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Merge pull request #1158 from scurest/evalorder

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Add lint for reads and writes that depend on evaluation order
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Oliver Schneider 2016-08-11 13:41:32 +02:00 committed by GitHub
commit 4aae2b60bc
5 changed files with 306 additions and 1 deletions
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1
CHANGELOG.md
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@ -170,6 +170,7 @@ All notable changes to this project will be documented in this file.
[`enum_glob_use`]: https://github.com/Manishearth/rust-clippy/wiki#enum_glob_use
[`enum_variant_names`]: https://github.com/Manishearth/rust-clippy/wiki#enum_variant_names
[`eq_op`]: https://github.com/Manishearth/rust-clippy/wiki#eq_op
[`eval_order_dependence`]: https://github.com/Manishearth/rust-clippy/wiki#eval_order_dependence
[`expl_impl_clone_on_copy`]: https://github.com/Manishearth/rust-clippy/wiki#expl_impl_clone_on_copy
[`explicit_counter_loop`]: https://github.com/Manishearth/rust-clippy/wiki#explicit_counter_loop
[`explicit_iter_loop`]: https://github.com/Manishearth/rust-clippy/wiki#explicit_iter_loop

3
README.md
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@ -17,7 +17,7 @@ Table of contents:
## Lints
There are 162 lints included in this crate:
There are 163 lints included in this crate:
name | default | triggers on
---------------------------------------------------------------------------------------------------------------------|---------|----------------------------------------------------------------------------------------------------------------------------------
@ -57,6 +57,7 @@ name
[enum_glob_use](https://github.com/Manishearth/rust-clippy/wiki#enum_glob_use) | allow | use items that import all variants of an enum
[enum_variant_names](https://github.com/Manishearth/rust-clippy/wiki#enum_variant_names) | warn | enums where all variants share a prefix/postfix
[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`)
[eval_order_dependence](https://github.com/Manishearth/rust-clippy/wiki#eval_order_dependence) | warn | whether a variable read occurs before a write depends on sub-expression evaluation order
[expl_impl_clone_on_copy](https://github.com/Manishearth/rust-clippy/wiki#expl_impl_clone_on_copy) | warn | implementing `Clone` explicitly on `Copy` types
[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

250
clippy_lints/src/eval_order_dependence.rs Normal file
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@ -0,0 +1,250 @@
use rustc::hir::def_id::DefId;
use rustc::hir::intravisit::{Visitor, walk_expr};
use rustc::hir::*;
use rustc::lint::*;
use utils::{get_parent_expr, span_note_and_lint};
/// **What it does:** Checks for a read and a write to the same variable where
/// whether the read occurs before or after the write depends on the evaluation
/// order of sub-expressions.
///
/// **Why is this bad?** It is often confusing to read. In addition, the
/// sub-expression evaluation order for Rust is not well documented.
///
/// **Known problems:** Code which intentionally depends on the evaluation
/// order, or which is correct for any evaluation order.
///
/// **Example:**
/// ```rust
/// let mut x = 0;
/// let a = {x = 1; 1} + x;
/// // Unclear whether a is 1 or 2.
/// ```
declare_lint! {
pub EVAL_ORDER_DEPENDENCE,
Warn,
"whether a variable read occurs before a write depends on sub-expression evaluation order"
}
#[derive(Copy,Clone)]
pub struct EvalOrderDependence;
impl LintPass for EvalOrderDependence {
fn get_lints(&self) -> LintArray {
lint_array!(EVAL_ORDER_DEPENDENCE)
}
}
impl LateLintPass for EvalOrderDependence {
fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
// Find a write to a local variable.
match expr.node {
ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => {
if let ExprPath(None, ref path) = lhs.node {
if path.segments.len() == 1 {
let var = cx.tcx.expect_def(lhs.id).def_id();
let mut visitor = ReadVisitor {
cx: cx,
var: var,
write_expr: expr,
last_expr: expr,
};
check_for_unsequenced_reads(&mut visitor);
}
}
}
_ => {}
}
}
}
/// Walks up the AST from the the given write expression (`vis.write_expr`)
/// looking for reads to the same variable that are unsequenced relative to the
/// write.
///
/// This means reads for which there is a common ancestor between the read and
/// the write such that
///
/// * evaluating the ancestor necessarily evaluates both the read and the write
/// (for example, `&x` and `|| x = 1` don't necessarily evaluate `x`), and
///
/// * which one is evaluated first depends on the order of sub-expression
/// evaluation. Blocks, `if`s, loops, `match`es, and the short-circuiting
/// logical operators are considered to have a defined evaluation order.
///
/// When such a read is found, the lint is triggered.
fn check_for_unsequenced_reads(vis: &mut ReadVisitor) {
let map = &vis.cx.tcx.map;
let mut cur_id = vis.write_expr.id;
loop {
let parent_id = map.get_parent_node(cur_id);
if parent_id == cur_id {
break;
}
let parent_node = match map.find(parent_id) {
Some(parent) => parent,
None => break,
};
let stop_early = match parent_node {
map::Node::NodeExpr(expr) => check_expr(vis, expr),
map::Node::NodeStmt(stmt) => check_stmt(vis, stmt),
map::Node::NodeItem(_) => {
// We reached the top of the function, stop.
break;
},
_ => { StopEarly::KeepGoing }
};
match stop_early {
StopEarly::Stop => break,
StopEarly::KeepGoing => {},
}
cur_id = parent_id;
}
}
/// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
/// `check_expr` weren't an independent function, this would be unnecessary and
/// we could just use `break`).
enum StopEarly {
KeepGoing,
Stop,
}
fn check_expr<'v, 't>(vis: & mut ReadVisitor<'v, 't>, expr: &'v Expr) -> StopEarly {
if expr.id == vis.last_expr.id {
return StopEarly::KeepGoing;
}
match expr.node {
ExprVec(_) |
ExprTup(_) |
ExprMethodCall(_, _, _) |
ExprCall(_, _) |
ExprAssign(_, _) |
ExprIndex(_, _) |
ExprRepeat(_, _) |
ExprStruct(_, _, _) => {
walk_expr(vis, expr);
}
ExprBinary(op, _, _) |
ExprAssignOp(op, _, _) => {
if op.node == BiAnd || op.node == BiOr {
// x && y and x || y always evaluate x first, so these are
// strictly sequenced.
} else {
walk_expr(vis, expr);
}
}
ExprClosure(_, _, _, _) => {
// Either
//
// * `var` is defined in the closure body, in which case we've
// reached the top of the enclosing function and can stop, or
//
// * `var` is captured by the closure, in which case, because
// evaluating a closure does not evaluate its body, we don't
// necessarily have a write, so we need to stop to avoid
// generating false positives.
//
// This is also the only place we need to stop early (grrr).
return StopEarly::Stop;
}
// All other expressions either have only one child or strictly
// sequence the evaluation order of their sub-expressions.
_ => {}
}
vis.last_expr = expr;
StopEarly::KeepGoing
}
fn check_stmt<'v, 't>(vis: &mut ReadVisitor<'v, 't>, stmt: &'v Stmt) -> StopEarly {
match stmt.node {
StmtExpr(ref expr, _) |
StmtSemi(ref expr, _) => check_expr(vis, expr),
StmtDecl(ref decl, _) => {
// If the declaration is of a local variable, check its initializer
// expression if it has one. Otherwise, keep going.
let local = match decl.node {
DeclLocal(ref local) => Some(local),
_ => None,
};
local.and_then(|local| local.init.as_ref())
.map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr))
}
}
}
/// A visitor that looks for reads from a variable.
struct ReadVisitor<'v, 't: 'v> {
cx: &'v LateContext<'v, 't>,
/// The id of the variable we're looking for.
var: DefId,
/// The expressions where the write to the variable occurred (for reporting
/// in the lint).
write_expr: &'v Expr,
/// The last (highest in the AST) expression we've checked, so we know not
/// to recheck it.
last_expr: &'v Expr,
}
impl<'v, 't> Visitor<'v> for ReadVisitor<'v, 't> {
fn visit_expr(&mut self, expr: &'v Expr) {
if expr.id == self.last_expr.id {
return;
}
match expr.node {
ExprPath(None, ref path) => {
if path.segments.len() == 1 && self.cx.tcx.expect_def(expr.id).def_id() == self.var {
if is_in_assignment_position(self.cx, expr) {
// This is a write, not a read.
} else {
span_note_and_lint(
self.cx,
EVAL_ORDER_DEPENDENCE,
expr.span,
"unsequenced read of a variable",
self.write_expr.span,
"whether read occurs before this write depends on evaluation order"
);
}
}
}
// We're about to descend a closure. Since we don't know when (or
// if) the closure will be evaluated, any reads in it might not
// occur here (or ever). Like above, bail to avoid false positives.
ExprClosure(_, _, _, _) |
// We want to avoid a false positive when a variable name occurs
// only to have its address taken, so we stop here. Technically,
// this misses some weird cases, eg.
//
// ```rust
// let mut x = 0;
// let a = foo(&{x = 1; x}, x);
// ```
//
// TODO: fix this
ExprAddrOf(_, _) => {
return;
}
_ => {}
}
walk_expr(self, expr);
}
}
/// Returns true if `expr` is the LHS of an assignment, like `expr = ...`.
fn is_in_assignment_position(cx: &LateContext, expr: &Expr) -> bool {
if let Some(parent) = get_parent_expr(cx, expr) {
if let ExprAssign(ref lhs, _) = parent.node {
return lhs.id == expr.id;
}
}
false
}

3
clippy_lints/src/lib.rs
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@ -78,6 +78,7 @@ pub mod enum_variants;
pub mod eq_op;
pub mod escape;
pub mod eta_reduction;
pub mod eval_order_dependence;
pub mod format;
pub mod formatting;
pub mod functions;
@ -256,6 +257,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
reg.register_late_lint_pass(box arithmetic::Arithmetic::default());
reg.register_late_lint_pass(box assign_ops::AssignOps);
reg.register_late_lint_pass(box let_if_seq::LetIfSeq);
reg.register_late_lint_pass(box eval_order_dependence::EvalOrderDependence);
reg.register_lint_group("clippy_restrictions", vec![
arithmetic::FLOAT_ARITHMETIC,
@ -325,6 +327,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
eq_op::EQ_OP,
escape::BOXED_LOCAL,
eta_reduction::REDUNDANT_CLOSURE,
eval_order_dependence::EVAL_ORDER_DEPENDENCE,
format::USELESS_FORMAT,
formatting::SUSPICIOUS_ASSIGNMENT_FORMATTING,
formatting::SUSPICIOUS_ELSE_FORMATTING,

50
tests/compile-fail/eval_order_dependence.rs Normal file
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@ -0,0 +1,50 @@
#![feature(plugin)]
#![plugin(clippy)]
#[deny(eval_order_dependence)]
#[allow(unused_assignments, unused_variables, many_single_char_names, no_effect, dead_code, blacklisted_name)]
fn main() {
let mut x = 0;
let a = { x = 1; 1 } + x;
//~^ ERROR unsequenced read
// Example from iss#277
x += { x = 20; 2 }; //~ERROR unsequenced read
// Does it work in weird places?
// ...in the base for a struct expression?
struct Foo { a: i32, b: i32 };
let base = Foo { a: 4, b: 5 };
let foo = Foo { a: x, .. { x = 6; base } };
//~^ ERROR unsequenced read
// ...inside a closure?
let closure = || {
let mut x = 0;
x += { x = 20; 2 }; //~ERROR unsequenced read
};
// ...not across a closure?
let mut y = 0;
let b = (y, || { y = 1 });
// && and || evaluate left-to-right.
let a = { x = 1; true } && (x == 3);
let a = { x = 1; true } || (x == 3);
// Make sure we don't get confused by alpha conversion.
let a = { let mut x = 1; x = 2; 1 } + x;
// No warning if we don't read the variable...
x = { x = 20; 2 };
// ...if the assignment is in a closure...
let b = { || { x = 1; }; 1 } + x;
// ... or the access is under an address.
let b = ({ let p = &x; 1 }, { x = 1; x });
// Limitation: l-values other than simple variables don't trigger
// the warning.
let mut tup = (0, 0);
let c = { tup.0 = 1; 1 } + tup.0;
// Limitation: you can get away with a read under address-of.
let mut z = 0;
let b = (&{ z = x; x }, { x = 3; x });
}