spec: clarify the difference between &T{} and new(T)

Add a small paragraph and example pointing out
the difference for the case where T is a slice
or map. This is a common error for Go novices.

Fixes #29425.

Change-Id: Icdb59f25361e9f6a09b190fbfcc9ae0c7d90077b
Reviewed-on: https://go-review.googlesource.com/c/go/+/176338
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/go_spec.html b/doc/go_spec.html
index 158146b9c0..dea3afe498 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -2513,11 +2513,25 @@ For array and slice literals the following rules apply:
 generates a pointer to a unique <a href="#Variables">variable</a> initialized
 with the literal's value.
 </p>
+
 <pre>
 var pointer *Point3D = &amp;Point3D{y: 1000}
 </pre>
 
 <p>
+Note that the <a href="#The_zero_value">zero value</a> for a slice or map
+type is not the same as an initialized but empty value of the same type.
+Consequently, taking the address of an empty slice or map composite literal
+does not have the same effect as allocating a new slice or map value with
+<a href="#Allocation">new</a>.
+</p>
+
+<pre>
+p1 := &[]int{}    // p1 points to an initialized, empty slice with value []int{} and length 0
+p2 := new([]int)  // p2 points to an uninitialized slice with value nil and length 0
+</pre>
+
+<p>
 The length of an array literal is the length specified in the literal type.
 If fewer elements than the length are provided in the literal, the missing
 elements are set to the zero value for the array element type.