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add documentation
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@buttercrab
buttercrab committed Feb 13, 2022
1 parent 2b3e7fa commit 2ebe751
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Showing 2 changed files with 51 additions and 14 deletions.
43 changes: 40 additions & 3 deletions clippy_lints/src/only_used_in_recursion.rs
View file
Expand Up @@ -21,7 +21,7 @@ use rustc_span::Span;

declare_clippy_lint! {
/// ### What it does
/// Checks for arguments that is only used in recursion with no side-effects.
/// Checks for arguments that are only used in recursion with no side-effects.
/// The arguments can be involved in calculations and assignments but as long as
/// the calculations have no side-effects (function calls or mutating dereference)
/// and the assigned variables are also only in recursion, it is useless.
Expand All @@ -30,7 +30,21 @@ declare_clippy_lint! {
/// The could contain a useless calculation and can make function simpler.
///
/// ### Known problems
/// It could not catch the variable that has no side effects but only used in recursion.
/// In some cases, this would not catch all useless arguments.
///
/// ```rust
/// fn foo(a: usize, b: usize) -> usize {
/// let f = |x| x + 1;
///
/// if a == 0 {
/// 1
/// } else {
/// foo(a - 1, f(b))
/// }
/// }
/// ```
///
/// For example, the argument `b` is only used in recursion, but the lint would not catch it.
///
/// ### Example
/// ```rust
Expand Down Expand Up @@ -111,10 +125,12 @@ impl<'tcx> LateLintPass<'tcx> for OnlyUsedInRecursion {

visitor.visit_expr(&body.value);
let vars = std::mem::take(&mut visitor.ret_vars);
// this would set the return variables to side effect
visitor.add_side_effect(vars);

let mut queue = visitor.has_side_effect.iter().copied().collect::<VecDeque<_>>();

// a simple BFS to check all the variables that have side effect
while let Some(id) = queue.pop_front() {
if let Some(next) = visitor.graph.get(&id) {
for i in next {
Expand All @@ -134,6 +150,8 @@ impl<'tcx> LateLintPass<'tcx> for OnlyUsedInRecursion {

queue.push_back(id);

// a simple BFS to check the graph can reach to itself
// if it can't, it means the variable is never used in recursion
while let Some(id) = queue.pop_front() {
if let Some(next) = visitor.graph.get(&id) {
for i in next {
Expand All @@ -150,7 +168,7 @@ impl<'tcx> LateLintPass<'tcx> for OnlyUsedInRecursion {
cx,
ONLY_USED_IN_RECURSION,
span,
"parameter is only used in recursion with no side-effects",
"parameter is only used in recursion",
"if this is intentional, prefix with an underscore",
format!("_{}", ident.name.as_str()),
Applicability::MaybeIncorrect,
Expand Down Expand Up @@ -178,6 +196,21 @@ pub fn is_array(ty: Ty<'_>) -> bool {
}
}

/// This builds the graph of side effect.
/// The edge `a -> b` means if `a` has side effect, `b` will have side effect.
///
/// There are some exmaple in following code:
/// ```rust, ignore
/// let b = 1;
/// let a = b; // a -> b
/// let (c, d) = (a, b); // c -> b, d -> b
///
/// let e = if a == 0 { // e -> a
/// c // e -> c
/// } else {
/// d // e -> d
/// };
/// ```
pub struct SideEffectVisit<'tcx> {
graph: FxHashMap<HirId, FxHashSet<HirId>>,
has_side_effect: FxHashSet<HirId>,
Expand Down Expand Up @@ -241,6 +274,7 @@ impl<'tcx> Visitor<'tcx> for SideEffectVisit<'tcx> {
self.visit_if(bind, then_expr, else_expr);
},
ExprKind::Match(expr, arms, _) => self.visit_match(expr, arms),
// since analysing the closure is not easy, just set all variables in it to side-effect
ExprKind::Closure(_, _, body_id, _, _) => {
let body = self.ty_ctx.hir().body(body_id);
self.visit_body(body);
Expand Down Expand Up @@ -359,6 +393,9 @@ impl<'tcx> SideEffectVisit<'tcx> {
let mut ret_vars = std::mem::take(&mut self.ret_vars);
self.visit_expr(rhs);
self.ret_vars.append(&mut ret_vars);

// the binary operation between non primitive values are overloaded operators
// so they can have side-effects
if !is_primitive(self.ty_res.expr_ty(lhs)) || !is_primitive(self.ty_res.expr_ty(rhs)) {
self.ret_vars.iter().for_each(|id| {
self.has_side_effect.insert(id.0);
Expand Down
22 changes: 11 additions & 11 deletions tests/ui/only_used_in_recursion.stderr
View file
@@ -1,66 +1,66 @@
error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:3:21
|
LL | fn simple(a: usize, b: usize) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`
|
= note: `-D clippy::only-used-in-recursion` implied by `-D warnings`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:7:24
|
LL | fn with_calc(a: usize, b: isize) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:11:14
|
LL | fn tuple((a, b): (usize, usize)) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:15:24
|
LL | fn let_tuple(a: usize, b: usize) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:20:14
|
LL | fn array([a, b]: [usize; 2]) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:24:20
|
LL | fn index(a: usize, mut b: &[usize], c: usize) -> usize {
| ^^^^^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:24:37
|
LL | fn index(a: usize, mut b: &[usize], c: usize) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_c`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:28:21
|
LL | fn break_(a: usize, mut b: usize, mut c: usize) -> usize {
| ^^^^^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:46:23
|
LL | fn mut_ref2(a: usize, b: &mut usize) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:51:28
|
LL | fn not_primitive(a: usize, b: String) -> usize {
| ^ help: if this is intentional, prefix with an underscore: `_b`

error: parameter is only used in recursion with no side-effects
error: parameter is only used in recursion
--> $DIR/only_used_in_recursion.rs:64:32
|
LL | fn method(&self, a: usize, b: usize) -> usize {
Expand Down

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