Make rustc implicitly use panic=abort for the panic_abort crate

The panic_abort crate must be compiled with panic=abort, but cargo
doesn't allow setting the panic strategy for a single crate. Bootstrap
handles this in its rustc wrapper, but for example the build systems of
cg_clif and cg_gcc don't use a rustc wrapper, so they would either need
to add one or be unable to build a sysroot suitable for both panic=abort
and panic=unwind (as is currently the case).

Author: bjorn3

compiler/rustc_session/src/session.rs

@@ -704,6 +704,11 @@ impl Session {
     /// Returns the panic strategy for this compile session. If the user explicitly selected one
     /// using '-C panic', use that, otherwise use the panic strategy defined by the target.
     pub fn panic_strategy(&self) -> PanicStrategy {
+        if self.opts.crate_name.as_deref() == Some("panic_abort") {
+            // The panic_abort crate must always be compiled with panic=abort.
+            return PanicStrategy::Abort;
+        }
+
         self.opts.cg.panic.unwrap_or(self.target.panic_strategy)
     }
 

src/bootstrap/src/bin/rustc.rs

@@ -161,7 +161,8 @@ fn main() {
         // This... is a bit of a hack how we detect this. Ideally this
         // information should be encoded in the crate I guess? Would likely
         // require an RFC amendment to RFC 1513, however.
-        if crate_name == Some("panic_abort") {
+        // cfg(not(bootstrap))
+        if crate_name == Some("panic_abort") && stage == "0" {
             cmd.arg("-C").arg("panic=abort");
         }