rust-clippy/clippy_lints/src/no_effect.rs

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use crate::utils::{snippet_opt, span_lint, span_lint_and_sugg};
use clippy_utils::ty::has_drop;
use rustc_errors::Applicability;
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use rustc_hir::def::{DefKind, Res};
use rustc_hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use std::ops::Deref;
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declare_clippy_lint! {
/// **What it does:** Checks for statements which have no effect.
///
/// **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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// 0;
/// ```
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pub NO_EFFECT,
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complexity,
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"statements with no effect"
}
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declare_clippy_lint! {
/// **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.
///
/// **Example:**
/// ```rust,ignore
/// compute_array()[0];
/// ```
pub UNNECESSARY_OPERATION,
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complexity,
"outer expressions with no effect"
}
fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
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if expr.span.from_expansion() {
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return false;
}
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match expr.kind {
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ExprKind::Lit(..) | ExprKind::Closure(..) => true,
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ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().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)
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| 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.typeck_results().expr_ty(expr))
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&& fields.iter().all(|field| has_no_effect(cx, &field.expr))
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&& base.as_ref().map_or(true, |base| has_no_effect(cx, base))
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},
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ExprKind::Call(ref callee, ref args) => {
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if let ExprKind::Path(ref qpath) = callee.kind {
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let res = cx.qpath_res(qpath, callee.hir_id);
match res {
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Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
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!has_drop(cx, cx.typeck_results().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, _) => {
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block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
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},
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_ => false,
}
}
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declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
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impl<'tcx> LateLintPass<'tcx> for NoEffect {
fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
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if let StmtKind::Semi(ref expr) = stmt.kind {
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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 {
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if e.span.from_expansion() {
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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);
}
}
}
}
fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
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if expr.span.from_expansion() {
return None;
}
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match expr.kind {
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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)
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| 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.typeck_results().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) => {
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if let ExprKind::Path(ref qpath) = callee.kind {
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let res = cx.qpath_res(qpath, callee.hir_id);
match res {
Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
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if !has_drop(cx, cx.typeck_results().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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}
}