Convert tarpc::service! function macro to #[tarpc::service] attribute macro

[tikue]

Fixes #244

I am leaning toward making this change. I think, overall, it's a net positive. The proc macro is also much easier to read than the macro_rules code (not that that couldn't have been changed without making it an attribute).

[vorot93]

Is this necessary?

[tikue]

I keep forgetting to update examples before pushing code. This will make that harder. I could make this change outside this pr if you'd like.

[vorot93]

Should be enabled by default

[tikue]

I tried to get this to work. Unfortunately, it presents a footgun for users who don't need serde but don't opt out of the default features. They'll get the following errors if they don't have serde in their Cargo.toml dependencies:

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:11:1
   |
11 | #[tarpc::service]
   | ^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:11:1
   |
11 | #[tarpc::service]
   | ^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:11:1
   |
11 | #[tarpc::service]
   | ^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:11:1
   |
11 | #[tarpc::service]
   | ^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:14:20
   |
14 |     async fn hello(name: String) -> String;
   |                    ^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> example-service/src/lib.rs:14:37
   |
14 |     async fn hello(name: String) -> String;
   |                                     ^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable

error: aborting due to 6 previous errors

For more information about this error, try `rustc --explain E0658`.
error: Could not compile `tarpc-example-service`.

This comment has more context.

[vorot93]

Should be enabled by default

[vorot93]

One thing to note is that serde integration should be always enabled by default. This is what users expect and it is extremely confusing when they get compilation errors having followed the API to a letter.

[tikue]

Let me experiment with making serde default enabled. One issue is that the feature currently requires users to depend on the serde crate with the derive feature enabled. While it's not a great experience right now if they want to use serde and it's not enabled, it's arguably worse if they don't care about serde at all but get a compile error because they are missing a crate they aren't even going to use.

[tikue]

If I can make it work by reexporting serde from tarpc, then I'm in favor of that.

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