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pprust: fix parenthesization of exprs The pretty printer in `syntax::print::pprust` currently relies on the presence of `ExprKind::Paren` hints in order to correctly parenthesize expressions in its output. If `Paren` nodes are missing, it sometimes produces wrong output, such as printing `1 - (2 - 3)` as `1 - 2 - 3`. This PR fixes `pprust` to correctly print expressions regardless of the presence or absence of `Paren` nodes. This should make `pprust` easier to use with programmatically constructed ASTs. A few notes: * I added a function for assigning precedence values to exprs in `syntax::util::parser`, since there is already code there for assigning precedence values to binops. Let me know if I should move this somewhere more `pprust`-specific. * I also moved the `contains_exterior_struct_lit` function from `rustc_lint::unused::UnusedParens` into `syntax::util::parser`, since it's needed for determining the correct parenthesization of `if`/`while` conditional expressions. * I couldn't find a good way to compare two exprs for equivalence while ignoring semantically-irrelevant details like spans. So the test for the new behavior relies on a slight hack: it adds `Paren` nodes everywhere, so that the pretty-printed version exactly reflects the structure of the AST, and then compares the printed strings. This works, but let me know if there's a better way.