debuginfo: by-value function parameters are not described correctly.

[michaelwoerister]

Currently, debug info for the following code will not be correct:

struct Struct {
    a: int,
    b: float
}

fn struct_fun(by_val: Struct) {
    {...}
}

When breaking in struct_fun() GDB will report that there is no variable with name by_val. The reason is that by_val, having a non-immediate type, is really passed by reference as far as LLVM is concerned (see trans::base::copy_args_to_allocas()). The ty::t associated with the value, however, is still the non-pointer type. LLVM does not seem to like this during codegen and silently omits the debug info entry for the parameter (see CompileUnit::constructVariableDIE() in llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp). Consequently, GDB won't find anything for the name by_val.

A fix will probably involve DIBuilder::createComplexVariable().

[michaelwoerister]

Update: After some research it seems that this is not fixable without LLVM modifications. That's the bad news. The good news is that clang has the same problem and there is some work under way to support this case in the DIBuilder: llvm-mirror/llvm@c971cb8

The fix does not seem to be finished yet though.

[alexcrichton]

That llvm commit will probably get included in #8328