clarify why Vec::new() + resize is worse than vec![0; N]

clippy_lints/src/slow_vector_initialization.rs

@@ -20,6 +20,20 @@ declare_clippy_lint! {
     /// These structures are non-idiomatic and less efficient than simply using
     /// `vec![0; len]`.
     ///
+    /// More specifically, for `vec![0; len]`, the compiler can use a more specialized type of allocation
+    /// that also zero-initializes the allocated memory in the same call
+    /// (see: [alloc_zeroed](https://doc.rust-lang.org/stable/std/alloc/trait.GlobalAlloc.html#method.alloc_zeroed)).
+    ///
+    /// Writing `Vec::new()` followed by `vec.resize(len, 0)` is suboptimal because,
+    /// while it does do the same number of allocations,
+    /// it involves two operations for allocating and initializing.
+    /// The `resize` call first allocates memory (since `Vec::new()` did not), and only *then* zero-initializes it.
+    ///
+    /// Writing `Vec::with_capacity(size)` followed by `vec.resize(len, 0)` is similar.
+    /// The allocation shifts from `resize` to `with_capacity`,
+    /// but the zero-initialization still happens separately,
+    /// when it could be done in one call with `vec![0; len]` (`alloc_zeroed`).
+    ///
     /// ### Example
     /// ```rust
     /// # use core::iter::repeat;
@@ -32,6 +46,9 @@ declare_clippy_lint! {
     ///
     /// let mut vec2 = Vec::with_capacity(len);
     /// vec2.extend(repeat(0).take(len));
+    ///
+    /// let mut vec3 = Vec::new();
+    /// vec3.resize(len, 0);
     /// ```
     ///
     /// Use instead:
@@ -39,6 +56,7 @@ declare_clippy_lint! {
     /// # let len = 4;
     /// let mut vec1 = vec![0; len];
     /// let mut vec2 = vec![0; len];
+    /// let mut vec3 = vec![0; len];
     /// ```
     #[clippy::version = "1.32.0"]
     pub SLOW_VECTOR_INITIALIZATION,