fix(vrl): mark set_semantic_meaning impure

[sfackler]

Summary

The VRL compiler previously warned about the unused result of set_semantic_meaning, even though it doesn't return anything due to it being incorrectly considered a pure function.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Please read our Vector contributor resources.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run dd-rust-license-tool write to regenerate the license inventory and commit the changes (if any). More details here.

References

Closes #21887

[bits-bot]

All committers have signed the CLA.

[pront]

Thanks @sfackler! Did you have a chance to test that the warning went away?

[sfackler]

I have not - what's the easiest way to do that?

[pront]

I have not - what's the easiest way to do that?

You can build Vector from this PR and use a Remap with set_semantic_meaning in its source. Something like:

sources:
  demo_logs_1:
    type: demo_logs
    format: json
transforms:
  remap0:
    type: remap
    inputs: ["demo_logs_1"]
    source: |
      set_semantic_meaning(.host, "a")
sinks:
  console:
    type: console
    inputs: ["remap0"]
    encoding:
      codec: json

[sfackler]

Huh - it seems like this change doesn't actually fix the warning? I had to add a second statement to the VRL script (does it treat the last expression as an implicit return to the caller?) for the warning to trigger at all, but with that setup it warns both on master and this branch:

$ cat config.yaml
sources:
  demo_logs:
    type: demo_logs
    format: json
transforms:
  remap:
    type: remap
    inputs: [demo_logs]
    source: |
      set_semantic_meaning(.foo, "a")
      .foo = "bar"
sinks:
  console:
    type: console
    inputs: [remap]
    encoding:
      codec: json
$ cargo run -p vector -- --config config.yaml
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.33s
     Running `target/debug/vector --config config.yaml`
2024-11-26T21:09:00.300855Z  INFO vector::app: Log level is enabled. level="info"
2024-11-26T21:09:00.319805Z  INFO vector::app: Loading configs. paths=["config.yaml"]
2024-11-26T21:09:00.329959Z  WARN transform{component_kind="transform" component_id=remap component_type=remap}: vector::transforms::remap: VRL compilation warning. warnings=
warning[E900]: unused result for function call `set_semantic_meaning(.foo, "a")`
  ┌─ :1:1
  │
1 │ set_semantic_meaning(.foo, "a")
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the result of this expression or remove it
  │
  = this expression has no side-effects
  = see language documentation at https://vrl.dev
  = try your code in the VRL REPL, learn more at https://vrl.dev/examples

2024-11-26T21:09:00.331415Z  INFO vector::topology::running: Running healthchecks.
2024-11-26T21:09:00.331730Z  INFO vector::topology::builder: Healthcheck passed.
2024-11-26T21:09:00.331916Z  INFO vector: Vector has started. debug="true" version="0.43.0" arch="x86_64" revision=""
2024-11-26T21:09:00.332007Z  INFO vector::app: API is disabled, enable by setting `api.enabled` to `true` and use commands like `vector top`.
{"foo":"bar"}

[sfackler]

Oh - looks like the lint doesn't actually check if the function is pure or not: https://github.com/vectordotdev/vrl/blob/e3852056d7f2f52a6127c5407ab840f52e545a88/src/compiler/unused_expression_checker.rs#L354-L372

[pront]

does it treat the last expression as an implicit return to the caller?

Yes, the result of the last expression is considered "used". I think we need to add set_semantic_meaning to this list.

But we should also merge this PR, since this a 'strange' function which modifies internal state.