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};
9 use std::collections::{HashSet,HashMap};
10 use syntax::ast::Lit_::*;
12 use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type,
13 in_external_macro, expr_block, span_help_and_lint, is_integer_literal};
14 use utils::{VEC_PATH, LL_PATH};
16 declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
17 "for-looping over a range of indices where an iterator over items would do" }
19 declare_lint!{ pub EXPLICIT_ITER_LOOP, Warn,
20 "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do" }
22 declare_lint!{ pub ITER_NEXT_LOOP, Warn,
23 "for-looping over `_.next()` which is probably not intended" }
25 declare_lint!{ pub WHILE_LET_LOOP, Warn,
26 "`loop { if let { ... } else break }` can be written as a `while let` loop" }
28 declare_lint!{ pub UNUSED_COLLECT, Warn,
29 "`collect()`ing an iterator without using the result; this is usually better \
30 written as a for loop" }
32 declare_lint!{ pub REVERSE_RANGE_LOOP, Warn,
33 "Iterating over an empty range, such as `10..0` or `5..5`" }
35 declare_lint!{ pub EXPLICIT_COUNTER_LOOP, Warn,
36 "for-looping with an explicit counter when `_.enumerate()` would do" }
38 #[derive(Copy, Clone)]
41 impl LintPass for LoopsPass {
42 fn get_lints(&self) -> LintArray {
43 lint_array!(NEEDLESS_RANGE_LOOP, EXPLICIT_ITER_LOOP, ITER_NEXT_LOOP,
44 WHILE_LET_LOOP, UNUSED_COLLECT, REVERSE_RANGE_LOOP, EXPLICIT_COUNTER_LOOP)
48 impl LateLintPass for LoopsPass {
49 fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
50 if let Some((pat, arg, body)) = recover_for_loop(expr) {
51 // check for looping over a range and then indexing a sequence with it
52 // -> the iteratee must be a range literal
53 if let ExprRange(Some(ref l), _) = arg.node {
54 // Range should start with `0`
55 if let ExprLit(ref lit) = l.node {
56 if let LitInt(0, _) = lit.node {
58 // the var must be a single name
59 if let PatIdent(_, ref ident, _) = pat.node {
60 let mut visitor = VarVisitor { cx: cx, var: ident.node.name,
61 indexed: HashSet::new(), nonindex: false };
62 walk_expr(&mut visitor, body);
63 // linting condition: we only indexed one variable
64 if visitor.indexed.len() == 1 {
65 let indexed = visitor.indexed.into_iter().next().expect(
66 "Len was nonzero, but no contents found");
68 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
69 "the loop variable `{}` is used to index `{}`. Consider using \
70 `for ({}, item) in {}.iter().enumerate()` or similar iterators",
71 ident.node.name, indexed, ident.node.name, indexed));
73 span_lint(cx, NEEDLESS_RANGE_LOOP, expr.span, &format!(
74 "the loop variable `{}` is only used to index `{}`. \
75 Consider using `for item in &{}` or similar iterators",
76 ident.node.name, indexed, indexed));
84 // if this for loop is iterating over a two-sided range...
85 if let ExprRange(Some(ref start_expr), Some(ref stop_expr)) = arg.node {
86 // ...and both sides are compile-time constant integers...
87 if let Some(Constant::ConstantInt(start_idx, _)) = constant_simple(start_expr) {
88 if let Some(Constant::ConstantInt(stop_idx, _)) = constant_simple(stop_expr) {
89 // ...and the start index is greater than the stop index,
90 // this loop will never run. This is often confusing for developers
91 // who think that this will iterate from the larger value to the
93 if start_idx > stop_idx {
94 span_help_and_lint(cx, REVERSE_RANGE_LOOP, expr.span,
95 "this range is empty so this for loop will never run",
96 &format!("Consider using `({}..{}).rev()` if you are attempting to \
97 iterate over this range in reverse", stop_idx, start_idx));
98 } else if start_idx == stop_idx {
99 // if they are equal, it's also problematic - this loop
101 span_lint(cx, REVERSE_RANGE_LOOP, expr.span,
102 "this range is empty so this for loop will never run");
108 if let ExprMethodCall(ref method, _, ref args) = arg.node {
109 // just the receiver, no arguments
111 let method_name = method.node;
112 // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
113 if method_name.as_str() == "iter" || method_name.as_str() == "iter_mut" {
114 if is_ref_iterable_type(cx, &args[0]) {
115 let object = snippet(cx, args[0].span, "_");
116 span_lint(cx, EXPLICIT_ITER_LOOP, expr.span, &format!(
117 "it is more idiomatic to loop over `&{}{}` instead of `{}.{}()`",
118 if method_name.as_str() == "iter_mut" { "mut " } else { "" },
119 object, object, method_name));
122 // check for looping over Iterator::next() which is not what you want
123 else if method_name.as_str() == "next" &&
124 match_trait_method(cx, arg, &["core", "iter", "Iterator"]) {
125 span_lint(cx, ITER_NEXT_LOOP, expr.span,
126 "you are iterating over `Iterator::next()` which is an Option; \
127 this will compile but is probably not what you want");
132 // Look for variables that are incremented once per loop iteration.
133 let mut visitor = IncrementVisitor { cx: cx, states: HashMap::new(), depth: 0, done: false };
134 walk_expr(&mut visitor, body);
136 // For each candidate, check the parent block to see if
137 // it's initialized to zero at the start of the loop.
138 let map = &cx.tcx.map;
139 let parent_scope = map.get_enclosing_scope(expr.id).and_then(|id| map.get_enclosing_scope(id) );
140 if let Some(parent_id) = parent_scope {
141 if let NodeBlock(block) = map.get(parent_id) {
142 for (id, _) in visitor.states.iter().filter( |&(_,v)| *v == VarState::IncrOnce) {
143 let mut visitor2 = InitializeVisitor { cx: cx, end_expr: expr, var_id: id.clone(),
144 state: VarState::IncrOnce, name: None,
145 depth: 0, done: false };
146 walk_block(&mut visitor2, block);
148 if visitor2.state == VarState::Warn {
149 if let Some(name) = visitor2.name {
150 span_lint(cx, EXPLICIT_COUNTER_LOOP, expr.span,
151 &format!("the variable `{0}` is used as a loop counter. Consider \
152 using `for ({0}, item) in {1}.enumerate()` \
153 or similar iterators",
154 name, snippet(cx, arg.span, "_")));
161 // check for `loop { if let {} else break }` that could be `while let`
162 // (also matches explicit "match" instead of "if let")
163 if let ExprLoop(ref block, _) = expr.node {
164 // extract a single expression
165 if let Some(inner) = extract_single_expr(block) {
166 if let ExprMatch(ref matchexpr, ref arms, ref source) = inner.node {
167 // ensure "if let" compatible match structure
169 MatchSource::Normal | MatchSource::IfLetDesugar{..} => if
171 arms[0].pats.len() == 1 && arms[0].guard.is_none() &&
172 arms[1].pats.len() == 1 && arms[1].guard.is_none() &&
173 // finally, check for "break" in the second clause
174 is_break_expr(&arms[1].body)
176 if in_external_macro(cx, expr.span) { return; }
177 span_help_and_lint(cx, WHILE_LET_LOOP, expr.span,
178 "this loop could be written as a `while let` loop",
179 &format!("try\nwhile let {} = {} {}",
180 snippet(cx, arms[0].pats[0].span, ".."),
181 snippet(cx, matchexpr.span, ".."),
182 expr_block(cx, &arms[0].body, "..")));
191 fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
192 if let StmtSemi(ref expr, _) = stmt.node {
193 if let ExprMethodCall(ref method, _, ref args) = expr.node {
194 if args.len() == 1 && method.node.as_str() == "collect" &&
195 match_trait_method(cx, expr, &["core", "iter", "Iterator"]) {
196 span_lint(cx, UNUSED_COLLECT, expr.span, &format!(
197 "you are collect()ing an iterator and throwing away the result. \
198 Consider using an explicit for loop to exhaust the iterator"));
205 /// Recover the essential nodes of a desugared for loop:
206 /// `for pat in arg { body }` becomes `(pat, arg, body)`.
207 fn recover_for_loop(expr: &Expr) -> Option<(&Pat, &Expr, &Expr)> {
210 let ExprMatch(ref iterexpr, ref arms, _) = expr.node,
211 let ExprCall(_, ref iterargs) = iterexpr.node,
212 iterargs.len() == 1 && arms.len() == 1 && arms[0].guard.is_none(),
213 let ExprLoop(ref block, _) = arms[0].body.node,
214 block.stmts.is_empty(),
215 let Some(ref loopexpr) = block.expr,
216 let ExprMatch(_, ref innerarms, MatchSource::ForLoopDesugar) = loopexpr.node,
217 innerarms.len() == 2 && innerarms[0].pats.len() == 1,
218 let PatEnum(_, Some(ref somepats)) = innerarms[0].pats[0].node,
221 return Some((&somepats[0],
223 &innerarms[0].body));
229 struct VarVisitor<'v, 't: 'v> {
230 cx: &'v LateContext<'v, 't>, // context reference
231 var: Name, // var name to look for as index
232 indexed: HashSet<Name>, // indexed variables
233 nonindex: bool, // has the var been used otherwise?
236 impl<'v, 't> Visitor<'v> for VarVisitor<'v, 't> {
237 fn visit_expr(&mut self, expr: &'v Expr) {
238 if let ExprPath(None, ref path) = expr.node {
239 if path.segments.len() == 1 && path.segments[0].identifier.name == self.var {
240 // we are referencing our variable! now check if it's as an index
243 let Some(parexpr) = get_parent_expr(self.cx, expr),
244 let ExprIndex(ref seqexpr, _) = parexpr.node,
245 let ExprPath(None, ref seqvar) = seqexpr.node,
246 seqvar.segments.len() == 1
248 self.indexed.insert(seqvar.segments[0].identifier.name);
249 return; // no need to walk further
252 // we are not indexing anything, record that
253 self.nonindex = true;
257 walk_expr(self, expr);
261 /// Return true if the type of expr is one that provides IntoIterator impls
262 /// for &T and &mut T, such as Vec.
263 fn is_ref_iterable_type(cx: &LateContext, e: &Expr) -> bool {
264 // no walk_ptrs_ty: calling iter() on a reference can make sense because it
265 // will allow further borrows afterwards
266 let ty = cx.tcx.expr_ty(e);
267 is_iterable_array(ty) ||
268 match_type(cx, ty, &VEC_PATH) ||
269 match_type(cx, ty, &LL_PATH) ||
270 match_type(cx, ty, &["std", "collections", "hash", "map", "HashMap"]) ||
271 match_type(cx, ty, &["std", "collections", "hash", "set", "HashSet"]) ||
272 match_type(cx, ty, &["collections", "vec_deque", "VecDeque"]) ||
273 match_type(cx, ty, &["collections", "binary_heap", "BinaryHeap"]) ||
274 match_type(cx, ty, &["collections", "btree", "map", "BTreeMap"]) ||
275 match_type(cx, ty, &["collections", "btree", "set", "BTreeSet"])
278 fn is_iterable_array(ty: ty::Ty) -> bool {
279 //IntoIterator is currently only implemented for array sizes <= 32 in rustc
281 ty::TyArray(_, 0...32) => true,
286 /// If block consists of a single expression (with or without semicolon), return it.
287 fn extract_single_expr(block: &Block) -> Option<&Expr> {
288 match (&block.stmts.len(), &block.expr) {
289 (&1, &None) => match block.stmts[0].node {
290 StmtExpr(ref expr, _) |
291 StmtSemi(ref expr, _) => Some(expr),
294 (&0, &Some(ref expr)) => Some(expr),
299 /// Return true if expr contains a single break expr (maybe within a block).
300 fn is_break_expr(expr: &Expr) -> bool {
302 ExprBreak(None) => true,
303 ExprBlock(ref b) => match extract_single_expr(b) {
304 Some(ref subexpr) => is_break_expr(subexpr),
311 // To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
312 // incremented exactly once in the loop body, and initialized to zero
313 // at the start of the loop.
316 Initial, // Not examined yet
317 IncrOnce, // Incremented exactly once, may be a loop counter
318 Declared, // Declared but not (yet) initialized to zero
323 // Scan a for loop for variables that are incremented exactly once.
324 struct IncrementVisitor<'v, 't: 'v> {
325 cx: &'v LateContext<'v, 't>, // context reference
326 states: HashMap<NodeId, VarState>, // incremented variables
327 depth: u32, // depth of conditional expressions
331 impl<'v, 't> Visitor<'v> for IncrementVisitor<'v, 't> {
332 fn visit_expr(&mut self, expr: &'v Expr) {
337 // If node is a variable
338 if let Some(def_id) = var_def_id(self.cx, expr) {
339 if let Some(parent) = get_parent_expr(self.cx, expr) {
340 let state = self.states.entry(def_id).or_insert(VarState::Initial);
343 ExprAssignOp(op, ref lhs, ref rhs) =>
344 if lhs.id == expr.id {
345 if op.node == BiAdd && is_integer_literal(rhs, 1) {
346 *state = match *state {
347 VarState::Initial if self.depth == 0 => VarState::IncrOnce,
348 _ => VarState::DontWarn
352 // Assigned some other value
353 *state = VarState::DontWarn;
356 ExprAssign(ref lhs, _) if lhs.id == expr.id => *state = VarState::DontWarn,
357 ExprAddrOf(mutability,_) if mutability == MutMutable => *state = VarState::DontWarn,
362 // Give up if there are nested loops
363 else if is_loop(expr) {
368 // Keep track of whether we're inside a conditional expression
369 else if is_conditional(expr) {
371 walk_expr(self, expr);
375 walk_expr(self, expr);
379 // Check whether a variable is initialized to zero at the start of a loop.
380 struct InitializeVisitor<'v, 't: 'v> {
381 cx: &'v LateContext<'v, 't>, // context reference
382 end_expr: &'v Expr, // the for loop. Stop scanning here.
386 depth: u32, // depth of conditional expressions
390 impl<'v, 't> Visitor<'v> for InitializeVisitor<'v, 't> {
391 fn visit_decl(&mut self, decl: &'v Decl) {
392 // Look for declarations of the variable
393 if let DeclLocal(ref local) = decl.node {
394 if local.pat.id == self.var_id {
395 if let PatIdent(_, ref ident, _) = local.pat.node {
396 self.name = Some(ident.node.name);
398 self.state = if let Some(ref init) = local.init {
399 if is_integer_literal(init, 0) {
411 walk_decl(self, decl);
414 fn visit_expr(&mut self, expr: &'v Expr) {
415 if self.state == VarState::DontWarn || expr == self.end_expr {
418 // No need to visit expressions before the variable is
419 // declared or after we've rejected it.
420 if self.state == VarState::IncrOnce || self.done {
424 // If node is the desired variable, see how it's used
425 if var_def_id(self.cx, expr) == Some(self.var_id) {
426 if let Some(parent) = get_parent_expr(self.cx, expr) {
428 ExprAssignOp(_, ref lhs, _) if lhs.id == expr.id => {
429 self.state = VarState::DontWarn;
431 ExprAssign(ref lhs, ref rhs) if lhs.id == expr.id => {
432 self.state = if is_integer_literal(rhs, 0) && self.depth == 0 {
437 ExprAddrOf(mutability,_) if mutability == MutMutable => self.state = VarState::DontWarn,
442 // If there are other loops between the declaration and the target loop, give up
443 else if is_loop(expr) {
444 self.state = VarState::DontWarn;
448 // Keep track of whether we're inside a conditional expression
449 else if is_conditional(expr) {
451 walk_expr(self, expr);
455 walk_expr(self, expr);
459 fn var_def_id(cx: &LateContext, expr: &Expr) -> Option<NodeId> {
460 if let Some(path_res) = cx.tcx.def_map.borrow().get(&expr.id) {
461 if let DefLocal(node_id) = path_res.base_def {
468 fn is_loop(expr: &Expr) -> bool {
470 ExprLoop(..) | ExprWhile(..) => true,
475 fn is_conditional(expr: &Expr) -> bool {
477 ExprIf(..) | ExprMatch(..) => true,