[cg_ssa] TerminatorCodegenHelper::do_call assumes that destination doesnt have more than one predecessor

[bjorn3]

rust/src/librustc_codegen_ssa/mir/block.rs

Lines 127 to 131 in 4abb0ad

	if let Some((ret_dest, target)) = destination {
	let mut ret_bx = fx.build_block(target);
	fx.set_debug_loc(&mut ret_bx, self.terminator.source_info);
	fx.store_return(&mut ret_bx, ret_dest, &fn_abi.ret, invokeret);
	}

It appends the ret value storage code directly to the destination bb. This means that if two calls have the same destination, the destination bb will contain the ret value storage code for both calls.

[matthewjasper]

There's a MIR pass that should prevent this from ever happening. This deserves a comment to that effect though.

[bjorn3]

The MIR pass is https://github.com/rust-lang/rust/blob/80a65bcaf2f2b8a5c659b21b32b42bc300338a0e/src/librustc_mir/transform/add_call_guards.rs

While this is not an issue for cg_llvm, because of this pass, the current code won't work for cg_clif, as Cranelift doesn't allow building a new block before the current is terminated.