chore: switch to Rust 2024 edition

[DavSanchez]

What this PR does / why we need it

Updates the project to use Rust 2024, which includes some syntax and semantic breaking changes but will allow us to use additional new features such as let_chains in Rust 1.88.

The codebase was migrated automatically using cargo fix --edition --all-features so there might be changes done conservatively, like the match expressions replacing if lets in dafa43c. Where appropriate, I'll try to rework the syntax for it to be more clear before applying the final cargo fmt that will change a lot of files.

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[DavSanchez]

For reference of the use<>: https://doc.rust-lang.org/stable/reference/types/impl-trait.html#precise-capturing

[danielorihuela]

I've read some documentation and blog posts. I'm still a bit puzzled. Was this added by the migrator? Is it extremely necessary or the function would still work if we removed the use bound, but using it is more correct?

[DavSanchez]

Yeah it was added automatically by the migrator. In Rust 2024 if you remove this the project stops compiling due to borrowed data escaping some functions in the tracing_layers module. Let me retrieve the exact lints to give more context.

[danielorihuela]

Don't worry, I can do that on my computer and check it out. I was curious if it was automatically done and potentially not required.

[DavSanchez]

$ cargo build
# ...
error[E0521]: borrowed data escapes outside of function
  --> agent-control/src/instrumentation/tracing_layers/file.rs:29:25
   |
18 |       config: &LoggingConfig,
   |       ------  - let's call the lifetime of this reference `'1`
   |       |
   |       `config` is a reference that is only valid in the function body
...
29 |               let layer = tracing_subscriber::fmt::layer()
   |  _________________________^
30 | |                 .with_writer(file_writer)
31 | |                 .with_ansi(false) // Disable colors for file
32 | |                 .with_target(target)
...  |
35 | |                 .with_filter(config.filter()?)
36 | |                 .boxed();
   | |                        ^
   | |                        |
   | |________________________`config` escapes the function body here
   |                          argument requires that `'1` must outlive `'static`

error[E0521]: borrowed data escapes outside of function
  --> agent-control/src/instrumentation/tracing_layers/stdout.rs:12:17
   |
8  |   pub fn stdout(config: &LoggingConfig) -> Result<LayerBox, LoggingConfigError> {
   |                 ------  - let's call the lifetime of this reference `'1`
   |                 |
   |                 `config` is a reference that is only valid in the function body
...
12 |       let layer = tracing_subscriber::fmt::layer()
   |  _________________^
13 | |         .with_writer(std::io::stdout)
14 | |         .with_ansi(config.format.ansi_colors)
15 | |         .with_target(target)
...  |
18 | |         .with_filter(config.filter()?)
19 | |         .boxed();
   | |                ^
   | |                |
   | |________________`config` escapes the function body here
   |                  argument requires that `'1` must outlive `'static`

For more information about this error, try `rustc --explain E0521`.
error: could not compile `newrelic_agent_control` (lib) due to 2 previous errors

So now apparently we have to make explicit that the type impl Filter<Registry> doesn't capture any lifetimes of the &LoggingConfig/&self. I will try to search for more info to better clarify how this changed in Rust 2024.

[danielorihuela]

I found a stackoverflow link pointing to other three resources

https://blog.rust-lang.org/2024/10/17/Rust-1.82.0/#precise-capturing-use-syntax
https://blog.rust-lang.org/2024/09/05/impl-trait-capture-rules/
https://github.com/rust-lang/rfcs/blob/master/text/3617-precise-capturing.md

Those are helpful, but I never used that so it's confusing.

[DavSanchez]

This is the exact change related to "impl traits in return position (RPIT)": https://doc.rust-lang.org/nightly/edition-guide/rust-2024/rpit-lifetime-capture.html

In Rust 2024, all in-scope generic parameters, including lifetime parameters, are implicitly captured when the use<..> bound is not present.