Add slow zero-filled vector initialization lint

Add lint to detect slow zero-filled vector initialization. It detects
when a vector is zero-filled with extended with `repeat(0).take(len)`
or `resize(len, 0)`.
This zero-fillings are usually slower than simply using `vec![0; len]`.

clippy_lints/src/lib.rs

@@ -190,6 +190,7 @@ pub mod replace_consts;
 pub mod returns;
 pub mod serde_api;
 pub mod shadow;
+pub mod slow_vector_initialization;
 pub mod strings;
 pub mod suspicious_trait_impl;
 pub mod swap;
@@ -459,6 +460,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
     reg.register_late_lint_pass(box non_copy_const::NonCopyConst);
     reg.register_late_lint_pass(box ptr_offset_with_cast::Pass);
     reg.register_late_lint_pass(box redundant_clone::RedundantClone);
+    reg.register_late_lint_pass(box slow_vector_initialization::Pass);
 
     reg.register_lint_group("clippy::restriction", Some("clippy_restriction"), vec![
         arithmetic::FLOAT_ARITHMETIC,
@@ -980,6 +982,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         methods::SINGLE_CHAR_PATTERN,
         misc::CMP_OWNED,
         mutex_atomic::MUTEX_ATOMIC,
+        slow_vector_initialization::SLOW_VECTOR_INITIALIZATION,
         trivially_copy_pass_by_ref::TRIVIALLY_COPY_PASS_BY_REF,
         types::BOX_VEC,
         vec::USELESS_VEC,

clippy_lints/src/slow_vector_initialization.rs

@@ -0,0 +1,304 @@
+// 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::rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use crate::rustc::{declare_tool_lint, lint_array};
+use crate::rustc::hir::*;
+use if_chain::if_chain;
+use crate::syntax_pos::symbol::Symbol;
+use crate::syntax::ast::{LitKind, NodeId};
+use crate::syntax::source_map::Span;
+use crate::utils::{match_qpath, span_lint_and_then, SpanlessEq};
+use crate::utils::get_enclosing_block;
+use crate::rustc_errors::{Applicability};
+
+/// **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 or unsafe vector initialization"
+}
+
+#[derive(Copy, Clone, Default)]
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(SLOW_VECTOR_INITIALIZATION)
+    }
+}
+
+/// VecInitialization contains data regarding a vector initialized 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 VecInitialization<'tcx> {
+    /// Symbol of the local variable name
+    variable_name: Symbol,
+
+    /// Reference to the expression which initializes the vector
+    initialization_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
+            if let Some(ref len_arg) = Pass::is_vec_with_capacity(right);
+
+            then {
+                let vi = VecInitialization {
+                    variable_name: variable_name.ident.name,
+                    initialization_expr: right,
+                    len_expr: len_arg,
+                };
+
+                Pass::search_slow_zero_filling(cx, vi, expr.id, expr.span);
+            }
+        }
+    }
+
+    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;
+            if let Some(ref len_arg) = Pass::is_vec_with_capacity(init);
+
+            then {
+                let vi = VecInitialization {
+                    variable_name: variable_name.name,
+                    initialization_expr: init,
+                    len_expr: len_arg,
+                };
+
+                Pass::search_slow_zero_filling(cx, vi, stmt.node.id(), stmt.span);
+            }
+        }
+    }
+}
+
+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 for slow zero filling vector initialization for the given vector
+    fn search_slow_zero_filling<'tcx>(
+        cx: &LateContext<'_, 'tcx>,
+        vec_initialization: VecInitialization<'tcx>,
+        parent_node: NodeId,
+        parent_span: Span
+    ) {
+        let enclosing_body = get_enclosing_block(cx, parent_node);
+
+        if enclosing_body.is_none() {
+            return;
+        }
+
+        let mut v = SlowInitializationVisitor {
+            cx,
+            vec_ini: vec_initialization,
+            slow_expression: None,
+            initialization_found: false,
+        };
+
+        v.visit_block(enclosing_body.unwrap());
+
+        if let Some(ref repeat_expr) = v.slow_expression {
+            span_lint_and_then(
+                cx,
+                SLOW_VECTOR_INITIALIZATION,
+                parent_span,
+                "detected slow zero-filling initialization",
+                |db| {
+                    db.span_suggestion_with_applicability(v.vec_ini.initialization_expr.span, "consider replacing with", "vec![0; ..]".to_string(), Applicability::Unspecified);
+
+                    match repeat_expr {
+                        InitializationType::Extend(e) => {
+                            db.span_note(e.span, "extended here with .. 0");
+                        },
+                        InitializationType::Resize(e) => {
+                            db.span_note(e.span, "resize here with .. 0");
+                        }
+                    }
+                }
+            );
+        }
+    }
+}
+
+/// SlowInitializationVisitor searches for slow zero filling vector initialization, for the given
+/// vector.
+struct SlowInitializationVisitor<'a, 'tcx: 'a> {
+    cx: &'a LateContext<'a, 'tcx>,
+
+    /// Contains the information
+    vec_ini: VecInitialization<'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> SlowInitializationVisitor<'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_ini.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_ini.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_ini.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_ini.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 SlowInitializationVisitor<'a, 'tcx> {
+    fn visit_expr(&mut self, expr: &'tcx Expr) {
+        // Stop the search if we already found a slow zero-filling initialization
+        if self.slow_expression.is_some() {
+            return
+        }
+
+        // Skip all the expressions previous to the vector initialization
+        if self.vec_ini.initialization_expr.id == expr.id {
+            self.initialization_found = true;
+        }
+        
+        self.search_slow_extend_filling(expr);
+        self.search_slow_resize_filling(expr);
+
+        walk_expr(self, expr);
+    }
+
+    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
+        NestedVisitorMap::None
+    }
+}

tests/ui/slow_vector_initialization.rs

@@ -0,0 +1,63 @@
+// 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 std::iter::repeat;
+
+fn main() {
+    resize_vector();
+    extend_vector();
+    mixed_extend_resize_vector();
+}
+
+fn extend_vector() {
+    // Extend with constant expression
+    let len = 300;
+    let mut vec1 = Vec::with_capacity(len);
+    vec1.extend(repeat(0).take(len));
+
+    // Extend with len expression
+    let mut vec2 = Vec::with_capacity(len - 10);
+    vec2.extend(repeat(0).take(len - 10));
+
+    // Extend with mismatching expression should not be warned
+    let mut vec3 = Vec::with_capacity(24322);
+    vec3.extend(repeat(0).take(2));
+}
+
+fn mixed_extend_resize_vector() {
+    // Mismatching len
+    let mut mismatching_len = Vec::with_capacity(30);
+
+    // Slow initialization
+    let mut resized_vec = Vec::with_capacity(30);
+    let mut extend_vec = Vec::with_capacity(30);
+
+    resized_vec.resize(30, 0);
+    mismatching_len.extend(repeat(0).take(40));
+    extend_vec.extend(repeat(0).take(30));
+}
+
+fn resize_vector() {
+    // Resize with constant expression
+    let len = 300;
+    let mut vec1 = Vec::with_capacity(len);
+    vec1.resize(len, 0);
+
+    // Resize mismatch len
+    let mut vec2 = Vec::with_capacity(200);
+    vec2.resize(10, 0);
+
+    // Resize with len expression
+    let mut vec3 = Vec::with_capacity(len - 10);
+    vec3.resize(len - 10, 0);
+
+    // Reinitialization should be warned
+    vec1 = Vec::with_capacity(10);
+    vec1.resize(10, 0);
+}

tests/ui/slow_vector_initialization.stderr

@@ -0,0 +1,101 @@
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:21:5
+   |
+21 |     let mut vec1 = Vec::with_capacity(len);
+   |     ^^^^^^^^^^^^^^^-----------------------^
+   |                    |
+   |                    help: consider replacing with: `vec![0; ..]`
+   |
+   = note: `-D clippy::slow-vector-initialization` implied by `-D warnings`
+note: extended here with .. 0
+  --> $DIR/slow_vector_initialization.rs:22:5
+   |
+22 |     vec1.extend(repeat(0).take(len));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:25:5
+   |
+25 |     let mut vec2 = Vec::with_capacity(len - 10);
+   |     ^^^^^^^^^^^^^^^----------------------------^
+   |                    |
+   |                    help: consider replacing with: `vec![0; ..]`
+   |
+note: extended here with .. 0
+  --> $DIR/slow_vector_initialization.rs:26:5
+   |
+26 |     vec2.extend(repeat(0).take(len - 10));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:38:5
+   |
+38 |     let mut resized_vec = Vec::with_capacity(30);
+   |     ^^^^^^^^^^^^^^^^^^^^^^----------------------^
+   |                           |
+   |                           help: consider replacing with: `vec![0; ..]`
+   |
+note: resize here with .. 0
+  --> $DIR/slow_vector_initialization.rs:41:5
+   |
+41 |     resized_vec.resize(30, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:39:5
+   |
+39 |     let mut extend_vec = Vec::with_capacity(30);
+   |     ^^^^^^^^^^^^^^^^^^^^^----------------------^
+   |                          |
+   |                          help: consider replacing with: `vec![0; ..]`
+   |
+note: extended here with .. 0
+  --> $DIR/slow_vector_initialization.rs:43:5
+   |
+43 |     extend_vec.extend(repeat(0).take(30));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:49:5
+   |
+49 |     let mut vec1 = Vec::with_capacity(len);
+   |     ^^^^^^^^^^^^^^^-----------------------^
+   |                    |
+   |                    help: consider replacing with: `vec![0; ..]`
+   |
+note: resize here with .. 0
+  --> $DIR/slow_vector_initialization.rs:50:5
+   |
+50 |     vec1.resize(len, 0);
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:57:5
+   |
+57 |     let mut vec3 = Vec::with_capacity(len - 10);
+   |     ^^^^^^^^^^^^^^^----------------------------^
+   |                    |
+   |                    help: consider replacing with: `vec![0; ..]`
+   |
+note: resize here with .. 0
+  --> $DIR/slow_vector_initialization.rs:58:5
+   |
+58 |     vec3.resize(len - 10, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: detected slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:61:5
+   |
+61 |     vec1 = Vec::with_capacity(10);
+   |     ^^^^^^^----------------------
+   |            |
+   |            help: consider replacing with: `vec![0; ..]`
+   |
+note: resize here with .. 0
+  --> $DIR/slow_vector_initialization.rs:62:5
+   |
+62 |     vec1.resize(10, 0);
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+