Needs a rebase... I'm on it

Rebased!

You should never clone AST nodes if you can avoid it.
Thankfully, the AST is borrowed by tcx.map so you can freely pass &'tcx ast::Expr around and whatnot.

When I first tried storing an &ast::Expr, I got stuck in lifetime hell because the new lifetime parameter of const_val required adding lifetimes to other enums as well. I think a better solution is to store the NodeId in the Struct and Tuple variants and to use tcx.map.expect_expr(id) to retrieve the &Expr back. Do you think this is a correct approach?

I am building rustc again to see if it works. It will take some time because for some reason LLVM is being compiled again.

@aochagavia that's a good idea, for some reason I didn't expect adding a lifetime const_val to propagate outside that module (or only to types that already had 'tcx), my bad.

@eddyb I have pushed the changes now

@bors rollup r+

@bors r=eddyb a83db81