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doc: document new iteration variable semantics in spec

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For #56010.

Change-Id: Icca987a03d80587dd0e901f596ff7788584893ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/551095
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
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Robert Griesemer 2023-12-18 17:09:18 -08:00 committed by Gopher Robot
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@ -6541,7 +6541,6 @@ additionally it may specify an <i>init</i>
and a <i>post</i> statement, such as an assignment,
an increment or decrement statement. The init statement may be a
<a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
Variables declared by the init statement are re-used in each iteration.
</p>
<pre class="ebnf">
@ -6573,6 +6572,48 @@ for cond { S() } is the same as for ; cond ; { S() }
for { S() } is the same as for true { S() }
</pre>
<p>
Each iteration has its own separate declared variable (or variables)
[<a href="#Go_1.22">Go 1.22</a>].
The variable used by the first iteration is declared by the init statement.
The variable used by each subsequent iteration is declared implicitly before
executing the post statement and initialized to the value of the previous
iteration's variable at that moment.
</p>
<pre>
var prints []func()
for i := 0; i < 5; i++ {
prints = append(prints, func() { println(i) })
i++
}
for _, p := range prints {
p()
}
</pre>
<p>
prints
</p>
<pre>
0
3
5
</pre>
<p>
Prior to [<a href="#Go_1.22">Go 1.22</a>], iterations share one set of variables
instead of having their own separate variables.
In that case, the example above prints
</p>
<pre>
6
6
6
</pre>
<h4 id="For_range">For statements with <code>range</code> clause</h4>
<p>
@ -6677,9 +6718,10 @@ The iteration variables may be declared by the "range" clause using a form of
<a href="#Short_variable_declarations">short variable declaration</a>
(<code>:=</code>).
In this case their types are set to the types of the respective iteration values
and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
statement; they are re-used in each iteration.
If the iteration variables are declared outside the "for" statement,
and their <a href="#Declarations_and_scope">scope</a> is the block of the "for" statement;
each iteration has its own separate variables [<a href="#Go_1.22">Go 1.22</a>]
(see also <a href="#For_clause">"for" statements with a ForClause</a>).
If the iteration variables are declared outside the “for” statement,
after execution their values will be those of the last iteration.
</p>
@ -8550,7 +8592,11 @@ passed as arguments to other (possibly generic) functions.
<h4 id="Go_1.22">Go 1.22</h4>
<ul>
<li>
A <a href="#For_range">"for" statement with a "range" clause</a> may iterate over
In a <a href="#For_statements">"for" statement</a>, each iteration has its own set of iteration
variables rather than sharing the same variables in each iteration.
</li>
<li>
A "for" statement with <a href="#For_range">"range" clause</a> may iterate over
integer values from zero to an upper limit.
</li>
</ul>