feat: decompose macros feature as macros-decl and macros-proc

[BastiDood]

Motivation

As its name suggests, the macros feature enables the use of various macros in Tokio. Among the most beloved are the #[tokio::main] and the select! macros.

However, one major difference between attribute macros and declarative macros is that the former requires the tokio-macros crate, which is the home for various procedural macros.

tokio/tokio/Cargo.toml

Line 48 in d8cad13

macros = ["tokio-macros"]

In turn, a dependency on the tokio-macros crate also pulls in the (relatively heavy) syn and quote crates.

tokio/tokio-macros/Cargo.toml

Lines 24 to 27 in d8cad13

	[dependencies]
	proc-macro2 = "1.0.7"
	quote = "1"
	syn = { version = "1.0.56", features = ["full"] }

The main issue arises when attribute macros are not needed, but the user still wishes to use the declarative macros (i.e. select!, join!, and try_join!). They are thus forced to enable the macros feature, which undesirably pulls in the attribute macros from the tokio-macros crate anyway.

Solution

To avoid this issue, this PR decomposes the macros feature into two sub-features: macros-decl and macros-proc.¹ The macros-decl feature enables the declarative macros while the macros-proc feature enables the procedural macros. To maintain backwards-compatibility, the macros feature is now just an alias for ["macros-decl", "macros-proc"].

Most notably, the macros-proc feature enables dep:tokio-macros.² The explicit decomposition allows the user to use the declarative macros without being forced to pull in the heavier attribute macros.³

Please do let me know if there are any issues with my decomposition. In particular, I would love to resolve any compilation issues (if any) that arise due to outdated references to the old macros feature. Thanks! 🎉

Footnotes

1. I'm open to changing the sub-feature names if anyone prefers otherwise. I figured that this was an appropriate naming convention since there is already precedent with ["io-std", "io-util"] and ["rt", "rt-multi-thread"]. ↩

2. In the actual PR, I did not use the dep: prefix to respect the MSRV. ↩

3. It is worth noting, however, that the syn and quote crates would have been pulled in by other crates, anyway. This is especially true for projects with many dependencies (most notably serde_derive). Nevertheless, it is still good to give the user extra control over the compilation of dependencies. ↩

[taiki-e]

Note that the compile of tokio-macros will be fast enough once #4513 is merged.

[BastiDood]

Looking forward to it! Though, I must say that there is still some value in decomposing the features anyway. This is so that users do not need to download nor compile tokio-macros in the first place. For instance, consider a situation where it is possible for Tokio to compile earlier in the topologically-ordered tree of dependencies by removing tokio-macros (if not needed). A little less work for the compiler goes a long way. 👌

[Darksonn]

The select! macro depends on a proc macro.

[Darksonn]

I'm going to close this for now. It's not clear that we want to do this because it exposes implementation details of the tokio::select! macro. Additionally, discussing that kind of stuff should generally happen on an issue rather than a PR.

[gustav3d]

Note that the compile of tokio-macros will be fast enough once #4513 is merged.

Will it ever be merged ?.

[Darksonn]

Well, it requires that someone sits down and spends the time to figure out what the PR is missing, and then to fix those missing things.