rust-clippy/clippy_lints/src/slow_vector_initialization.rs

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// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use crate::utils::sugg::Sugg;
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use crate::utils::{get_enclosing_block, match_qpath, span_lint_and_then, SpanlessEq};
use if_chain::if_chain;
use rustc::hir::intravisit::{walk_block, walk_expr, walk_stmt, NestedVisitorMap, Visitor};
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, Lint, LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};
use rustc_errors::Applicability;
use syntax::ast::{LitKind, NodeId};
use syntax_pos::symbol::Symbol;
/// **What it does:** Checks slow zero-filled vector initialization
///
/// **Why is this bad?** This structures are non-idiomatic and less efficient than simply using
/// `vec![len; 0]`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let mut vec1 = Vec::with_capacity(len);
/// vec1.resize(len, 0);
///
/// let mut vec2 = Vec::with_capacity(len);
/// vec2.extend(repeat(0).take(len))
/// ```
declare_clippy_lint! {
pub SLOW_VECTOR_INITIALIZATION,
perf,
"slow vector initialization"
}
#[derive(Copy, Clone, Default)]
pub struct Pass;
impl LintPass for Pass {
fn get_lints(&self) -> LintArray {
lint_array!(SLOW_VECTOR_INITIALIZATION,)
}
}
/// `VecAllocation` contains data regarding a vector allocated with `with_capacity` and then
/// assigned to a variable. For example, `let mut vec = Vec::with_capacity(0)` or
/// `vec = Vec::with_capacity(0)`
struct VecAllocation<'tcx> {
/// Symbol of the local variable name
variable_name: Symbol,
/// Reference to the expression which allocates the vector
allocation_expr: &'tcx Expr,
/// Reference to the expression used as argument on `with_capacity` call. This is used
/// to only match slow zero-filling idioms of the same length than vector initialization.
len_expr: &'tcx Expr,
}
/// Type of slow initialization
enum InitializationType<'tcx> {
/// Extend is a slow initialization with the form `vec.extend(repeat(0).take(..))`
Extend(&'tcx Expr),
/// Resize is a slow initialization with the form `vec.resize(.., 0)`
Resize(&'tcx Expr),
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
// Matches initialization on reassignements. For example: `vec = Vec::with_capacity(100)`
if_chain! {
if let ExprKind::Assign(ref left, ref right) = expr.node;
// Extract variable name
if let ExprKind::Path(QPath::Resolved(_, ref path)) = left.node;
if let Some(variable_name) = path.segments.get(0);
// Extract len argument
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if let Some(ref len_arg) = Self::is_vec_with_capacity(right);
then {
let vi = VecAllocation {
variable_name: variable_name.ident.name,
allocation_expr: right,
len_expr: len_arg,
};
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Self::search_initialization(cx, vi, expr.id);
}
}
}
fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
// Matches statements which initializes vectors. For example: `let mut vec = Vec::with_capacity(10)`
if_chain! {
if let StmtKind::Decl(ref decl, _) = stmt.node;
if let DeclKind::Local(ref local) = decl.node;
if let PatKind::Binding(BindingAnnotation::Mutable, _, variable_name, None) = local.pat.node;
if let Some(ref init) = local.init;
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if let Some(ref len_arg) = Self::is_vec_with_capacity(init);
then {
let vi = VecAllocation {
variable_name: variable_name.name,
allocation_expr: init,
len_expr: len_arg,
};
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Self::search_initialization(cx, vi, stmt.node.id());
}
}
}
}
impl Pass {
/// Checks if the given expression is `Vec::with_capacity(..)`. It will return the expression
/// of the first argument of `with_capacity` call if it matches or `None` if it does not.
fn is_vec_with_capacity(expr: &Expr) -> Option<&Expr> {
if_chain! {
if let ExprKind::Call(ref func, ref args) = expr.node;
if let ExprKind::Path(ref path) = func.node;
if match_qpath(path, &["Vec", "with_capacity"]);
if args.len() == 1;
then {
return Some(&args[0]);
}
}
None
}
/// Search initialization for the given vector
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fn search_initialization<'tcx>(cx: &LateContext<'_, 'tcx>, vec_alloc: VecAllocation<'tcx>, parent_node: NodeId) {
let enclosing_body = get_enclosing_block(cx, parent_node);
if enclosing_body.is_none() {
return;
}
let mut v = VectorInitializationVisitor {
cx,
vec_alloc,
slow_expression: None,
initialization_found: false,
};
v.visit_block(enclosing_body.unwrap());
if let Some(ref allocation_expr) = v.slow_expression {
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Self::lint_initialization(cx, allocation_expr, &v.vec_alloc);
}
}
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fn lint_initialization<'tcx>(
cx: &LateContext<'_, 'tcx>,
initialization: &InitializationType<'tcx>,
vec_alloc: &VecAllocation<'_>,
) {
match initialization {
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InitializationType::Extend(e) | InitializationType::Resize(e) => Self::emit_lint(
cx,
e,
vec_alloc,
"slow zero-filling initialization",
SLOW_VECTOR_INITIALIZATION,
),
};
}
fn emit_lint<'tcx>(
cx: &LateContext<'_, 'tcx>,
slow_fill: &Expr,
vec_alloc: &VecAllocation<'_>,
msg: &str,
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lint: &'static Lint,
) {
let len_expr = Sugg::hir(cx, vec_alloc.len_expr, "len");
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span_lint_and_then(cx, lint, slow_fill.span, msg, |db| {
db.span_suggestion_with_applicability(
vec_alloc.allocation_expr.span,
"consider replace allocation with",
format!("vec![0; {}]", len_expr),
Applicability::Unspecified,
);
});
}
}
/// `VectorInitializationVisitor` searches for unsafe or slow vector initializations for the given
/// vector.
struct VectorInitializationVisitor<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
/// Contains the information
vec_alloc: VecAllocation<'tcx>,
/// Contains, if found, the slow initialization expression
slow_expression: Option<InitializationType<'tcx>>,
/// true if the initialization of the vector has been found on the visited block
initialization_found: bool,
}
impl<'a, 'tcx> VectorInitializationVisitor<'a, 'tcx> {
/// Checks if the given expression is extending a vector with `repeat(0).take(..)`
fn search_slow_extend_filling(&mut self, expr: &'tcx Expr) {
if_chain! {
if self.initialization_found;
if let ExprKind::MethodCall(ref path, _, ref args) = expr.node;
if let ExprKind::Path(ref qpath_subj) = args[0].node;
if match_qpath(&qpath_subj, &[&self.vec_alloc.variable_name.to_string()]);
if path.ident.name == "extend";
if let Some(ref extend_arg) = args.get(1);
if self.is_repeat_take(extend_arg);
then {
self.slow_expression = Some(InitializationType::Extend(expr));
}
}
}
/// Checks if the given expression is resizing a vector with 0
fn search_slow_resize_filling(&mut self, expr: &'tcx Expr) {
if_chain! {
if self.initialization_found;
if let ExprKind::MethodCall(ref path, _, ref args) = expr.node;
if let ExprKind::Path(ref qpath_subj) = args[0].node;
if match_qpath(&qpath_subj, &[&self.vec_alloc.variable_name.to_string()]);
if path.ident.name == "resize";
if let (Some(ref len_arg), Some(fill_arg)) = (args.get(1), args.get(2));
// Check that is filled with 0
if let ExprKind::Lit(ref lit) = fill_arg.node;
if let LitKind::Int(0, _) = lit.node;
// Check that len expression is equals to `with_capacity` expression
if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
then {
self.slow_expression = Some(InitializationType::Resize(expr));
}
}
}
/// Returns `true` if give expression is `repeat(0).take(...)`
fn is_repeat_take(&self, expr: &Expr) -> bool {
if_chain! {
if let ExprKind::MethodCall(ref take_path, _, ref take_args) = expr.node;
if take_path.ident.name == "take";
// Check that take is applied to `repeat(0)`
if let Some(ref repeat_expr) = take_args.get(0);
if self.is_repeat_zero(repeat_expr);
// Check that len expression is equals to `with_capacity` expression
if let Some(ref len_arg) = take_args.get(1);
if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
then {
return true;
}
}
false
}
/// Returns `true` if given expression is `repeat(0)`
fn is_repeat_zero(&self, expr: &Expr) -> bool {
if_chain! {
if let ExprKind::Call(ref fn_expr, ref repeat_args) = expr.node;
if let ExprKind::Path(ref qpath_repeat) = fn_expr.node;
if match_qpath(&qpath_repeat, &["repeat"]);
if let Some(ref repeat_arg) = repeat_args.get(0);
if let ExprKind::Lit(ref lit) = repeat_arg.node;
if let LitKind::Int(0, _) = lit.node;
then {
return true
}
}
false
}
}
impl<'a, 'tcx> Visitor<'tcx> for VectorInitializationVisitor<'a, 'tcx> {
fn visit_stmt(&mut self, stmt: &'tcx Stmt) {
if self.initialization_found {
match stmt.node {
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StmtKind::Expr(ref expr, _) | StmtKind::Semi(ref expr, _) => {
self.search_slow_extend_filling(expr);
self.search_slow_resize_filling(expr);
},
_ => (),
}
self.initialization_found = false;
} else {
walk_stmt(self, stmt);
}
}
fn visit_block(&mut self, block: &'tcx Block) {
if self.initialization_found {
if let Some(ref s) = block.stmts.get(0) {
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self.visit_stmt(s)
}
self.initialization_found = false;
} else {
walk_block(self, block);
}
}
fn visit_expr(&mut self, expr: &'tcx Expr) {
// Skip all the expressions previous to the vector initialization
if self.vec_alloc.allocation_expr.id == expr.id {
self.initialization_found = true;
}
walk_expr(self, expr);
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}