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Merge pull request #314 from nweston/loop-counter

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Lint for loops with explicit counter variable (#159)
This commit is contained in:
Manish Goregaokar 2015-09-15 21:37:37 +05:30
commit 68d4b3af12
7 changed files with 293 additions and 30 deletions
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3
README.md
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@ -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`

1
src/lib.rs
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@ -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,

213
src/loops.rs
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@ -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 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 @@ fn check_expr(&mut self, cx: &Context, expr: &Expr) {
}
}
}
// 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
}
}

13
src/misc.rs
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@ -7,7 +7,7 @@
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 @@ fn get_lints(&self) -> LintArray {
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 @@ fn check_expr(&mut self, cx: &Context, expr: &Expr) {
}
}
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)]

14
src/ranges.rs
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@ -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 @@ fn check_expr(&mut self, cx: &Context, expr: &Expr) {
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
}

11
src/utils.rs
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@ -244,6 +244,17 @@ fn inner(ty: ty::Ty, depth: usize) -> (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! {

68
tests/compile-fail/for_loop.rs
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@ -14,9 +14,9 @@ fn iter(&self) -> std::slice::Iter<u8> {
}
}
#[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 }
}