rust-clippy/src/loops.rs

use rustc::lint::*;
use syntax::ast::*;
use syntax::visit::{Visitor, walk_expr};
use std::collections::HashSet;

use utils::{span_lint, get_parent_expr};

declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
               "for-looping over a range of indices where an iterator over items would do" }

#[derive(Copy, Clone)]
pub struct LoopsPass;

impl LintPass for LoopsPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(NEEDLESS_RANGE_LOOP)
    }

    fn check_expr(&mut self, cx: &Context, expr: &Expr) {
        if let Some((pat, arg, body)) = recover_for_loop(expr) {
            // the var must be a single name
            if let PatIdent(_, ref ident, _) = pat.node {
                // the iteratee must be a range literal
                if let ExprRange(_, _) = arg.node {
                    let mut visitor = VarVisitor { cx: cx, var: ident.node.name,
                                                   indexed: HashSet::new(), nonindex: false };
                    walk_expr(&mut visitor, body);
                    // linting condition: we only indexed one variable
                    if visitor.indexed.len() == 1 {
                        let indexed = visitor.indexed.into_iter().next().unwrap();
                        if visitor.nonindex {
                            span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
                                "the loop variable `{}` is used to index `{}`. Consider using \
                                 `for ({}, item) in {}.iter().enumerate()` or similar iterators.",
                                ident.node.name.as_str(), indexed.as_str(),
                                ident.node.name.as_str(), indexed.as_str()));
                        } else {
                            span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
                                "the loop variable `{}` is only used to index `{}`. \
                                 Consider using `for item in &{}` or similar iterators.",
                                ident.node.name.as_str(), indexed.as_str(), indexed.as_str()));
                        }
                    }
                }
            }
        }
    }
}

/// Recover the essential nodes of a desugared for loop:
/// `for pat in arg { body }` becomes `(pat, arg, body)`.
fn recover_for_loop<'a>(expr: &'a Expr) -> Option<(&'a Pat, &'a Expr, &'a Expr)> {
    if_let_chain! {
        [
            let ExprMatch(ref iterexpr, ref arms, _) = expr.node,
            let ExprCall(_, ref iterargs) = iterexpr.node,
            iterargs.len() == 1,
            arms.len() == 1 && arms[0].guard.is_none(),
            let ExprLoop(ref block, _) = arms[0].body.node,
            block.stmts.is_empty(),
            let Some(ref loopexpr) = block.expr,
            let ExprMatch(_, ref innerarms, MatchSource::ForLoopDesugar) = loopexpr.node,
            innerarms.len() == 2 && innerarms[0].pats.len() == 1,
            let PatEnum(_, Some(ref somepats)) = innerarms[0].pats[0].node,
            somepats.len() == 1
        ], {
            return Some((&*somepats[0],
                         &*iterargs[0],
                         &*innerarms[0].body));
        }
    }
    None
}

struct VarVisitor<'v, 't: 'v> {
    cx: &'v Context<'v, 't>, // context reference
    var: Name,               // var name to look for as index
    indexed: HashSet<Name>,  // indexed variables
    nonindex: bool,          // has the var been used otherwise?
}

impl<'v, 't> Visitor<'v> for VarVisitor<'v, 't> {
    fn visit_expr(&mut self, expr: &'v Expr) {
        if let ExprPath(None, ref path) = expr.node {
            if path.segments.len() == 1 && path.segments[0].identifier.name == self.var {
                // we are referencing our variable! now check if it's as an index
                if_let_chain! {
                    [
                        let Some(parexpr) = get_parent_expr(self.cx, expr),
                        let ExprIndex(ref seqexpr, _) = parexpr.node,
                        let ExprPath(None, ref seqvar) = seqexpr.node,
                        seqvar.segments.len() == 1
                    ], {
                        self.indexed.insert(seqvar.segments[0].identifier.name);
                        return;  // no need to walk further
                    }
                }
                // we are not indexing anything, record that
                self.nonindex = true;
                return;
            }
        }
        walk_expr(self, expr);
    }
}