at all. The path I followed to get here was pretty convoluted.

[dtolnay]

Sounds good. On the plus side, when we stabilize const generics and variadic generics I look forward to the 50% compile time improvement from doing the array impls and tuple impls that way!