Use public R C APIs for R 4.6 compatibility

[programLyrique]

Replace legacy/non-API R internals that are no longer available under R 4.6 headers. Allocate language objects directly with Rf_allocLang(), avoid Rf_findVarInFrame(), and rebuild transformed closures with R_mkClosure() while preserving the original environment and attributes.

This fixes installation failures such as:

imputesrcref.so: undefined symbol: SET_BODY

The srcref coordinate output was compared against the previous implementation under R 4.5.2 and matched after normalizing srcfile timestamps.

[Copilot]

Pull request overview

This PR updates the package’s C code to avoid relying on R internals that are no longer available with R 4.6 headers, replacing them with public R APIs to restore build/install compatibility.

Changes:

• Rebuild transformed functions using R_mkClosure() (preserving the original closure environment) and reapply attributes via attributes().
• Replace legacy allocList(...); SET_TYPEOF(..., LANGSXP) patterns with Rf_allocLang() for language object allocation.
• Replace Rf_findVarInFrame() usage with R_tryEvalSilent() when probing base bindings for SPECIALSXP.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/transform.c	Uses public APIs (Rf_allocLang, R_mkClosure, attributes()) to construct transformed closures without internal setters.
src/srcref_sites.c	Allocates language objects with Rf_allocLang() for R 4.6 header compatibility.
src/source_impute.c	Allocates the sys.source call with Rf_allocLang() instead of mutating a list into a LANGSXP.
src/blacklist.c	Avoids Rf_findVarInFrame() by evaluating symbols in R_BaseEnv via R_tryEvalSilent().

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[MatejKocourek]

Are there performance implications for this? Time to impute is included in our results for comparison against covr.
For example for avoiding Rf_findVarInFrame and instead calling eval?

[programLyrique]

I have not benchmarked yet but I expect there would be a performance impact as it now copies instead of mutating place for the closures. I don't think the eval is a big cost though, as it is doing it on a symbol, and eval has a specific path for this.

No need to merge this now, but anyway, some of the functions used in the package are not public anymore in R 4.6 so the package will break.