Explain tests and setting cfgs

Author: saethlin

compiler/rustc_session/src/config/cfg.rs

@@ -205,6 +205,10 @@ pub(crate) fn default_configuration(sess: &Session) -> Cfg {
         ins_none!(sym::overflow_checks);
     }
 
+    // We insert a cfg for the name of session's panic strategy.
+    // Since the ImmediateAbort strategy is new, it also sets cfg(panic="abort"), so that code
+    // which is trying to detect whether unwinding is enabled by checking for cfg(panic="abort")
+    // does not need to be updated.
     ins_sym!(sym::panic, sess.panic_strategy().desc_symbol());
     if sess.panic_strategy() == PanicStrategy::ImmediateAbort {
         ins_sym!(sym::panic, PanicStrategy::Abort.desc_symbol());

tests/run-make-cargo/panic-immediate-abort-codegen/rmake.rs

@@ -1,3 +1,8 @@
+// This is a codegen test which checks that when code is compiled with panic=immediate-abort,
+// we get a `tail call void @llvm.trap()` in user code instead of a call into the standard
+// library's panic formatting code (such as panic_fmt) or one of the numerous panic outlining shims
+// (such as slice_index_fail).
+
 #![deny(warnings)]
 
 use run_make_support::{cargo, llvm_filecheck, path, rfs, target};

tests/run-make-cargo/panic-immediate-abort-works/rmake.rs

@@ -1,3 +1,9 @@
+// This test ensures we are able to compile and link a simple binary with panic=immediate-abort.
+// The test panic-immediate-abort-codegen checks that panic strategy produces the desired codegen,
+// but is based on compiling a library crate (which is the norm for codegen tests because it is
+// cleaner and more portable). So this test ensures that we didn't mix up a cfg or a compiler
+// implementation detail in a way that makes panic=immediate-abort encounter errors at link time.
+
 //@ needs-target-std
 
 #![deny(warnings)]