bindings: rust: feature gate unreleased features

`gpiod_line_request_get_chip_name()` is not released yet. Still, libgpiod-sys
will just happily generate bindings for it if it sees the definition in the
header file.

This guards the unreleased features behind an optional feature `vnext`.

To sketch the process of what happens once these features get into an
assumed "2.1" release:

libgpiod-sys will get updated with a `v2_1` feature. That feature would
then raise the minimum version that is attempted to query from pkg-
config. An identical feature will then be introduced on the `libgpiod`
crate and `vnext` guards will be changed to `v2_1` guards. The `vnext`
feature will then be updated to require the new `v2_1` feature.

Eventually, we will probably raise the minimum supported version for the
rust bindings and drop all the version gates before that.

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

bindings/rust/libgpiod/Cargo.toml

@@ -18,6 +18,9 @@ exclude = [
     "Makefile.am",
 ]
 
+[features]
+vnext = []
+
 [dependencies]
 errno = "0.2.8"
 intmap = "2.0.0"

bindings/rust/libgpiod/Makefile.am

@@ -8,7 +8,7 @@ command = SYSTEM_DEPS_LIBGPIOD_NO_PKG_CONFIG=1 \
 		SYSTEM_DEPS_LIBGPIOD_SEARCH_NATIVE="${PWD}/../../../lib/.libs/" \
 		SYSTEM_DEPS_LIBGPIOD_LIB=gpiod \
 		SYSTEM_DEPS_LIBGPIOD_INCLUDE="${PWD}/../../../include/"  \
-		cargo build --release --lib
+		cargo build --features=vnext --release --lib
 
 if WITH_TESTS
 command += --tests

bindings/rust/libgpiod/README.md

@@ -17,6 +17,19 @@ By default, `libgpiod-sys` builds against the libgpiod version identified via
 `pkg-config`. See the `README.md` of `libgpiod-sys` for options to override
 that.
 
+Currently at least libgpiod 2.0 is required with the default feature set.
+
+## Features
+
+The Rust bindings will usually be built against whatever libgpiod version a
+system provides. Hence, only the functionality of the oldest supported libgpiod
+C library will be exposed by default.
+
+Setting flags allows to increase the base version and export features of newer
+versions:
+
+- `vnext`: The upcoming, still unreleased version of the C lib
+
 ## License
 
 This project is licensed under either of

bindings/rust/libgpiod/src/line_request.rs

@@ -2,6 +2,7 @@
 // SPDX-FileCopyrightText: 2022 Linaro Ltd.
 // SPDX-FileCopyrightText: 2022 Viresh Kumar <viresh.kumar@linaro.org>
 
+#[cfg(feature = "vnext")]
 use std::ffi::CStr;
 use std::os::unix::prelude::AsRawFd;
 use std::time::Duration;
@@ -31,6 +32,7 @@ impl Request {
     }
 
     /// Get the name of the chip this request was made on.
+    #[cfg(feature = "vnext")]
     pub fn chip_name(&self) -> Result<&str> {
         // SAFETY: The `gpiod_line_request` is guaranteed to be live as long
         // as `&self`

bindings/rust/libgpiod/tests/line_request.rs

@@ -60,6 +60,7 @@ mod line_request {
         use super::*;
 
         #[test]
+        #[cfg(feature = "vnext")]
         fn chip_name() {
             const GPIO: Offset = 2;
             let mut config = TestConfig::new(NGPIO).unwrap();