2 use rustc_front::hir::*;
4 use rustc_front::visit::{Visitor, walk_expr, walk_block, walk_decl};
6 use rustc::middle::def::DefLocal;
7 use consts::{constant_simple, Constant};
8 use rustc::front::map::Node::{NodeBlock};
10 use std::collections::{HashSet,HashMap};
11 use syntax::ast::Lit_::*;
13 use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type,
14 in_external_macro, expr_block, span_help_and_lint, is_integer_literal};
15 use utils::{VEC_PATH, LL_PATH};
17 declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
18 "for-looping over a range of indices where an iterator over items would do" }
20 declare_lint!{ pub EXPLICIT_ITER_LOOP, Warn,
21 "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do" }
23 declare_lint!{ pub ITER_NEXT_LOOP, Warn,
24 "for-looping over `_.next()` which is probably not intended" }
26 declare_lint!{ pub WHILE_LET_LOOP, Warn,
27 "`loop { if let { ... } else break }` can be written as a `while let` loop" }
29 declare_lint!{ pub UNUSED_COLLECT, Warn,
30 "`collect()`ing an iterator without using the result; this is usually better \
31 written as a for loop" }
33 declare_lint!{ pub REVERSE_RANGE_LOOP, Warn,
34 "Iterating over an empty range, such as `10..0` or `5..5`" }
36 declare_lint!{ pub EXPLICIT_COUNTER_LOOP, Warn,
37 "for-looping with an explicit counter when `_.enumerate()` would do" }
39 declare_lint!{ pub EMPTY_LOOP, Warn, "empty `loop {}` detected" }
41 declare_lint!{ pub WHILE_LET_ON_ITERATOR, Warn, "using a while-let loop instead of a for loop on an iterator" }
43 #[derive(Copy, Clone)]
46 impl LintPass for LoopsPass {
47 fn get_lints(&self) -> LintArray {
48 lint_array!(NEEDLESS_RANGE_LOOP, EXPLICIT_ITER_LOOP, ITER_NEXT_LOOP,
49 WHILE_LET_LOOP, UNUSED_COLLECT, REVERSE_RANGE_LOOP,
50 EXPLICIT_COUNTER_LOOP, EMPTY_LOOP,
51 WHILE_LET_ON_ITERATOR)
55 impl LateLintPass for LoopsPass {
56 fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
57 if let Some((pat, arg, body)) = recover_for_loop(expr) {
58 // check for looping over a range and then indexing a sequence with it
59 // -> the iteratee must be a range literal
60 if let ExprRange(Some(ref l), _) = arg.node {
61 // Range should start with `0`
62 if let ExprLit(ref lit) = l.node {
63 if let LitInt(0, _) = lit.node {
65 // the var must be a single name
66 if let PatIdent(_, ref ident, _) = pat.node {
67 let mut visitor = VarVisitor { cx: cx, var: ident.node.name,
68 indexed: HashSet::new(), nonindex: false };
69 walk_expr(&mut visitor, body);
70 // linting condition: we only indexed one variable
71 if visitor.indexed.len() == 1 {
72 let indexed = visitor.indexed.into_iter().next().expect(
73 "Len was nonzero, but no contents found");
75 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
76 "the loop variable `{}` is used to index `{}`. Consider using \
77 `for ({}, item) in {}.iter().enumerate()` or similar iterators",
78 ident.node.name, indexed, ident.node.name, indexed));
80 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
81 "the loop variable `{}` is only used to index `{}`. \
82 Consider using `for item in &{}` or similar iterators",
83 ident.node.name, indexed, indexed));
91 // if this for loop is iterating over a two-sided range...
92 if let ExprRange(Some(ref start_expr), Some(ref stop_expr)) = arg.node {
93 // ...and both sides are compile-time constant integers...
94 if let Some(start_idx @ Constant::ConstantInt(..)) = constant_simple(start_expr) {
95 if let Some(stop_idx @ Constant::ConstantInt(..)) = constant_simple(stop_expr) {
96 // ...and the start index is greater than the stop index,
97 // this loop will never run. This is often confusing for developers
98 // who think that this will iterate from the larger value to the
100 if start_idx > stop_idx {
101 span_help_and_lint(cx, REVERSE_RANGE_LOOP, expr.span,
102 "this range is empty so this for loop will never run",
103 &format!("Consider using `({}..{}).rev()` if you are attempting to \
104 iterate over this range in reverse", stop_idx, start_idx));
105 } else if start_idx == stop_idx {
106 // if they are equal, it's also problematic - this loop
108 span_lint(cx, REVERSE_RANGE_LOOP, expr.span,
109 "this range is empty so this for loop will never run");
115 if let ExprMethodCall(ref method, _, ref args) = arg.node {
116 // just the receiver, no arguments
118 let method_name = method.node;
119 // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
120 if method_name.as_str() == "iter" || method_name.as_str() == "iter_mut" {
121 if is_ref_iterable_type(cx, &args[0]) {
122 let object = snippet(cx, args[0].span, "_");
123 span_lint(cx, EXPLICIT_ITER_LOOP, expr.span, &format!(
124 "it is more idiomatic to loop over `&{}{}` instead of `{}.{}()`",
125 if method_name.as_str() == "iter_mut" { "mut " } else { "" },
126 object, object, method_name));
129 // check for looping over Iterator::next() which is not what you want
130 else if method_name.as_str() == "next" &&
131 match_trait_method(cx, arg, &["core", "iter", "Iterator"]) {
132 span_lint(cx, ITER_NEXT_LOOP, expr.span,
133 "you are iterating over `Iterator::next()` which is an Option; \
134 this will compile but is probably not what you want");
139 // Look for variables that are incremented once per loop iteration.
140 let mut visitor = IncrementVisitor { cx: cx, states: HashMap::new(), depth: 0, done: false };
141 walk_expr(&mut visitor, body);
143 // For each candidate, check the parent block to see if
144 // it's initialized to zero at the start of the loop.
145 let map = &cx.tcx.map;
146 let parent_scope = map.get_enclosing_scope(expr.id).and_then(|id| map.get_enclosing_scope(id) );
147 if let Some(parent_id) = parent_scope {
148 if let NodeBlock(block) = map.get(parent_id) {
149 for (id, _) in visitor.states.iter().filter( |&(_,v)| *v == VarState::IncrOnce) {
150 let mut visitor2 = InitializeVisitor { cx: cx, end_expr: expr, var_id: id.clone(),
151 state: VarState::IncrOnce, name: None,
152 depth: 0, done: false };
153 walk_block(&mut visitor2, block);
155 if visitor2.state == VarState::Warn {
156 if let Some(name) = visitor2.name {
157 span_lint(cx, EXPLICIT_COUNTER_LOOP, expr.span,
158 &format!("the variable `{0}` is used as a loop counter. Consider \
159 using `for ({0}, item) in {1}.enumerate()` \
160 or similar iterators",
161 name, snippet(cx, arg.span, "_")));
168 // check for `loop { if let {} else break }` that could be `while let`
169 // (also matches an explicit "match" instead of "if let")
170 // (even if the "match" or "if let" is used for declaration)
171 if let ExprLoop(ref block, _) = expr.node {
172 // also check for empty `loop {}` statements
173 if block.stmts.is_empty() && block.expr.is_none() {
174 span_lint(cx, EMPTY_LOOP, expr.span,
175 "empty `loop {}` detected. You may want to either \
176 use `panic!()` or add `std::thread::sleep(..);` to \
180 // extract the expression from the first statement (if any) in a block
181 let inner_stmt_expr = extract_expr_from_first_stmt(block);
182 // extract the first expression (if any) from the block
183 let inner_expr = extract_first_expr(block);
184 let (extracted, collect_expr) = match inner_stmt_expr {
185 Some(_) => (inner_stmt_expr, true), // check if an expression exists in the first statement
186 None => (inner_expr, false), // if not, let's go for the first expression in the block
189 if let Some(inner) = extracted {
190 if let ExprMatch(ref matchexpr, ref arms, ref source) = inner.node {
191 // collect the remaining statements below the match
192 let mut other_stuff = block.stmts
196 format!("{}", snippet(cx, stmt.span, ".."))
197 }).collect::<Vec<String>>();
198 if collect_expr { // if we have a statement which has a match,
199 match block.expr { // then collect the expression (without semicolon) below it
200 Some(ref expr) => other_stuff.push(format!("{}", snippet(cx, expr.span, ".."))),
205 // ensure "if let" compatible match structure
207 MatchSource::Normal | MatchSource::IfLetDesugar{..} => if
209 arms[0].pats.len() == 1 && arms[0].guard.is_none() &&
210 arms[1].pats.len() == 1 && arms[1].guard.is_none() &&
211 // finally, check for "break" in the second clause
212 is_break_expr(&arms[1].body)
214 if in_external_macro(cx, expr.span) { return; }
215 let loop_body = match inner_stmt_expr {
216 // FIXME: should probably be an ellipsis
217 // tabbing and newline is probably a bad idea, especially for large blocks
218 Some(_) => Cow::Owned(format!("{{\n {}\n}}", other_stuff.join("\n "))),
219 None => expr_block(cx, &arms[0].body,
220 Some(other_stuff.join("\n ")), ".."),
222 span_help_and_lint(cx, WHILE_LET_LOOP, expr.span,
223 "this loop could be written as a `while let` loop",
224 &format!("try\nwhile let {} = {} {}",
225 snippet(cx, arms[0].pats[0].span, ".."),
226 snippet(cx, matchexpr.span, ".."),
234 if let ExprMatch(ref expr, ref arms, MatchSource::WhileLetDesugar) = expr.node {
235 let pat = &arms[0].pats[0].node;
236 if let (&PatEnum(ref path, _), &ExprMethodCall(method_name, _, _)) = (pat, &expr.node) {
237 if method_name.node.as_str() == "next" &&
238 match_trait_method(cx, expr, &["core", "iter", "Iterator"]) &&
239 path.segments.last().unwrap().identifier.name.as_str() == "Some" {
240 span_lint(cx, WHILE_LET_ON_ITERATOR, expr.span,
241 "this loop could be written as a `for` loop");
247 fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
248 if let StmtSemi(ref expr, _) = stmt.node {
249 if let ExprMethodCall(ref method, _, ref args) = expr.node {
250 if args.len() == 1 && method.node.as_str() == "collect" &&
251 match_trait_method(cx, expr, &["core", "iter", "Iterator"]) {
252 span_lint(cx, UNUSED_COLLECT, expr.span, &format!(
253 "you are collect()ing an iterator and throwing away the result. \
254 Consider using an explicit for loop to exhaust the iterator"));
261 /// Recover the essential nodes of a desugared for loop:
262 /// `for pat in arg { body }` becomes `(pat, arg, body)`.
263 fn recover_for_loop(expr: &Expr) -> Option<(&Pat, &Expr, &Expr)> {
266 let ExprMatch(ref iterexpr, ref arms, _) = expr.node,
267 let ExprCall(_, ref iterargs) = iterexpr.node,
268 iterargs.len() == 1 && arms.len() == 1 && arms[0].guard.is_none(),
269 let ExprLoop(ref block, _) = arms[0].body.node,
270 block.stmts.is_empty(),
271 let Some(ref loopexpr) = block.expr,
272 let ExprMatch(_, ref innerarms, MatchSource::ForLoopDesugar) = loopexpr.node,
273 innerarms.len() == 2 && innerarms[0].pats.len() == 1,
274 let PatEnum(_, Some(ref somepats)) = innerarms[0].pats[0].node,
277 return Some((&somepats[0],
279 &innerarms[0].body));
285 struct VarVisitor<'v, 't: 'v> {
286 cx: &'v LateContext<'v, 't>, // context reference
287 var: Name, // var name to look for as index
288 indexed: HashSet<Name>, // indexed variables
289 nonindex: bool, // has the var been used otherwise?
292 impl<'v, 't> Visitor<'v> for VarVisitor<'v, 't> {
293 fn visit_expr(&mut self, expr: &'v Expr) {
294 if let ExprPath(None, ref path) = expr.node {
295 if path.segments.len() == 1 && path.segments[0].identifier.name == self.var {
296 // we are referencing our variable! now check if it's as an index
299 let Some(parexpr) = get_parent_expr(self.cx, expr),
300 let ExprIndex(ref seqexpr, _) = parexpr.node,
301 let ExprPath(None, ref seqvar) = seqexpr.node,
302 seqvar.segments.len() == 1
304 self.indexed.insert(seqvar.segments[0].identifier.name);
305 return; // no need to walk further
308 // we are not indexing anything, record that
309 self.nonindex = true;
313 walk_expr(self, expr);
317 /// Return true if the type of expr is one that provides IntoIterator impls
318 /// for &T and &mut T, such as Vec.
319 fn is_ref_iterable_type(cx: &LateContext, e: &Expr) -> bool {
320 // no walk_ptrs_ty: calling iter() on a reference can make sense because it
321 // will allow further borrows afterwards
322 let ty = cx.tcx.expr_ty(e);
323 is_iterable_array(ty) ||
324 match_type(cx, ty, &VEC_PATH) ||
325 match_type(cx, ty, &LL_PATH) ||
326 match_type(cx, ty, &["std", "collections", "hash", "map", "HashMap"]) ||
327 match_type(cx, ty, &["std", "collections", "hash", "set", "HashSet"]) ||
328 match_type(cx, ty, &["collections", "vec_deque", "VecDeque"]) ||
329 match_type(cx, ty, &["collections", "binary_heap", "BinaryHeap"]) ||
330 match_type(cx, ty, &["collections", "btree", "map", "BTreeMap"]) ||
331 match_type(cx, ty, &["collections", "btree", "set", "BTreeSet"])
334 fn is_iterable_array(ty: ty::Ty) -> bool {
335 // IntoIterator is currently only implemented for array sizes <= 32 in rustc
337 ty::TyArray(_, 0...32) => true,
342 /// If a block begins with a statement (possibly a `let` binding) and has an expression, return it.
343 fn extract_expr_from_first_stmt(block: &Block) -> Option<&Expr> {
344 if block.stmts.is_empty() { return None; }
345 if let StmtDecl(ref decl, _) = block.stmts[0].node {
346 if let DeclLocal(ref local) = decl.node {
347 if let Some(ref expr) = local.init { Some(expr) } else { None }
352 /// If a block begins with an expression (with or without semicolon), return it.
353 fn extract_first_expr(block: &Block) -> Option<&Expr> {
355 Some(ref expr) => Some(expr),
356 None if !block.stmts.is_empty() => match block.stmts[0].node {
357 StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => Some(expr),
364 /// Return true if expr contains a single break expr (maybe within a block).
365 fn is_break_expr(expr: &Expr) -> bool {
367 ExprBreak(None) => true,
368 // there won't be a `let <pat> = break` and so we can safely ignore the StmtDecl case
369 ExprBlock(ref b) => match extract_first_expr(b) {
370 Some(ref subexpr) => is_break_expr(subexpr),
377 // To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
378 // incremented exactly once in the loop body, and initialized to zero
379 // at the start of the loop.
382 Initial, // Not examined yet
383 IncrOnce, // Incremented exactly once, may be a loop counter
384 Declared, // Declared but not (yet) initialized to zero
389 // Scan a for loop for variables that are incremented exactly once.
390 struct IncrementVisitor<'v, 't: 'v> {
391 cx: &'v LateContext<'v, 't>, // context reference
392 states: HashMap<NodeId, VarState>, // incremented variables
393 depth: u32, // depth of conditional expressions
397 impl<'v, 't> Visitor<'v> for IncrementVisitor<'v, 't> {
398 fn visit_expr(&mut self, expr: &'v Expr) {
403 // If node is a variable
404 if let Some(def_id) = var_def_id(self.cx, expr) {
405 if let Some(parent) = get_parent_expr(self.cx, expr) {
406 let state = self.states.entry(def_id).or_insert(VarState::Initial);
409 ExprAssignOp(op, ref lhs, ref rhs) =>
410 if lhs.id == expr.id {
411 if op.node == BiAdd && is_integer_literal(rhs, 1) {
412 *state = match *state {
413 VarState::Initial if self.depth == 0 => VarState::IncrOnce,
414 _ => VarState::DontWarn
418 // Assigned some other value
419 *state = VarState::DontWarn;
422 ExprAssign(ref lhs, _) if lhs.id == expr.id => *state = VarState::DontWarn,
423 ExprAddrOf(mutability,_) if mutability == MutMutable => *state = VarState::DontWarn,
428 // Give up if there are nested loops
429 else if is_loop(expr) {
434 // Keep track of whether we're inside a conditional expression
435 else if is_conditional(expr) {
437 walk_expr(self, expr);
441 walk_expr(self, expr);
445 // Check whether a variable is initialized to zero at the start of a loop.
446 struct InitializeVisitor<'v, 't: 'v> {
447 cx: &'v LateContext<'v, 't>, // context reference
448 end_expr: &'v Expr, // the for loop. Stop scanning here.
452 depth: u32, // depth of conditional expressions
456 impl<'v, 't> Visitor<'v> for InitializeVisitor<'v, 't> {
457 fn visit_decl(&mut self, decl: &'v Decl) {
458 // Look for declarations of the variable
459 if let DeclLocal(ref local) = decl.node {
460 if local.pat.id == self.var_id {
461 if let PatIdent(_, ref ident, _) = local.pat.node {
462 self.name = Some(ident.node.name);
464 self.state = if let Some(ref init) = local.init {
465 if is_integer_literal(init, 0) {
477 walk_decl(self, decl);
480 fn visit_expr(&mut self, expr: &'v Expr) {
481 if self.state == VarState::DontWarn || expr == self.end_expr {
484 // No need to visit expressions before the variable is
485 // declared or after we've rejected it.
486 if self.state == VarState::IncrOnce || self.done {
490 // If node is the desired variable, see how it's used
491 if var_def_id(self.cx, expr) == Some(self.var_id) {
492 if let Some(parent) = get_parent_expr(self.cx, expr) {
494 ExprAssignOp(_, ref lhs, _) if lhs.id == expr.id => {
495 self.state = VarState::DontWarn;
497 ExprAssign(ref lhs, ref rhs) if lhs.id == expr.id => {
498 self.state = if is_integer_literal(rhs, 0) && self.depth == 0 {
503 ExprAddrOf(mutability,_) if mutability == MutMutable => self.state = VarState::DontWarn,
508 // If there are other loops between the declaration and the target loop, give up
509 else if is_loop(expr) {
510 self.state = VarState::DontWarn;
514 // Keep track of whether we're inside a conditional expression
515 else if is_conditional(expr) {
517 walk_expr(self, expr);
521 walk_expr(self, expr);
525 fn var_def_id(cx: &LateContext, expr: &Expr) -> Option<NodeId> {
526 if let Some(path_res) = cx.tcx.def_map.borrow().get(&expr.id) {
527 if let DefLocal(_, node_id) = path_res.base_def {
534 fn is_loop(expr: &Expr) -> bool {
536 ExprLoop(..) | ExprWhile(..) => true,
541 fn is_conditional(expr: &Expr) -> bool {
543 ExprIf(..) | ExprMatch(..) => true,