psm: allow manual opt out of #cfg[link("psm_s")]

Currenty, when using the `asm` configuration, the `lib.rs` file will manually
add a `link("psm_s")` attribute to the build causing a flag like `-lpsm_s`
to appear on the link line. `psm_s` is the library of assembly code, built by
`build.rs` in Cargo projects.

However, when using Cargo packages with build systems like Buck2, we have to
manually replace the `build.rs` script, and build the bits of C/assembly code
that come with `psm` and `stacker` as a separate build item (an actual library),
then link them into the Rust libraries.

The name of the library often can't be easily made identical to `psm_s` as
desired by the `link()` call, meaning that just blindly compiling this crate
causes an inevitable linking failure. But we're building the code and explicitly
linking the library anyway, removing the need for this `#[link]` clause.

Therefore, introduce a new `link_asm` cfg option to give "expert" users the
ability to link in code manually. This is always enabled by the Cargo build for
`asm`-compatible targets, but Buck users can leave it off.

Signed-off-by: Austin Seipp <[email protected]>

psm/build.rs

@@ -7,7 +7,7 @@ fn find_assembly(
     env: &str,
     masm: bool,
 ) -> Option<(&'static str, bool)> {
-    println!("cargo::rustc-check-cfg=cfg(switchable_stack,asm)");
+    println!("cargo::rustc-check-cfg=cfg(switchable_stack,asm,link_asm)");
     match (arch, endian, os, env) {
         // The implementations for stack switching exist, but, officially, doing so without Fibers
         // is not supported in Windows. For x86_64 the implementation actually works locally,
@@ -79,6 +79,7 @@ fn main() {
 
     let asm = if let Some((asm, canswitch)) = find_assembly(&arch, &endian, &os, &env, masm) {
         println!("cargo:rustc-cfg=asm");
+        println!("cargo:rustc-cfg=link_asm");
         if canswitch {
             println!("cargo:rustc-cfg=switchable_stack")
         }

psm/src/lib.rs

@@ -56,7 +56,7 @@ macro_rules! extern_item {
 // NB: this could be nicer across multiple blocks but we cannot do it because of
 // https://github.com/rust-lang/rust/issues/65847
 extern_item! { {
-    #![cfg_attr(asm, link(name="psm_s"))]
+    #![cfg_attr(link_asm, link(name="psm_s"))]
 
     #[cfg(asm)]
     fn rust_psm_stack_direction() -> u8;