Auto merge of rust-lang#11310 - y21:slow_vector_initialization_doc, r…

…=xFrednet,djc

[`slow_vector_initialization`]: clarify why `Vec::new()` + resize is worse

rust-lang#11198 extended this lint to also warn on `Vec::new()` + `resize(0, len)`, but did not update the lint documentation, so it left some confused (rust-lang#10938 (comment)).
This PR should make it a bit more clear. (cc `@djc` `@vi` what do you think about this?)

<details>
<summary>More details</summary>

Godbolt for `Vec::new()` + `.resize(x, 0)`: https://godbolt.org/z/e7q9xc9rG

The resize call first does a normal allocation (`__rust_alloc`):
```asm
alloc::raw_vec::finish_grow:
  ...
  cmp     qword ptr [rcx + 8], 0
  je      .LBB1_7  ; if capacity == 0 -> LBB1_7

.LBB1_7:
  ...
  call    qword ptr [rip + __rust_alloc@GOTPCREL]
```

*Then* a memset for zero initialization:
```asm
example::f:
  ...
  xor     esi, esi  ; 0
  call    qword ptr [rip + memset@GOTPCREL]
```
------------

Godbolt for `vec![0; len]`: https://godbolt.org/z/M3vr53vWY

Important bit:
```asm
example::f:
  ...
  call    qword ptr [rip + __rust_alloc_zeroed@GOTPCREL]
```

</details>

changelog: [`slow_vector_initialization`]: clarify why `Vec::new()` + resize is worse than `vec![0; len]`

clippy_lints/src/slow_vector_initialization.rs

@@ -20,25 +20,35 @@ declare_clippy_lint! {
     /// These structures are non-idiomatic and less efficient than simply using
     /// `vec![0; len]`.
     ///
+    /// Specifically, for `vec![0; len]`, the compiler can use a 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.
+    ///
     /// ### Example
     /// ```rust
     /// # use core::iter::repeat;
     /// # let len = 4;
-    /// let mut vec1 = Vec::with_capacity(len);
+    /// let mut vec1 = Vec::new();
     /// vec1.resize(len, 0);
     ///
-    /// let mut vec1 = Vec::with_capacity(len);
-    /// vec1.resize(vec1.capacity(), 0);
-    ///
     /// let mut vec2 = Vec::with_capacity(len);
-    /// vec2.extend(repeat(0).take(len));
+    /// vec2.resize(len, 0);
+    ///
+    /// let mut vec3 = Vec::with_capacity(len);
+    /// vec3.extend(repeat(0).take(len));
     /// ```
     ///
     /// Use instead:
     /// ```rust
     /// # 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,