use rustc::lint::*; use syntax::ast; use syntax::ast::*; use std::borrow::Cow; use utils::{snippet, snippet_block, span_help_and_lint}; declare_lint!(pub SINGLE_MATCH, Warn, "a match statement with a single nontrivial arm (i.e, where the other arm \ is `_ => {}`) is used; recommends `if let` instead"); #[allow(missing_copy_implementations)] pub struct MatchPass; impl LintPass for MatchPass { fn get_lints(&self) -> LintArray { lint_array!(SINGLE_MATCH) } fn check_expr(&mut self, cx: &Context, expr: &Expr) { if let ExprMatch(ref ex, ref arms, ast::MatchSource::Normal) = expr.node { // check preconditions: only two arms if arms.len() == 2 && // both of the arms have a single pattern and no guard arms[0].pats.len() == 1 && arms[0].guard.is_none() && arms[1].pats.len() == 1 && arms[1].guard.is_none() && // and the second pattern is a `_` wildcard: this is not strictly necessary, // since the exhaustiveness check will ensure the last one is a catch-all, // but in some cases, an explicit match is preferred to catch situations // when an enum is extended, so we don't consider these cases arms[1].pats[0].node == PatWild(PatWildSingle) && // finally, we don't want any content in the second arm (unit or empty block) is_unit_expr(&*arms[1].body) { let body_code = snippet_block(cx, arms[0].body.span, ".."); let body_code = if let ExprBlock(_) = arms[0].body.node { body_code } else { Cow::Owned(format!("{{ {} }}", body_code)) }; span_help_and_lint(cx, SINGLE_MATCH, expr.span, "you seem to be trying to use match for \ destructuring a single pattern. Did you mean to \ use `if let`?", &*format!("try\nif let {} = {} {}", snippet(cx, arms[0].pats[0].span, ".."), snippet(cx, ex.span, ".."), body_code) ); } } } } fn is_unit_expr(expr: &Expr) -> bool { match expr.node { ExprTup(ref v) if v.is_empty() => true, ExprBlock(ref b) if b.stmts.is_empty() && b.expr.is_none() => true, _ => false, } }