rust/clippy_lints/src/no_effect.rs
2017-02-04 22:22:33 -07:00

187 lines
6.3 KiB
Rust

use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::hir::def::Def;
use rustc::hir::{Expr, Expr_, Stmt, StmtSemi, BlockCheckMode, UnsafeSource, BiAnd, BiOr};
use utils::{in_macro, span_lint, snippet_opt, span_lint_and_then};
use std::ops::Deref;
/// **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;
/// ```
declare_lint! {
pub NO_EFFECT,
Warn,
"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.
///
/// **Example:**
/// ```rust
/// compute_array()[0];
/// ```
declare_lint! {
pub UNNECESSARY_OPERATION,
Warn,
"outer expressions with no effect"
}
fn has_no_effect(cx: &LateContext, expr: &Expr) -> bool {
if in_macro(cx, expr.span) {
return false;
}
match expr.node {
Expr_::ExprLit(..) |
Expr_::ExprClosure(..) |
Expr_::ExprPath(..) => true,
Expr_::ExprIndex(ref a, ref b) |
Expr_::ExprBinary(_, ref a, ref b) => has_no_effect(cx, a) && has_no_effect(cx, b),
Expr_::ExprArray(ref v) |
Expr_::ExprTup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
Expr_::ExprRepeat(ref inner, _) |
Expr_::ExprCast(ref inner, _) |
Expr_::ExprType(ref inner, _) |
Expr_::ExprUnary(_, ref inner) |
Expr_::ExprField(ref inner, _) |
Expr_::ExprTupField(ref inner, _) |
Expr_::ExprAddrOf(_, ref inner) |
Expr_::ExprBox(ref inner) => has_no_effect(cx, inner),
Expr_::ExprStruct(_, ref fields, ref base) => {
fields.iter().all(|field| has_no_effect(cx, &field.expr)) &&
match *base {
Some(ref base) => has_no_effect(cx, base),
None => true,
}
},
Expr_::ExprCall(ref callee, ref args) => {
if let Expr_::ExprPath(ref qpath) = callee.node {
let def = cx.tables.qpath_def(qpath, callee.id);
match def {
Def::Struct(..) |
Def::Variant(..) |
Def::StructCtor(..) |
Def::VariantCtor(..) => args.iter().all(|arg| has_no_effect(cx, arg)),
_ => false,
}
} else {
false
}
},
Expr_::ExprBlock(ref block) => {
block.stmts.is_empty() &&
if let Some(ref expr) = block.expr {
has_no_effect(cx, expr)
} else {
false
}
},
_ => false,
}
}
#[derive(Copy, Clone)]
pub struct Pass;
impl LintPass for Pass {
fn get_lints(&self) -> LintArray {
lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
if let StmtSemi(ref expr, _) = stmt.node {
if has_no_effect(cx, expr) {
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(cx, e.span) {
return;
}
if let Some(snip) = snippet_opt(cx, e.span) {
snippet.push_str(&snip);
snippet.push(';');
} else {
return;
}
}
span_lint_and_then(cx,
UNNECESSARY_OPERATION,
stmt.span,
"statement can be reduced",
|db| { db.span_suggestion(stmt.span, "replace it with", snippet); });
}
}
}
}
fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
if in_macro(cx, expr.span) {
return None;
}
match expr.node {
Expr_::ExprIndex(ref a, ref b) => Some(vec![&**a, &**b]),
Expr_::ExprBinary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
Some(vec![&**a, &**b])
},
Expr_::ExprArray(ref v) |
Expr_::ExprTup(ref v) => Some(v.iter().collect()),
Expr_::ExprRepeat(ref inner, _) |
Expr_::ExprCast(ref inner, _) |
Expr_::ExprType(ref inner, _) |
Expr_::ExprUnary(_, ref inner) |
Expr_::ExprField(ref inner, _) |
Expr_::ExprTupField(ref inner, _) |
Expr_::ExprAddrOf(_, ref inner) |
Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
Expr_::ExprStruct(_, ref fields, ref base) => {
Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
},
Expr_::ExprCall(ref callee, ref args) => {
if let Expr_::ExprPath(ref qpath) = callee.node {
let def = cx.tables.qpath_def(qpath, callee.id);
match def {
Def::Struct(..) |
Def::Variant(..) |
Def::StructCtor(..) |
Def::VariantCtor(..) => Some(args.iter().collect()),
_ => None,
}
} else {
None
}
},
Expr_::ExprBlock(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
}
},
_ => None,
}
}