Why Go 1.22’s New Loop Variable Scoping Fix Solves Common Closure Bugs

Go 1.22 Changes to for‑loop Variable Scope

In Go 1.22 the language specification was updated so that the loop variable in a for statement is re‑declared for each iteration. Earlier versions kept a single variable for the whole loop, which caused subtle bugs when the variable was captured by closures, especially in concurrent code.

What the change fixes

The previous behaviour meant that all closures created inside a loop would reference the same variable, leading to repeated or unexpected values when the closures were later executed. This issue affected many Go programs, from beginner code to production services such as Let’s Encrypt, which lost three million certificates in 2020 due to an incorrect loop handling.

Example before Go 1.22

for i := 0; i < 5; i++ {
    prints = append(prints, func() { println(i) })
    i++
}
for _, p := range prints {
    p()
}

Running the above on Go 1.21 or earlier prints:

6
6
6
1
2
3
6
6
6

Behavior in Go 1.22

With the new scoping rule each closure captures the value of i at the moment of the iteration, producing the expected sequence:

1
3
5
1
2
3
1
3
5

Simplified range syntax

Go 1.22 also introduces a more concise range form that can replace the traditional C‑style loop:

(i = 0; i < 5; i++)

Now you can write:

for i := range(5)

This iterates five times, yielding values 0‑4, and reduces boilerplate in benchmarks and repetitive code.

Enabling the experimental iterator feature

The new iterator capability can be turned on with the GOEXPERIMENT=rangefunc environment variable, allowing any function to be used as an iterator, similar to generators in other languages.

GOEXPERIMENT=rangefunc

Developers have praised the change for making concurrent testing and closure‑based patterns easier to write without extra linter work.

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