Provide access to a preprocessor

[kulp]

Proposal

I propose that Tool provide a way like to_command to invoke a C preprocessor.

I am not sure whether this is feasible.

Use case

GNU lightning is a portable C library for JIT compilation. It is wrapped by the lightning-sys crate. Most all of GNU lightning's API is provided by function-like macros. Some of the macros' definitions and/or existence depend on the target-architecture-specific #if branches (the API is slightly different on 64-bit architectures from on 32-bit architectures).

During build.rs in lightning-sys I want to find function-like macros like these :

#define jit_addi_d(u,v,w)   jit_new_node_wwd(jit_code_addi_d,u,v,w)
#define jit_beqi_d(v,w)     jit_new_node_pwd(jit_code_beqi_d,NULL,v,w)

so that the build step can generate Rust macros like these :

jit_entry!( jit_addi_d(u,v,w) => add => [ i, d ] => jit_new_node_wwd(jit_code_addi_d,u,v,w)    )
jit_entry!( jit_beqi_d(v,w)   => beq => [ i, d ] => jit_new_node_pwd(jit_code_beqi_d,NULL,v,w) )

which can then generate Rust entry points in a less manual and error-prone way than is currently used. I can already generate these Rust macros from a manually-preprocessed C header file, but I need some portable mechanism for determining which macros to generate and with what expansions. If cc provided access to a preprocessor, I would have a solution.

I recognize that this request does not fit into the stated purpose of the crate :

The purpose of this crate is to provide the utility functions necessary to compile C code into a static archive which is then linked into a Rust crate.

so if this is out of scope for this crate, then I understand.