rust-clippy/src/matches.rs

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,
    }
}