extend the sixth requirement

Author: lcnr

src/doc/rustc-dev-guide/src/solve/trait-solving.md

@@ -96,6 +96,10 @@ Trait solving during codegen should have the same result as during typeck. As we
 all free regions during codegen we must not rely on them during typeck. A noteworthy example
 is special behavior for `'static`.
 
+We also have to be careful with relying on equality of regions in the trait solver.
+This is fine for codegen, as we treat all erased regions are equal. We can however
+loose equality information from HIR to MIR typeck.
+
 ### 7. Removing ambiguity makes strictly more things compile
 
 We *should* not rely on ambiguity for things to compile.