Rollup merge of #76441 - jyn514:parallel, r=jonas-schievink

Note that parallel-compiler = true causes tests to fail

Mentioning #75760.

Author: jonas-schievink

config.toml.example

@@ -393,6 +393,7 @@
 #incremental = false
 
 # Build a multi-threaded rustc
+# FIXME(#75760): Some UI tests fail when this option is enabled.
 #parallel-compiler = false
 
 # The default linker that will be hard-coded into the generated compiler for