[slow_vector_initialization]: only warn on vec![] expn

tests/ui/slow_vector_initialization.rs

@@ -1,5 +1,5 @@
-use std::iter::repeat;
 //@no-rustfix
+use std::iter::repeat;
 fn main() {
     resize_vector();
     extend_vector();
@@ -86,6 +86,20 @@ fn from_empty_vec() {
     vec1 = Vec::new();
     vec1.resize(10, 0);
     //~^ ERROR: slow zero-filling initialization
+
+    vec1 = vec![];
+    vec1.resize(10, 0);
+    //~^ ERROR: slow zero-filling initialization
+
+    macro_rules! x {
+        () => {
+            vec![]
+        };
+    }
+
+    // `vec![]` comes from another macro, don't warn
+    vec1 = x!();
+    vec1.resize(10, 0);
 }
 
 fn do_stuff(vec: &mut [u8]) {}

tests/ui/slow_vector_initialization.stderr

@@ -96,13 +96,21 @@ LL |     vec1 = Vec::new();
 LL |     vec1.resize(10, 0);
    |     ^^^^^^^^^^^^^^^^^^
 
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:91:5
+   |
+LL |     vec1 = vec![];
+   |            ------ help: consider replacing this with: `vec![0; 10]`
+LL |     vec1.resize(10, 0);
+   |     ^^^^^^^^^^^^^^^^^^
+
 error: this argument is a mutable reference, but not used mutably
-  --> $DIR/slow_vector_initialization.rs:91:18
+  --> $DIR/slow_vector_initialization.rs:105:18
    |
 LL | fn do_stuff(vec: &mut [u8]) {}
    |                  ^^^^^^^^^ help: consider changing to: `&[u8]`
    |
    = note: `-D clippy::needless-pass-by-ref-mut` implied by `-D warnings`
 
-error: aborting due to 13 previous errors
+error: aborting due to 14 previous errors