Running a recent r-devel:

• checking compiled code ... NOTE
  File ‘Rcpp/libs/Rcpp.so’:
  Found non-API calls to R: ‘STRING_PTR’, ‘VECTOR_PTR’

Compiled code should not call non-API entry points in R.

See ‘Writing portable packages’ in the ‘Writing R Extensions’ manual,
and section ‘Moving into C API compliance’ for issues with the use of non-API entry points.

• checking sizes of PDF files under ‘inst/doc’ ... OK

The new sectionMoving into C API compliance mentions read-only variantsSTRING_PTR_RO andVECTOR_PTR_RO which look promising.