doc/go1.15: mention vet warning for string(x)
For #32479
Change-Id: I974709d471021d370aa9bdc5f24b02afa8bd9b54
Reviewed-on: https://go-review.googlesource.com/c/go/+/234517
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index af0b3ba..7271755 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -116,6 +116,47 @@
<code>GODEBUG=modcacheunzipinplace=1</code>.
</p>
+<h3 id="vet">Vet</h3>
+
+<h4 id="vet-string-int">New warning for string(x)</h4>
+
+<p><!-- CL 212919, 232660 -->
+ The vet tool now warns about conversions of the
+ form <code>string(x)</code> where <code>x</code> has an integer type
+ other than <code>rune</code> or <code>byte</code>.
+ Experience with Go has shown that many conversions of this form
+ erroneously assume that <code>string(x)</code> evaluates to the
+ string representation of the integer <code>x</code>.
+ It actually evaluates to a string containing the UTF-8 encoding of
+ the value of <code>x</code>.
+ For example, <code>string(9786)</code> does not evaluate to the
+ string <code>"9786"</code>; it evaluates to the
+ string <code>"\xe2\x98\xba"</code>, or <code>"☺"</code>.
+</p>
+
+<p>
+ Code that is using <code>string(x)</code> correctly can be rewritten
+ to <code>string(rune(x))</code>.
+ Or, in some cases, calling <code>utf8.EncodeRune(buf, x)</code> with
+ a suitable byte slice <code>buf</code> may be the right solution.
+ Other code should most likely use <code>strconv.Itoa</code>
+ or <code>fmt.Sprint</code>.
+</p>
+
+<p>
+ This new vet check is enabled by default when using <code>go test</code>.
+</p>
+
+<p>
+ We are considering prohibiting the conversion in a future release of Go.
+ That is, the language would change to only
+ permit <code>string(x)</code> for integer <code>x</code> when the
+ type of <code>x</code> is <code>rune</code> or <code>byte</code>.
+ Such a language change would not be backward compatible.
+ We are using this vet check as a first trial step toward changing
+ the language.
+</p>
+
<h2 id="runtime">Runtime</h2>
<p>
