doc/go_spec.html: document new(expr)

Also, add a release note.

For #45624

Change-Id: I1a0e111e00885c9640c073000afb72731d0930fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/704737
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

doc/go_spec.html

@@ -7806,12 +7806,32 @@ min(x, y, z) == min(min(x, y), z)
 <h3 id="Allocation">Allocation</h3>
 
 <p>
-The built-in function <code>new</code> takes a type <code>T</code>,
-allocates storage for a <a href="#Variables">variable</a> of that type
-at run time, and returns a value of type <code>*T</code>
-<a href="#Pointer_types">pointing</a> to it.
-The variable is initialized as described in the section on
-<a href="#The_zero_value">initial values</a>.
+  The built-in function <code>new</code> creates a new, initialized
+  <a href="#Variables">variable</a> and returns
+  a <a href="#Pointer_types">pointer</a> to it.
+
+  It accepts a single argument, which may be either an expression or a type.
+</p>
+<p>
+  If the argument <code>expr</code> is an expression of
+  type <code>T</code>, or an untyped constant expression
+  whose <a href="#Constants">default type</a> is <code>T</code>,
+  then <code>new(expr)</code> allocates a variable of
+  type <code>T</code>, initializes it to the value
+  of <code>expr</code>, and returns its address, a value of
+  type <code>*T</code>.
+</p>
+<p>
+  If the argument is a type <code>T</code>, then <code>new(T)</code>
+  allocates a variable initialized to
+  the <a href="#The_zero_value">zero value</a> of type <code>T</code>.
+</p>
+<p>
+  For example, <code>new(123)</code> and <code>new(int)</code> each
+  return a pointer to a new variable of type <code>int</code>.
+
+  The value of the first variable is <code>123</code>, and the value
+  of the second is <code>0</code>.
 </p>
 
 <pre class="grammar">

doc/next/2-language.md

@@ -1,3 +1,28 @@
 ## Changes to the language {#language}
 
+<!-- https://go.dev/issue/45624 --->
 
+The built-in `new` function, which creates a new variable, now allows
+its operand to be an expression, specifying the initial value of the
+variable.
+
+This feature is particularly useful when working with serialization
+packages such as `encoding/json` or protocol buffers that use a
+pointer to represent an optional value, as it enables an optional
+field to be populated in a simple expression, for example:
+
+```go
+import "encoding/json"
+
+type Person struct {
+	Name string   `json:"name"`
+	Age  *int     `json:"age"` // age if known; nil otherwise
+}
+
+func personJSON(name string, age int) ([]byte, error) {
+	return json.Marshal(Person{
+		Name: name,
+		Age:  new(age),
+	})
+}
+```