rust-clippy/clippy_lints/src/slow_vector_initialization.rs

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use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::sugg::Sugg;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::{get_enclosing_block, is_expr_path_def_path, path_to_local, path_to_local_id, paths, SpanlessEq};
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use if_chain::if_chain;
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use rustc_ast::ast::LitKind;
use rustc_errors::Applicability;
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use rustc_hir::intravisit::{walk_block, walk_expr, walk_stmt, NestedVisitorMap, Visitor};
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use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, PatKind, QPath, Stmt, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::map::Map;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::sym;
declare_clippy_lint! {
/// **What it does:** Checks slow zero-filled vector initialization
///
/// **Why is this bad?** These structures are non-idiomatic and less efficient than simply using
/// `vec![0; len]`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// # use core::iter::repeat;
/// # let len = 4;
///
/// // Bad
/// let mut vec1 = Vec::with_capacity(len);
/// vec1.resize(len, 0);
///
/// let mut vec2 = Vec::with_capacity(len);
/// vec2.extend(repeat(0).take(len));
///
/// // Good
/// let mut vec1 = vec![0; len];
/// let mut vec2 = vec![0; len];
/// ```
pub SLOW_VECTOR_INITIALIZATION,
perf,
"slow vector initialization"
}
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declare_lint_pass!(SlowVectorInit => [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> {
/// HirId of the variable
local_id: HirId,
/// Reference to the expression which allocates the vector
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allocation_expr: &'tcx Expr<'tcx>,
/// 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.
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len_expr: &'tcx Expr<'tcx>,
}
/// Type of slow initialization
enum InitializationType<'tcx> {
/// Extend is a slow initialization with the form `vec.extend(repeat(0).take(..))`
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Extend(&'tcx Expr<'tcx>),
/// Resize is a slow initialization with the form `vec.resize(.., 0)`
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Resize(&'tcx Expr<'tcx>),
}
impl<'tcx> LateLintPass<'tcx> for SlowVectorInit {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
// Matches initialization on reassignements. For example: `vec = Vec::with_capacity(100)`
if_chain! {
if let ExprKind::Assign(left, right, _) = expr.kind;
// Extract variable
if let Some(local_id) = path_to_local(left);
// Extract len argument
if let Some(len_arg) = Self::is_vec_with_capacity(cx, right);
then {
let vi = VecAllocation {
local_id,
allocation_expr: right,
len_expr: len_arg,
};
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Self::search_initialization(cx, vi, expr.hir_id);
}
}
}
fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
// Matches statements which initializes vectors. For example: `let mut vec = Vec::with_capacity(10)`
if_chain! {
if let StmtKind::Local(local) = stmt.kind;
if let PatKind::Binding(BindingAnnotation::Mutable, local_id, _, None) = local.pat.kind;
if let Some(init) = local.init;
if let Some(len_arg) = Self::is_vec_with_capacity(cx, init);
then {
let vi = VecAllocation {
local_id,
allocation_expr: init,
len_expr: len_arg,
};
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Self::search_initialization(cx, vi, stmt.hir_id);
}
}
}
}
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impl SlowVectorInit {
/// 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<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
if_chain! {
if let ExprKind::Call(func, [arg]) = expr.kind;
if let ExprKind::Path(QPath::TypeRelative(ty, name)) = func.kind;
if name.ident.as_str() == "with_capacity";
if is_type_diagnostic_item(cx, cx.typeck_results().node_type(ty.hir_id), sym::vec_type);
then {
Some(arg)
} else {
None
}
}
}
/// Search initialization for the given vector
fn search_initialization<'tcx>(cx: &LateContext<'tcx>, vec_alloc: VecAllocation<'tcx>, parent_node: HirId) {
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>,
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initialization: &InitializationType<'tcx>,
vec_alloc: &VecAllocation<'_>,
) {
match initialization {
InitializationType::Extend(e) | InitializationType::Resize(e) => {
Self::emit_lint(cx, e, vec_alloc, "slow zero-filling initialization")
},
};
}
fn emit_lint<'tcx>(cx: &LateContext<'tcx>, slow_fill: &Expr<'_>, vec_alloc: &VecAllocation<'_>, msg: &str) {
let len_expr = Sugg::hir(cx, vec_alloc.len_expr, "len");
span_lint_and_then(cx, SLOW_VECTOR_INITIALIZATION, slow_fill.span, msg, |diag| {
diag.span_suggestion(
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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> {
cx: &'a LateContext<'tcx>,
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/// Contains the information.
vec_alloc: VecAllocation<'tcx>,
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/// Contains the slow initialization expression, if one was found.
slow_expression: Option<InitializationType<'tcx>>,
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/// `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(..)`
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fn search_slow_extend_filling(&mut self, expr: &'tcx Expr<'_>) {
if_chain! {
if self.initialization_found;
if let ExprKind::MethodCall(path, _, [self_arg, extend_arg], _) = expr.kind;
if path_to_local_id(self_arg, self.vec_alloc.local_id);
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if path.ident.name == sym!(extend);
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
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fn search_slow_resize_filling(&mut self, expr: &'tcx Expr<'_>) {
if_chain! {
if self.initialization_found;
if let ExprKind::MethodCall(path, _, [self_arg, len_arg, fill_arg], _) = expr.kind;
if path_to_local_id(self_arg, self.vec_alloc.local_id);
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if path.ident.name == sym!(resize);
// Check that is filled with 0
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if let ExprKind::Lit(ref lit) = fill_arg.kind;
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(...)`
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fn is_repeat_take(&self, expr: &Expr<'_>) -> bool {
if_chain! {
if let ExprKind::MethodCall(take_path, _, take_args, _) = expr.kind;
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if take_path.ident.name == sym!(take);
// Check that take is applied to `repeat(0)`
if let Some(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(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(fn_expr, [repeat_arg]) = expr.kind;
if is_expr_path_def_path(self.cx, fn_expr, &paths::ITER_REPEAT);
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if let ExprKind::Lit(ref lit) = repeat_arg.kind;
if let LitKind::Int(0, _) = lit.node;
then {
true
} else {
false
}
}
}
}
impl<'a, 'tcx> Visitor<'tcx> for VectorInitializationVisitor<'a, 'tcx> {
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type Map = Map<'tcx>;
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fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
if self.initialization_found {
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match stmt.kind {
StmtKind::Expr(expr) | StmtKind::Semi(expr) => {
self.search_slow_extend_filling(expr);
self.search_slow_resize_filling(expr);
},
_ => (),
}
self.initialization_found = false;
} else {
walk_stmt(self, stmt);
}
}
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fn visit_block(&mut self, block: &'tcx Block<'_>) {
if self.initialization_found {
if let Some(s) = block.stmts.get(0) {
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self.visit_stmt(s)
}
self.initialization_found = false;
} else {
walk_block(self, block);
}
}
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fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
// Skip all the expressions previous to the vector initialization
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if self.vec_alloc.allocation_expr.hir_id == expr.hir_id {
self.initialization_found = true;
}
walk_expr(self, expr);
}
fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}