Rework structured value casting #396
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This commit also removes some unsafe that we don't need
build.rs
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| #[cfg(feature = "kv_unstable")] | ||
| into_primitive::generate(); | ||
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| println!("cargo:rustc-cfg=src_build"); |
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I’m using this so the into_primitive module can tell if it’s in the build script or not. Is there a cfg that exists already for this?
src/lib.rs
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| @@ -276,6 +276,9 @@ | |||
| #![cfg_attr(rustbuild, feature(staged_api, rustc_private))] | |||
| #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))] | |||
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| #![feature(is_sorted)] | |||
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This seems nice for debug assertions. I’ll look into where stabilisation is at.
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See: rust-lang/rust#53485
src/lib.rs
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| @@ -276,6 +276,9 @@ | |||
| #![cfg_attr(rustbuild, feature(staged_api, rustc_private))] | |||
| #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))] | |||
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| #![feature(is_sorted)] | |||
| #![feature(const_type_id)] | |||
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I think we just stabilise these const methods on-demand. I can’t see why this one shouldn’t be stabilised, but will look into it.
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See: rust-lang/rust#72488
| @@ -0,0 +1,109 @@ | |||
| // NOTE: With specialization we could potentially avoid this call using a blanket | |||
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Specialization is possible because we use impl Trait instead of dyn Trait for inputs. This approach would still be necessary if we switched to dyn Trait. The trade-off would be that with dyn Trait we could generate less code in calling libraries at the cost of still requiring this runtime check.
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cc @hawkw sorry for the random ping but thought you might find this interesting for tracing 🙂
This is using a static set of type ids sorted at build time and binary searches them for interesting types given a generic T: Debug + ‘static input and treats the input as that concrete type. The runtime cost per argument is small (I measured ~9ns on my machine for a hit and ~5ns for a miss) but is not negligible. It could also use specialisation to remove that runtime cost, which I think might land as min_specialization sometime over the next year. Maybe you could consider it for something like the #[instrument] macro, to keep the nice compatible Debug bound, but also retain numbers and booleans as structured values?
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This is fascinating, thanks for pinging me!
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So...
If rust-lang/rust#72437 and rust-lang/rust#72488 are both merged then you’d be able to do this at compile time without specialization :)
There’s an example in the const_type_id PR.
Cargo.toml
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| @@ -47,6 +47,7 @@ std = [] | |||
| # this will have a tighter MSRV before stabilization | |||
| kv_unstable = [] | |||
| kv_unstable_sval = ["kv_unstable", "sval/fmt"] | |||
| kv_unstable_const_primitive = [] | |||
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Just testing different strategies for destructuring.
| // Use consts to match a type with a conversion fn | ||
| // Pros: fast, will work on stable soon (possibly 1.45.0) | ||
| // Cons: requires a `'static` bound | ||
| #[cfg(all(src_build, feature = "kv_unstable_const_primitive"))] |
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@hawkw I’ve got a few different strategies for capturing the value from a generic T here now.
For tracing, if this is something you wanted to do you might end up needing to look more at the specialization one, because the others require a ’static bound in order to get the type id, which from what I can see isn’t compatible with how the #[instrument] macro currently works.
src/lib.rs
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| @@ -275,6 +275,8 @@ | |||
| // an unstable crate | |||
| #![cfg_attr(rustbuild, feature(staged_api, rustc_private))] | |||
| #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))] | |||
| // TODO: Remove once https://github.com/rust-lang/rust/pull/72488 is merged | |||
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I'll wait for rust-lang/rust#72488 before merging this PR so this unstable feature can be removed.
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Once a new |
This is a spike to see if we can improve the way we cast values to concrete types. It does more work when capturing
DebugandDisplayto retain the underlying primitive so that a new macro could reasonably default to creating values through these std traits while retaining numbers.It does some wacky build script code generation to create a static sorted set of type ids we care about so they can be binary searched quickly.