But in principle if this definition appears in the interface file, it may be inlined, and then the local RULES may spring into action, no? What you say about dropping local rules is true if the defn doesn't appear in the interface file.
Moreover, even if it does appear in the interface file, there is no need to feed those unused local let-bindings down through CorePrep and the code generator.
Hmm. Consider a defn that does appear in an interface file. We tidy it, and should presumably include RULES on local let-bindings. Now I suppose we want to walk over it again, dropping the RULES (and INLINE pragmas incidentally) and doing occurrence analysis.
The easy way to do this is as follows.
- Don't change CoreTidy
- In CorePrep dump all RULES and INLINE pragmas, whether top-level or nested. (Remember, interface files see the result of CoreTidy; CorePrep is the start of the codegen pipeline.)
- Do occurrence analysis on the result oc CorePrep. This step will drop all dead code. NB that a top-level Id f mentioned only in a RULE for an externally-visible Id g will itself be marked as externally-visible by CoreTidy. And the occurrence ahalyser should keep all such things. (Need to check that.)