Lint only the first statment/expression after alloc

Instead of searching for all the successive expressions after a vector
allocation, check only the first expression.

This is done to minimize the amount of false positives of the lint.

tests/ui/slow_vector_initialization.rs

@@ -34,13 +34,13 @@ fn extend_vector() {
 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);
-    let mut extend_vec = Vec::with_capacity(30);
-
     resized_vec.resize(30, 0);
-    mismatching_len.extend(repeat(0).take(40));
+
+    let mut extend_vec = Vec::with_capacity(30);
     extend_vec.extend(repeat(0).take(30));
 }
 
@@ -70,3 +70,14 @@ fn unsafe_vector() {
         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));
+}

tests/ui/slow_vector_initialization.stderr

@@ -17,20 +17,18 @@ error: slow zero-filling initialization
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:42:5
+  --> $DIR/slow_vector_initialization.rs:41:5
    |
-39 |     let mut resized_vec = Vec::with_capacity(30);
+40 |     let mut resized_vec = Vec::with_capacity(30);
    |                           ---------------------- help: consider replace allocation with: `vec![0; 30]`
-...
-42 |     resized_vec.resize(30, 0);
+41 |     resized_vec.resize(30, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
   --> $DIR/slow_vector_initialization.rs:44:5
    |
-40 |     let mut extend_vec = Vec::with_capacity(30);
+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));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^