Cargo warnings on successful builds are silently dropped in rust_source() when echo = FALSE

[CGMossa]

Summary

In the dynamic-compilation path (rust_source(), rust_function(), rust_eval(), and the eng_extendr / eng_extendrsrc knitr engines), cargo warnings emitted on a successful build are not surfaced to the user when echo = FALSE (the default).

Where it happens

R/source.R invokes cargo build --lib with --message-format=json-diagnostic-rendered-ansi (source.R:354) and then runs it via run_cargo():

rlang::try_fetch(
  run_cargo(args, echo = echo, wd = NULL),
  error = function(cnd) {
    cli::cli_abort(
      "Rust code could not be compiled successfully. Aborting.",
      parent = cnd,
      class = "rextendr_error"
    )
  }
)

run_cargo() (R/run_cargo.R) only parses stdout when called with parse_json = TRUE; in the rust_source call site parse_json is left at its default FALSE. As a result:

• When echo = TRUE, cargo's rendered diagnostics stream to the terminal as usual.
• When echo = FALSE (default) and the build succeeds, the JSON diagnostics sit in out$stdout and are never re-emitted. Warnings (unused variables, deprecation notices, dead code, etc.) are invisible to the user.
• On failure, the chained parent = cnd carries processx's captured stderr into the cli abort, so errors do surface — this issue is specifically about success-with-warnings.

Proposed fix

Parse the JSON stdout from the successful run_cargo() invocation, filter for reason == "compiler-message" && message.level == "warning", and emit each rendered message via cli::cli_warn() (or print directly when quiet = FALSE). ANSI handling should mirror tty_has_colors(), matching what's already done in the error path of older versions.

This keeps the default echo = FALSE quiet for routine cargo chatter (Compiling/Finished lines on stderr) while still showing the diagnostics users actually need to act on.

Scope

Applies to anything that funnels through rust_source() — i.e. rust_function(), rust_eval() / rust_eval_deferred(), and both knitr engines in R/knitr_engine.R. The package-build path (Makevars + cargo build --lib + cargo run --bin document) is unaffected because compilation output is streamed live by R CMD INSTALL.