rust/clippy_lints/src/no_effect.rs

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use crate::utils::{has_drop, in_macro, snippet_opt, span_lint, span_lint_and_sugg};
use rustc::hir::def::Def;
use rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};
use rustc_errors::Applicability;
use std::ops::Deref;
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/// **What it does:** Checks for statements which have no effect.
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///
/// **Why is this bad?** Similar to dead code, these statements are actually
/// executed. However, as they have no effect, all they do is make the code less
/// readable.
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///
/// **Known problems:** None.
///
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/// **Example:**
/// ```rust
/// 0;
/// ```
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declare_clippy_lint! {
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pub NO_EFFECT,
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complexity,
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"statements with no effect"
}
/// **What it does:** Checks for expression statements that can be reduced to a
/// sub-expression.
///
/// **Why is this bad?** Expressions by themselves often have no side-effects.
/// Having such expressions reduces readability.
///
/// **Known problems:** None.
///
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/// **Example:**
/// ```rust
/// compute_array()[0];
/// ```
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declare_clippy_lint! {
pub UNNECESSARY_OPERATION,
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complexity,
"outer expressions with no effect"
}
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fn has_no_effect(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
if in_macro(expr.span) {
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return false;
}
match expr.node {
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ExprKind::Lit(..) | ExprKind::Closure(.., _) => true,
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ExprKind::Path(..) => !has_drop(cx, cx.tables.expr_ty(expr)),
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ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
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has_no_effect(cx, a) && has_no_effect(cx, b)
},
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ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
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ExprKind::Repeat(ref inner, _)
| ExprKind::Cast(ref inner, _)
| ExprKind::Type(ref inner, _)
| ExprKind::Unary(_, ref inner)
| ExprKind::Field(ref inner, _)
| ExprKind::AddrOf(_, ref inner)
| ExprKind::Box(ref inner) => has_no_effect(cx, inner),
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ExprKind::Struct(_, ref fields, ref base) => {
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!has_drop(cx, cx.tables.expr_ty(expr))
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&& fields.iter().all(|field| has_no_effect(cx, &field.expr))
&& match *base {
Some(ref base) => has_no_effect(cx, base),
None => true,
}
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},
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ExprKind::Call(ref callee, ref args) => {
if let ExprKind::Path(ref qpath) = callee.node {
let def = cx.tables.qpath_def(qpath, callee.hir_id);
match def {
Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
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!has_drop(cx, cx.tables.expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
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},
_ => false,
}
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} else {
false
}
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},
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ExprKind::Block(ref block, _) => {
block.stmts.is_empty()
&& if let Some(ref expr) = block.expr {
has_no_effect(cx, expr)
} else {
false
}
},
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_ => false,
}
}
#[derive(Copy, Clone)]
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pub struct Pass;
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impl LintPass for Pass {
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fn get_lints(&self) -> LintArray {
lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
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}
fn name(&self) -> &'static str {
"NoEffect"
}
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}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
if let StmtKind::Semi(ref expr) = stmt.node {
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if has_no_effect(cx, expr) {
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span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
} else if let Some(reduced) = reduce_expression(cx, expr) {
let mut snippet = String::new();
for e in reduced {
if in_macro(e.span) {
return;
}
if let Some(snip) = snippet_opt(cx, e.span) {
snippet.push_str(&snip);
snippet.push(';');
} else {
return;
}
}
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span_lint_and_sugg(
cx,
UNNECESSARY_OPERATION,
stmt.span,
"statement can be reduced",
"replace it with",
snippet,
Applicability::MachineApplicable,
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);
}
}
}
}
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fn reduce_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
if in_macro(expr.span) {
return None;
}
match expr.node {
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ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
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ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
Some(vec![&**a, &**b])
},
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ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
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ExprKind::Repeat(ref inner, _)
| ExprKind::Cast(ref inner, _)
| ExprKind::Type(ref inner, _)
| ExprKind::Unary(_, ref inner)
| ExprKind::Field(ref inner, _)
| ExprKind::AddrOf(_, ref inner)
| ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
ExprKind::Struct(_, ref fields, ref base) => {
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if has_drop(cx, cx.tables.expr_ty(expr)) {
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None
} else {
Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
}
},
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ExprKind::Call(ref callee, ref args) => {
if let ExprKind::Path(ref qpath) = callee.node {
let def = cx.tables.qpath_def(qpath, callee.hir_id);
match def {
Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
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if !has_drop(cx, cx.tables.expr_ty(expr)) =>
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{
Some(args.iter().collect())
},
_ => None,
}
} else {
None
}
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},
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ExprKind::Block(ref block, _) => {
if block.stmts.is_empty() {
block.expr.as_ref().and_then(|e| {
match block.rules {
BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
BlockCheckMode::DefaultBlock => Some(vec![&**e]),
// in case of compiler-inserted signaling blocks
_ => reduce_expression(cx, e),
}
})
} else {
None
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}
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},
_ => None,
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}
}