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doc/go1.15: mention vet warning for string(x)

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For #32479

Change-Id: I974709d471021d370aa9bdc5f24b02afa8bd9b54
Reviewed-on: https://go-review.googlesource.com/c/go/+/234517
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Ian Lance Taylor 2020-05-18 15:33:29 -07:00
parent 3b0882e838
commit b0bc18d5bc
1 changed files with 41 additions and 0 deletions
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@ -116,6 +116,47 @@ TODO
<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>