Merge pull request #3365 from gnieto/lint/slow-initialization

Add slow vector initializations lint

7 files changed, 561 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa05eca89d1..320a3511e5c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -842,6 +842,7 @@ All notable changes to this project will be documented in this file.
 [`single_char_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
+[`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
 [`str_to_string`]: https://rust-lang.github.io/rust-clippy/master/index.html#str_to_string
 [`string_add`]: https://rust-lang.github.io/rust-clippy/master/index.html#string_add
 [`string_add_assign`]: https://rust-lang.github.io/rust-clippy/master/index.html#string_add_assign
@@ -880,6 +881,7 @@ All notable changes to this project will be documented in this file.
 [`unneeded_field_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#unneeded_field_pattern
 [`unreadable_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#unreadable_literal
 [`unsafe_removed_from_name`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsafe_removed_from_name
+[`unsafe_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsafe_vector_initialization
 [`unseparated_literal_suffix`]: https://rust-lang.github.io/rust-clippy/master/index.html#unseparated_literal_suffix
 [`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
 [`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
diff --git a/README.md b/README.md
index f2b0da3ae71..9d142a2deee 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ We are currently in the process of discussing Clippy 1.0 via the RFC process in
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 288 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 290 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 2dbe448c950..35f47c1cde4 100644
--- a/clippy_lints/src/lib.rs
+++ b/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,
@@ -709,6 +711,8 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         returns::NEEDLESS_RETURN,
         returns::UNUSED_UNIT,
         serde_api::SERDE_API_MISUSE,
+        slow_vector_initialization::SLOW_VECTOR_INITIALIZATION,
+        slow_vector_initialization::UNSAFE_VECTOR_INITIALIZATION,
         strings::STRING_LIT_AS_BYTES,
         suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL,
         suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL,
@@ -955,6 +959,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         ranges::ITERATOR_STEP_BY_ZERO,
         regex::INVALID_REGEX,
         serde_api::SERDE_API_MISUSE,
+        slow_vector_initialization::UNSAFE_VECTOR_INITIALIZATION,
         suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL,
         suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL,
         swap::ALMOST_SWAPPED,
@@ -980,6 +985,7 @@ 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,
diff --git a/clippy_lints/src/slow_vector_initialization.rs b/clippy_lints/src/slow_vector_initialization.rs
new file mode 100644
index 00000000000..272047bc7cb
--- /dev/null
+++ b/clippy_lints/src/slow_vector_initialization.rs
@@ -0,0 +1,398 @@
+// 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, walk_stmt, walk_block, NestedVisitorMap, Visitor};
+use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass, Lint};
+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::utils::{match_qpath, span_lint_and_then, SpanlessEq, get_enclosing_block};
+use crate::utils::sugg::Sugg;
+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 vector initialization"
+}
+
+/// **What it does:** Checks unsafe vector initialization
+///
+/// **Why is this bad?** Changing the length of a vector may expose uninitialized memory, which
+/// can lead to memory safety issues
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// let mut vec1 = Vec::with_capacity(len);
+/// unsafe { vec1.set_len(len); }
+/// ```
+declare_clippy_lint! {
+    pub UNSAFE_VECTOR_INITIALIZATION,
+    correctness,
+    "unsafe vector initialization"
+}
+
+#[derive(Copy, Clone, Default)]
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(
+            SLOW_VECTOR_INITIALIZATION,
+            UNSAFE_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),
+
+    /// UnsafeSetLen is a slow initialization with the form `vec.set_len(..)`
+    UnsafeSetLen(&'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) = Self::is_vec_with_capacity(right);
+
+            then {
+                let vi = VecAllocation {
+                    variable_name: variable_name.ident.name,
+                    allocation_expr: right,
+                    len_expr: len_arg,
+                };
+
+                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;
+            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,
+                };
+
+                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
+    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 {
+            Self::lint_initialization(cx, allocation_expr, &v.vec_alloc);
+        }
+    }
+
+    fn lint_initialization<'tcx>(cx: &LateContext<'_, 'tcx>, initialization: &InitializationType<'tcx>, vec_alloc: &VecAllocation<'_>) {
+        match initialization {
+            InitializationType::UnsafeSetLen(e) =>
+                Self::emit_lint(
+                    cx,
+                    e,
+                    vec_alloc,
+                    "unsafe vector initialization",
+                    UNSAFE_VECTOR_INITIALIZATION
+                ),
+
+            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,
+        lint: &'static Lint
+    ) {
+        let len_expr = Sugg::hir(cx, vec_alloc.len_expr, "len");
+
+        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));
+            }
+        }
+    }
+
+    /// Checks if the given expression is using `set_len` to initialize the vector
+    fn search_unsafe_set_len(&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 == "set_len";
+            if let Some(ref len_arg) = args.get(1);
+
+            // 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::UnsafeSetLen(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 {
+                StmtKind::Expr(ref expr, _) |
+                StmtKind::Semi(ref expr, _) => {
+                    self.search_slow_extend_filling(expr);
+                    self.search_slow_resize_filling(expr);
+                    self.search_unsafe_set_len(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) {
+                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
+    }
+}
diff --git a/tests/ui/slow_vector_initialization.rs b/tests/ui/slow_vector_initialization.rs
new file mode 100644
index 00000000000..daa6b9c1376
--- /dev/null
+++ b/tests/ui/slow_vector_initialization.rs
@@ -0,0 +1,83 @@
+// 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();
+    unsafe_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);
+    mismatching_len.extend(repeat(0).take(40));
+
+    // Slow initialization
+    let mut resized_vec = Vec::with_capacity(30);
+    resized_vec.resize(30, 0);
+
+    let mut extend_vec = Vec::with_capacity(30);
+    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);
+}
+
+fn unsafe_vector() {
+    let mut unsafe_vec: Vec<u8> = Vec::with_capacity(200);
+
+    unsafe {
+        unsafe_vec.set_len(200);
+    }
+}
+
+fn do_stuff(vec: &mut Vec<u8>) {
+
+}
+
+fn extend_vector_with_manipulations_between() {
+    let len = 300;
+    let mut vec1:Vec<u8> = Vec::with_capacity(len);
+    do_stuff(&mut vec1);
+    vec1.extend(repeat(0).take(len));
+}
diff --git a/tests/ui/slow_vector_initialization.stderr b/tests/ui/slow_vector_initialization.stderr
new file mode 100644
index 00000000000..577cc82c6d5
--- /dev/null
+++ b/tests/ui/slow_vector_initialization.stderr
@@ -0,0 +1,71 @@
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:23:5
+   |
+22 |     let mut vec1 = Vec::with_capacity(len);
+   |                    ----------------------- help: consider replace allocation with: `vec![0; len]`
+23 |     vec1.extend(repeat(0).take(len));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::slow-vector-initialization` implied by `-D warnings`
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:27:5
+   |
+26 |     let mut vec2 = Vec::with_capacity(len - 10);
+   |                    ---------------------------- help: consider replace allocation with: `vec![0; len - 10]`
+27 |     vec2.extend(repeat(0).take(len - 10));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:41:5
+   |
+40 |     let mut resized_vec = Vec::with_capacity(30);
+   |                           ---------------------- help: consider replace allocation with: `vec![0; 30]`
+41 |     resized_vec.resize(30, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:44:5
+   |
+43 |     let mut extend_vec = Vec::with_capacity(30);
+   |                          ---------------------- help: consider replace allocation with: `vec![0; 30]`
+44 |     extend_vec.extend(repeat(0).take(30));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:51:5
+   |
+50 |     let mut vec1 = Vec::with_capacity(len);
+   |                    ----------------------- help: consider replace allocation with: `vec![0; len]`
+51 |     vec1.resize(len, 0);
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:59:5
+   |
+58 |     let mut vec3 = Vec::with_capacity(len - 10);
+   |                    ---------------------------- help: consider replace allocation with: `vec![0; len - 10]`
+59 |     vec3.resize(len - 10, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:63:5
+   |
+62 |     vec1 = Vec::with_capacity(10);
+   |            ---------------------- help: consider replace allocation with: `vec![0; 10]`
+63 |     vec1.resize(10, 0);
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: unsafe vector initialization
+  --> $DIR/slow_vector_initialization.rs:70:9
+   |
+67 |     let mut unsafe_vec: Vec<u8> = Vec::with_capacity(200);
+   |                                   ----------------------- help: consider replace allocation with: `vec![0; 200]`
+...
+70 |         unsafe_vec.set_len(200);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: #[deny(clippy::unsafe_vector_initialization)] on by default
+
+error: aborting due to 8 previous errors
+
diff --git a/tests/ui/slow_vector_initialization.stdout b/tests/ui/slow_vector_initialization.stdout
new file mode 100644
index 00000000000..e69de29bb2d
--- /dev/null
+++ b/tests/ui/slow_vector_initialization.stdout