3 use syntax::visit::{Visitor, walk_expr};
5 use std::collections::HashSet;
7 use utils::{snippet, span_lint, get_parent_expr, match_def_path};
9 declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
10 "for-looping over a range of indices where an iterator over items would do" }
12 declare_lint!{ pub EXPLICIT_ITER_LOOP, Warn,
13 "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do" }
15 declare_lint!{ pub ITER_NEXT_LOOP, Warn,
16 "for-looping over `_.next()` which is probably not intended" }
18 #[derive(Copy, Clone)]
21 impl LintPass for LoopsPass {
22 fn get_lints(&self) -> LintArray {
23 lint_array!(NEEDLESS_RANGE_LOOP, EXPLICIT_ITER_LOOP, ITER_NEXT_LOOP)
26 fn check_expr(&mut self, cx: &Context, expr: &Expr) {
27 if let Some((pat, arg, body)) = recover_for_loop(expr) {
28 // check for looping over a range and then indexing a sequence with it
29 // -> the iteratee must be a range literal
30 if let ExprRange(_, _) = arg.node {
31 // the var must be a single name
32 if let PatIdent(_, ref ident, _) = pat.node {
33 let mut visitor = VarVisitor { cx: cx, var: ident.node.name,
34 indexed: HashSet::new(), nonindex: false };
35 walk_expr(&mut visitor, body);
36 // linting condition: we only indexed one variable
37 if visitor.indexed.len() == 1 {
38 let indexed = visitor.indexed.into_iter().next().expect("Len was nonzero, but no contents found");
40 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
41 "the loop variable `{}` is used to index `{}`. Consider using \
42 `for ({}, item) in {}.iter().enumerate()` or similar iterators",
43 ident.node.name, indexed, ident.node.name, indexed));
45 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
46 "the loop variable `{}` is only used to index `{}`. \
47 Consider using `for item in &{}` or similar iterators",
48 ident.node.name, indexed, indexed));
54 if let ExprMethodCall(ref method, _, ref args) = arg.node {
55 // just the receiver, no arguments
57 let method_name = method.node.name;
58 // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
59 if method_name == "iter" {
60 let object = snippet(cx, args[0].span, "_");
61 span_lint(cx, EXPLICIT_ITER_LOOP, expr.span, &format!(
62 "it is more idiomatic to loop over `&{}` instead of `{}.iter()`",
64 } else if method_name == "iter_mut" {
65 let object = snippet(cx, args[0].span, "_");
66 span_lint(cx, EXPLICIT_ITER_LOOP, expr.span, &format!(
67 "it is more idiomatic to loop over `&mut {}` instead of `{}.iter_mut()`",
69 // check for looping over Iterator::next() which is not what you want
70 } else if method_name == "next" {
71 let method_call = ty::MethodCall::expr(arg.id);
72 let trt_id = cx.tcx.tables
73 .borrow().method_map.get(&method_call)
74 .and_then(|callee| cx.tcx.trait_of_item(callee.def_id));
75 if let Some(trt_id) = trt_id {
76 if match_def_path(cx, trt_id, &["core", "iter", "Iterator"]) {
77 span_lint(cx, ITER_NEXT_LOOP, expr.span,
78 "you are iterating over `Iterator::next()` which is an Option; \
79 this will compile but is probably not what you want");
89 /// Recover the essential nodes of a desugared for loop:
90 /// `for pat in arg { body }` becomes `(pat, arg, body)`.
91 fn recover_for_loop(expr: &Expr) -> Option<(&Pat, &Expr, &Expr)> {
94 let ExprMatch(ref iterexpr, ref arms, _) = expr.node,
95 let ExprCall(_, ref iterargs) = iterexpr.node,
96 iterargs.len() == 1 && arms.len() == 1 && arms[0].guard.is_none(),
97 let ExprLoop(ref block, _) = arms[0].body.node,
98 block.stmts.is_empty(),
99 let Some(ref loopexpr) = block.expr,
100 let ExprMatch(_, ref innerarms, MatchSource::ForLoopDesugar) = loopexpr.node,
101 innerarms.len() == 2 && innerarms[0].pats.len() == 1,
102 let PatEnum(_, Some(ref somepats)) = innerarms[0].pats[0].node,
105 return Some((&*somepats[0],
107 &*innerarms[0].body));
113 struct VarVisitor<'v, 't: 'v> {
114 cx: &'v Context<'v, 't>, // context reference
115 var: Name, // var name to look for as index
116 indexed: HashSet<Name>, // indexed variables
117 nonindex: bool, // has the var been used otherwise?
120 impl<'v, 't> Visitor<'v> for VarVisitor<'v, 't> {
121 fn visit_expr(&mut self, expr: &'v Expr) {
122 if let ExprPath(None, ref path) = expr.node {
123 if path.segments.len() == 1 && path.segments[0].identifier.name == self.var {
124 // we are referencing our variable! now check if it's as an index
127 let Some(parexpr) = get_parent_expr(self.cx, expr),
128 let ExprIndex(ref seqexpr, _) = parexpr.node,
129 let ExprPath(None, ref seqvar) = seqexpr.node,
130 seqvar.segments.len() == 1
132 self.indexed.insert(seqvar.segments[0].identifier.name);
133 return; // no need to walk further
136 // we are not indexing anything, record that
137 self.nonindex = true;
141 walk_expr(self, expr);