As long as your target language has a strict define-before-use rule and no advanced inference is required you will know the types of expressions, and can perform type-based optimizations. You can also do constant folding and (very rudimentary) inlining. But the best optimizations are done on IRs, which you don't have access to in an old-school single pass design. LICM, CSE, GVN, DCE, and all the countless loop opts are not available to you. You'll also spill to memory a lot, because you can't run a decent regalloc in a single pass.
I'm actually a big fan a function-by-function dual-pass compilation. You generate IR from the parser in one pass, and do codegen right after. Most intermediate state is thrown out (including the AST, for non-polymorphic functions) and you move on to the next function. This give you an extremely fast data-oriented baseline compiler with reasonable codegen (much better than something like tcc).
I'm actually a big fan a function-by-function dual-pass compilation. You generate IR from the parser in one pass, and do codegen right after. Most intermediate state is thrown out (including the AST, for non-polymorphic functions) and you move on to the next function. This give you an extremely fast data-oriented baseline compiler with reasonable codegen (much better than something like tcc).