Merge pull request #314 from nweston/loop-counter

Lint for loops with explicit counter variable (#159)

README.md

@@ -6,7 +6,7 @@ A collection of lints that give helpful tips to newbies and catch oversights.
 [Jump to usage instructions](#usage)
 
 ##Lints
-There are 57 lints included in this crate:
+There are 58 lints included in this crate:
 
 name                                                                                                   | default | meaning
 -------------------------------------------------------------------------------------------------------|---------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -21,6 +21,7 @@ name
 [cmp_owned](https://github.com/Manishearth/rust-clippy/wiki#cmp_owned)                                 | warn    | creating owned instances for comparing with others, e.g. `x == "foo".to_string()`
 [collapsible_if](https://github.com/Manishearth/rust-clippy/wiki#collapsible_if)                       | warn    | two nested `if`-expressions can be collapsed into one, e.g. `if x { if y { foo() } }` can be written as `if x && y { foo() }`
 [eq_op](https://github.com/Manishearth/rust-clippy/wiki#eq_op)                                         | warn    | equal operands on both sides of a comparison or bitwise combination (e.g. `x == x`)
+[explicit_counter_loop](https://github.com/Manishearth/rust-clippy/wiki#explicit_counter_loop)         | warn    | for-looping with an explicit counter when `_.enumerate()` would do
 [explicit_iter_loop](https://github.com/Manishearth/rust-clippy/wiki#explicit_iter_loop)               | warn    | for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do
 [float_cmp](https://github.com/Manishearth/rust-clippy/wiki#float_cmp)                                 | warn    | using `==` or `!=` on float values (as floating-point operations usually involve rounding errors, it is always better to check for approximate equality within small bounds)
 [identity_op](https://github.com/Manishearth/rust-clippy/wiki#identity_op)                             | warn    | using identity operations, e.g. `x + 0` or `y / 1`

src/lib.rs

@@ -119,6 +119,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         len_zero::LEN_WITHOUT_IS_EMPTY,
         len_zero::LEN_ZERO,
         lifetimes::NEEDLESS_LIFETIMES,
+        loops::EXPLICIT_COUNTER_LOOP,
         loops::EXPLICIT_ITER_LOOP,
         loops::ITER_NEXT_LOOP,
         loops::NEEDLESS_RANGE_LOOP,

src/loops.rs

@@ -1,13 +1,15 @@
 use rustc::lint::*;
 use rustc_front::hir::*;
 use reexport::*;
-use rustc_front::visit::{Visitor, walk_expr};
+use rustc_front::visit::{Visitor, walk_expr, walk_block, walk_decl};
 use rustc::middle::ty;
+use rustc::middle::def::DefLocal;
 use consts::{constant_simple, Constant};
-use std::collections::HashSet;
+use rustc::front::map::Node::{NodeBlock};
+use std::collections::{HashSet,HashMap};
 
 use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type,
-            in_external_macro, expr_block, span_help_and_lint};
+            in_external_macro, expr_block, span_help_and_lint, is_integer_literal};
 use utils::{VEC_PATH, LL_PATH};
 
 declare_lint!{ pub NEEDLESS_RANGE_LOOP, Warn,
@@ -29,13 +31,16 @@ declare_lint!{ pub UNUSED_COLLECT, Warn,
 declare_lint!{ pub REVERSE_RANGE_LOOP, Warn,
                "Iterating over an empty range, such as `10..0` or `5..5`" }
 
+declare_lint!{ pub EXPLICIT_COUNTER_LOOP, Warn,
+               "for-looping with an explicit counter when `_.enumerate()` would do" }
+
 #[derive(Copy, Clone)]
 pub struct LoopsPass;
 
 impl LintPass for LoopsPass {
     fn get_lints(&self) -> LintArray {
         lint_array!(NEEDLESS_RANGE_LOOP, EXPLICIT_ITER_LOOP, ITER_NEXT_LOOP,
-                    WHILE_LET_LOOP, UNUSED_COLLECT, REVERSE_RANGE_LOOP)
+                    WHILE_LET_LOOP, UNUSED_COLLECT, REVERSE_RANGE_LOOP, EXPLICIT_COUNTER_LOOP)
     }
 
     fn check_expr(&mut self, cx: &Context, expr: &Expr) {
@@ -120,6 +125,35 @@ impl LintPass for LoopsPass {
                     }
                 }
             }
+
+            // Look for variables that are incremented once per loop iteration.
+            let mut visitor = IncrementVisitor { cx: cx, states: HashMap::new(), depth: 0, done: false };
+            walk_expr(&mut visitor, body);
+
+            // For each candidate, check the parent block to see if
+            // it's initialized to zero at the start of the loop.
+            let map = &cx.tcx.map;
+            let parent_scope = map.get_enclosing_scope(expr.id).and_then(|id| map.get_enclosing_scope(id) );
+            if let Some(parent_id) = parent_scope {
+                if let NodeBlock(block) = map.get(parent_id) {
+                    for (id, _) in visitor.states.iter().filter( |&(_,v)| *v == VarState::IncrOnce) {
+                        let mut visitor2 = InitializeVisitor { cx: cx, end_expr: expr, var_id: id.clone(),
+                                                               state: VarState::IncrOnce, name: None,
+                                                               depth: 0, done: false };
+                        walk_block(&mut visitor2, block);
+
+                        if visitor2.state == VarState::Warn {
+                            if let Some(name) = visitor2.name {
+                                span_lint(cx, EXPLICIT_COUNTER_LOOP, expr.span,
+                                          &format!("the variable `{0}` is used as a loop counter. Consider \
+                                                    using `for ({0}, item) in {1}.enumerate()` \
+                                                    or similar iterators",
+                                                   name, snippet(cx, arg.span, "_")));
+                            }
+                        }
+                    }
+                }
+            }
         }
         // check for `loop { if let {} else break }` that could be `while let`
         // (also matches explicit "match" instead of "if let")
@@ -270,3 +304,174 @@ fn is_break_expr(expr: &Expr) -> bool {
         _ => false,
     }
 }
+
+// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
+// incremented exactly once in the loop body, and initialized to zero
+// at the start of the loop.
+#[derive(PartialEq)]
+enum VarState {
+    Initial,      // Not examined yet
+    IncrOnce,     // Incremented exactly once, may be a loop counter
+    Declared,     // Declared but not (yet) initialized to zero
+    Warn,
+    DontWarn
+}
+
+// Scan a for loop for variables that are incremented exactly once.
+struct IncrementVisitor<'v, 't: 'v> {
+    cx: &'v Context<'v, 't>,      // context reference
+    states: HashMap<NodeId, VarState>,  // incremented variables
+    depth: u32,                         // depth of conditional expressions
+    done: bool
+}
+
+impl<'v, 't> Visitor<'v> for IncrementVisitor<'v, 't> {
+    fn visit_expr(&mut self, expr: &'v Expr) {
+        if self.done {
+            return;
+        }
+
+        // If node is a variable
+        if let Some(def_id) = var_def_id(self.cx, expr) {
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                let state = self.states.entry(def_id).or_insert(VarState::Initial);
+
+                match parent.node {
+                    ExprAssignOp(op, ref lhs, ref rhs) =>
+                        if lhs.id == expr.id {
+                            if op.node == BiAdd && is_integer_literal(rhs, 1) {
+                                *state = match *state {
+                                    VarState::Initial if self.depth == 0 => VarState::IncrOnce,
+                                    _ => VarState::DontWarn
+                                };
+                            }
+                            else {
+                                // Assigned some other value
+                                *state = VarState::DontWarn;
+                            }
+                        },
+                    ExprAssign(ref lhs, _) if lhs.id == expr.id => *state = VarState::DontWarn,
+                    ExprAddrOf(mutability,_) if mutability == MutMutable => *state = VarState::DontWarn,
+                    _ => ()
+                }
+            }
+        }
+        // Give up if there are nested loops
+        else if is_loop(expr) {
+            self.states.clear();
+            self.done = true;
+            return;
+        }
+        // Keep track of whether we're inside a conditional expression
+        else if is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+            return;
+        }
+        walk_expr(self, expr);
+    }
+}
+
+// Check whether a variable is initialized to zero at the start of a loop.
+struct InitializeVisitor<'v, 't: 'v> {
+    cx: &'v Context<'v, 't>, // context reference
+    end_expr: &'v Expr,      // the for loop. Stop scanning here.
+    var_id: NodeId,
+    state: VarState,
+    name: Option<Name>,
+    depth: u32,              // depth of conditional expressions
+    done: bool
+}
+
+impl<'v, 't> Visitor<'v> for InitializeVisitor<'v, 't> {
+    fn visit_decl(&mut self, decl: &'v Decl) {
+        // Look for declarations of the variable
+        if let DeclLocal(ref local) = decl.node {
+            if local.pat.id == self.var_id {
+                if let PatIdent(_, ref ident, _) = local.pat.node {
+                    self.name = Some(ident.node.name);
+
+                    self.state = if let Some(ref init) = local.init {
+                        if is_integer_literal(init, 0) {
+                            VarState::Warn
+                        } else {
+                            VarState::Declared
+                        }
+                    }
+                    else {
+                        VarState::Declared
+                    }
+                }
+            }
+        }
+        walk_decl(self, decl);
+    }
+
+    fn visit_expr(&mut self, expr: &'v Expr) {
+        if self.state == VarState::DontWarn || expr == self.end_expr {
+            self.done = true;
+        }
+        // No need to visit expressions before the variable is
+        // declared or after we've rejected it.
+        if self.state == VarState::IncrOnce || self.done {
+            return;
+        }
+
+        // If node is the desired variable, see how it's used
+        if var_def_id(self.cx, expr) == Some(self.var_id) {
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                match parent.node {
+                    ExprAssignOp(_, ref lhs, _) if lhs.id == expr.id => {
+                        self.state = VarState::DontWarn;
+                    },
+                    ExprAssign(ref lhs, ref rhs) if lhs.id == expr.id => {
+                        self.state = if is_integer_literal(rhs, 0) && self.depth == 0 {
+                            VarState::Warn
+                        } else {
+                            VarState::DontWarn
+                        }},
+                    ExprAddrOf(mutability,_) if mutability == MutMutable => self.state = VarState::DontWarn,
+                    _ => ()
+                }
+            }
+        }
+        // If there are other loops between the declaration and the target loop, give up
+        else if is_loop(expr) {
+            self.state = VarState::DontWarn;
+            self.done = true;
+            return;
+        }
+        // Keep track of whether we're inside a conditional expression
+        else if is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+            return;
+        }
+        walk_expr(self, expr);
+    }
+}
+
+fn var_def_id(cx: &Context, expr: &Expr) -> Option<NodeId> {
+    if let Some(path_res) = cx.tcx.def_map.borrow().get(&expr.id) {
+        if let DefLocal(node_id) = path_res.base_def {
+            return Some(node_id)
+        }
+    }
+    None
+}
+
+fn is_loop(expr: &Expr) -> bool {
+    match expr.node {
+        ExprLoop(..) | ExprWhile(..)  => true,
+        _ => false
+    }
+}
+
+fn is_conditional(expr: &Expr) -> bool {
+    match expr.node {
+        ExprIf(..) | ExprMatch(..) => true,
+        _ => false
+    }
+}

src/misc.rs

@@ -7,7 +7,7 @@ use syntax::codemap::{Span, Spanned};
 use rustc_front::visit::FnKind;
 use rustc::middle::ty;
 
-use utils::{get_item_name, match_path, snippet, span_lint, walk_ptrs_ty};
+use utils::{get_item_name, match_path, snippet, span_lint, walk_ptrs_ty, is_integer_literal};
 use consts::constant;
 
 declare_lint!(pub TOPLEVEL_REF_ARG, Warn,
@@ -183,7 +183,7 @@ impl LintPass for ModuloOne {
     fn check_expr(&mut self, cx: &Context, expr: &Expr) {
         if let ExprBinary(ref cmp, _, ref right) = expr.node {
             if let &Spanned {node: BinOp_::BiRem, ..} = cmp {
-                if is_lit_one(right) {
+                if is_integer_literal(right, 1) {
                     cx.span_lint(MODULO_ONE, expr.span, "any number modulo 1 will be 0");
                 }
             }
@@ -191,15 +191,6 @@ impl LintPass for ModuloOne {
     }
 }
 
-fn is_lit_one(expr: &Expr) -> bool {
-    if let ExprLit(ref spanned) = expr.node {
-        if let LitInt(1, _) = spanned.node {
-            return true;
-        }
-    }
-    false
-}
-
 declare_lint!(pub REDUNDANT_PATTERN, Warn, "using `name @ _` in a pattern");
 
 #[derive(Copy,Clone)]

src/ranges.rs

@@ -1,7 +1,7 @@
 use rustc::lint::{Context, LintArray, LintPass};
 use rustc_front::hir::*;
 use syntax::codemap::Spanned;
-use utils::match_type;
+use utils::{match_type, is_integer_literal};
 
 declare_lint! {
     pub RANGE_STEP_BY_ZERO, Warn,
@@ -21,7 +21,7 @@ impl LintPass for StepByZero {
                               ref args) = expr.node {
             // Only warn on literal ranges.
             if ident.name == "step_by" && args.len() == 2 &&
-                is_range(cx, &args[0]) && is_lit_zero(&args[1]) {
+                is_range(cx, &args[0]) && is_integer_literal(&args[1], 0) {
                 cx.span_lint(RANGE_STEP_BY_ZERO, expr.span,
                              "Range::step_by(0) produces an infinite iterator. \
                               Consider using `std::iter::repeat()` instead")
@@ -37,13 +37,3 @@ fn is_range(cx: &Context, expr: &Expr) -> bool {
     // Note: RangeTo and RangeFull don't have step_by
     match_type(cx, ty, &["core", "ops", "Range"]) || match_type(cx, ty, &["core", "ops", "RangeFrom"])
 }
-
-fn is_lit_zero(expr: &Expr) -> bool {
-    // FIXME: use constant folding
-    if let ExprLit(ref spanned) = expr.node {
-        if let LitInt(0, _) = spanned.node {
-            return true;
-        }
-    }
-    false
-}

src/utils.rs

@@ -244,6 +244,17 @@ pub fn walk_ptrs_ty_depth(ty: ty::Ty) -> (ty::Ty, usize) {
     inner(ty, 0)
 }
 
+pub fn is_integer_literal(expr: &Expr, value: u64) -> bool
+{
+    // FIXME: use constant folding
+    if let ExprLit(ref spanned) = expr.node {
+        if let LitInt(v, _) = spanned.node {
+            return v == value;
+        }
+    }
+    false
+}
+
 /// Produce a nested chain of if-lets and ifs from the patterns:
 ///
 ///     if_let_chain! {

tests/compile-fail/for_loop.rs

@@ -14,9 +14,9 @@ impl Unrelated {
     }
 }
 
-#[deny(needless_range_loop, explicit_iter_loop, iter_next_loop, reverse_range_loop)]
+#[deny(needless_range_loop, explicit_iter_loop, iter_next_loop, reverse_range_loop, explicit_counter_loop)]
 #[deny(unused_collect)]
-#[allow(linkedlist)]
+#[allow(linkedlist,shadow_unrelated)]
 fn main() {
     let mut vec = vec![1, 2, 3, 4];
     let vec2 = vec![1, 2, 3, 4];
@@ -119,4 +119,68 @@ fn main() {
     let mut out = vec![];
     vec.iter().map(|x| out.push(x)).collect::<Vec<_>>(); //~ERROR you are collect()ing an iterator
     let _y = vec.iter().map(|x| out.push(x)).collect::<Vec<_>>(); // this is fine
+
+    // Loop with explicit counter variable
+    let mut _index = 0;
+    for _v in &vec { _index += 1 } //~ERROR the variable `_index` is used as a loop counter
+
+    let mut _index = 1;
+    _index = 0;
+    for _v in &vec { _index += 1 } //~ERROR the variable `_index` is used as a loop counter
+
+    let mut _index;
+    _index = 0;
+    for _v in &vec { _index += 1 } //~ERROR the variable `_index` is used as a loop counter
+    for _v in &vec { _index += 1 } // But this does not warn
+
+    // Potential false positives
+    let mut _index = 0;
+    _index = 1;
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    _index += 1;
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    if true { _index = 1 }
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    let mut _index = 1;
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    for _v in &vec { _index += 1; _index += 1 }
+
+    let mut _index = 0;
+    for _v in &vec { _index *= 2; _index += 1 }
+
+    let mut _index = 0;
+    for _v in &vec { _index = 1; _index += 1 }
+
+    let mut _index = 0;
+
+    for _v in &vec { let mut _index = 0; _index += 1 }
+
+    let mut _index = 0;
+    for _v in &vec { _index += 1; _index = 0; }
+
+    let mut _index = 0;
+    for _v in &vec { for _x in 0..1 { _index += 1; }; _index += 1 }
+
+    let mut _index = 0;
+    for x in &vec { if *x == 1 { _index += 1 } }
+
+    let mut _index = 0;
+    if true { _index = 1 };
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 1;
+    if false { _index = 0 };
+    for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    { let mut _x = &mut _index; }
+    for _v in &vec { _index += 1 }
 }