[refactoring] MIR stores the `return_ty` field in two places!

[nikomatsakis]

Right now, the MIR has a special field, return_ty:

rust/src/librustc/mir/mod.rs

Lines 85 to 86 in f93a492

	/// Return type of the function.
	pub return_ty: Ty<'tcx>,

However, the MIR also has an array of locals, and the 0th local is the "return slot". Its type is the return type:

rust/src/librustc/mir/mod.rs

Lines 97 to 102 in f93a492

	/// Declarations of locals.
	///
	/// The first local is the return value pointer, followed by `arg_count`
	/// locals for the function arguments, followed by any user-declared
	/// variables and temporaries.
	pub local_decls: LocalDecls<'tcx>,

It would be nice to eliminate the redundancy. My preference would be to remove the return_ty field, perhaps adding an accessor to Mir<'tcx> like:

fn return_ty(&self) -> Ty<'tcx> {
    self.local_vars[RETURN_POINTER].ty
}

[nikomatsakis]

I marked this as E-mentor, the mentoring instructions are inline above. Just remove the field in question and follow the thread of the compiler. Feel free to pop in on gitter with any questions that come up!

[j-haj]

I would like to give this a shot

[sodiumjoe]

looks like this is done and can be closed?