Fix unused_parens false positive when using binary operations

[mibac138]

Fixes#71290

r? @cuviper who provided instructions

[cuviper]

I think this is going to need some level of recursion -- for example, this still fails:

fn z() -> u8 {
({ 0 } + 1 + 2)
}

In the AST, that will associate like ( ({ 0 } + 1) + 2 ), so the LHS of the outer binop is another binop, which doesn't pass the expr_requires_semi_to_be_stmt test.

[mibac138]

I have now added recursion support and a relevant test case. I assume Expr can't create a cycle so this should be fine. Should I add an upper iteration bound anyway?

[cuviper]

I assume Expr can't create a cycle so this should be fine. Should I add an upper iteration bound anyway?

I think it's fine -- for example, contains_exterior_struct_lit uses plain recursion on binary lhs/rhs.

@bors r+

[bors]

📌 Commit 4b7a928 has been approved by cuviper