Reduce the complexity around all possible feature combinations (#107)

* Unconditionally enable the set_thread_id feature

If set_current_id_after_a_fork is isn't used, --gc-sections will already
automatically remove it from the executable.

* Fix the compile time assertion about origin-program requiring either origin-start or external-start

Also move a couple of feature deps from origin-start/external-start to
origin-program.

Author: bjorn3

Cargo.toml

@@ -65,29 +65,26 @@ rustc-dep-of-std = [
     "dep:compiler_builtins",
 ]
 
-# Enable the `set_current_id_after_a_fork` function.
-set_thread_id = []
-
 # Use origin's implementation of program startup and shutdown.
-origin-program = []
+origin-program = ["rustix/use-explicitly-provided-auxv", "rustix/runtime"]
 
 # Use origin's implementation of thread startup and shutdown.
 #
 # To use threads, it's also necessary to enable the "thread" feature.
-origin-thread = ["memoffset", "rustix/runtime", "origin-program"]
+origin-thread = ["memoffset", "origin-program"]
 
 # Use origin's implementation of signal handle registrtion.
 #
 # To use threads, it's also necessary to enable the "signal" feature.
 origin-signal = ["rustix/runtime"]
 
 # Use origin's `_start` definition.
-origin-start = ["rustix/use-explicitly-provided-auxv", "rustix/runtime", "origin-program"]
+origin-start = ["origin-program"]
 
 # Don't use origin's `_start` definition, but export a `start` function which
 # is meant to be run very early in program startup and passed a pointer to
 # the initial stack. Don't enable this when enabling "origin-start".
-external-start = ["rustix/use-explicitly-provided-auxv", "rustix/runtime", "origin-program"]
+external-start = ["origin-program"]
 
 # The loggers depend on a `.init_array` entry to initialize themselves, and
 # `env_logger` needs it so that `c-scape` can initialize environment variables

src/program.rs

@@ -38,7 +38,7 @@ use linux_raw_sys::ctypes::c_int;
 use rustix_futex_sync::Mutex;
 
 #[cfg(all(
-    feature = "program-start",
+    feature = "origin-program",
     not(any(feature = "origin-start", feature = "external-start"))
 ))]
 compile_error!("\"origin-program\" depends on either \"origin-start\" or \"external-start\".");

src/thread/libc.rs

@@ -196,7 +196,6 @@ pub fn current_id() -> ThreadId {
 /// This must only be called immediately after a `fork` before any other
 /// threads are created. `tid` must be the same value as what [`gettid`] would
 /// return.
-#[cfg(feature = "set_thread_id")]
 #[doc(hidden)]
 #[inline]
 pub unsafe fn set_current_id_after_a_fork(tid: ThreadId) {

src/thread/linux_raw.rs

@@ -989,8 +989,6 @@ pub fn current_id() -> ThreadId {
 /// This must only be called immediately after a `fork` before any other
 /// threads are created. `tid` must be the same value as what [`gettid`] would
 /// return.
-#[cfg(feature = "set_thread_id")]
-#[cfg_attr(doc_cfg, doc(cfg(feature = "set_thread_id")))]
 #[doc(hidden)]
 #[inline]
 pub unsafe fn set_current_id_after_a_fork(tid: ThreadId) {