diff --git a/Cargo.toml b/Cargo.toml
index a10cf12e..2aabd3b8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,6 +16,13 @@ links = "R"
 documentation = "https://docs.rs/libR-sys/latest/libR_sys/"
 repository = "https://github.com/extendr/libR-sys"
 
+[features]
+default = []
+# Expose a small set of R "non-API" entry points (promise/environment/closure
+# accessors) in the `non_api` module. Off by default: these are not part of R's
+# stable C API and may not be available on every R version.
+non-api = []
+
 [lib]
 # Some code comments on R's source code might be accidentally treated as Rust's
 # doc test. See https://github.com/extendr/libR-sys/issues/194 for the details.
diff --git a/src/lib.rs b/src/lib.rs
index 273f76e4..eadbfb2b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -66,6 +66,9 @@
 
 include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
 
+#[cfg(feature = "non-api")]
+pub mod non_api;
+
 #[non_exhaustive]
 #[repr(transparent)]
 #[derive(Debug)]
diff --git a/src/non_api.rs b/src/non_api.rs
new file mode 100644
index 00000000..7740e406
--- /dev/null
+++ b/src/non_api.rs
@@ -0,0 +1,32 @@
+//! R "non-API" entry points, gated behind the off-by-default `non-api` feature.
+//!
+//! These are not part of R's stable C API; some are not exported by every R
+//! version (e.g. `SET_FORMALS`, `SET_BODY`, `SET_CLOENV`, `PRSEEN`,
+//! `Rf_isValidStringF` are not exported by R 4.6), so calling them may fail to
+//! link depending on the R you build against. Use with care.
+//!
+//! Ported from `extendr-ffi` (`src/non_api.rs`).
+use crate::{Rboolean, SEXP, SEXPTYPE};
+extern "C" {
+    pub fn Rf_isValidString(arg1: SEXP) -> Rboolean;
+    pub fn Rf_isValidStringF(arg1: SEXP) -> Rboolean;
+    pub fn SYMVALUE(x: SEXP) -> SEXP;
+    pub fn PRSEEN(x: SEXP) -> ::std::os::raw::c_int;
+    pub fn SET_ENCLOS(x: SEXP, v: SEXP);
+    pub fn ENVFLAGS(x: SEXP) -> ::std::os::raw::c_int;
+    pub fn SET_ENVFLAGS(x: SEXP, v: ::std::os::raw::c_int);
+    pub fn ENCLOS(x: SEXP) -> SEXP;
+    pub fn HASHTAB(x: SEXP) -> SEXP;
+    pub fn FRAME(x: SEXP) -> SEXP;
+    pub fn Rf_allocSExp(arg1: SEXPTYPE) -> SEXP;
+    pub fn SET_FORMALS(x: SEXP, v: SEXP);
+    pub fn SET_BODY(x: SEXP, v: SEXP);
+    pub fn SET_CLOENV(x: SEXP, v: SEXP);
+    pub fn SET_PRCODE(x: SEXP, v: SEXP);
+    pub fn SET_PRENV(x: SEXP, v: SEXP);
+    pub fn SET_PRVALUE(x: SEXP, v: SEXP);
+    #[doc = "Promise Access Functions"]
+    pub fn PRCODE(x: SEXP) -> SEXP;
+    pub fn PRENV(x: SEXP) -> SEXP;
+    pub fn PRVALUE(x: SEXP) -> SEXP;
+}