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doc: document change in nil-ptr checking behavior

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This could bite people during the 1.25 release, so make sure it
has good documentation in the release notes.

Update #72860

Change-Id: Ie9aaa219025a631e81ebc48461555c5fb898f43f
Reviewed-on: https://go-review.googlesource.com/c/go/+/658955
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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Keith Randall 2025-03-18 14:04:26 -07:00 committed by Gopher Robot
parent bfb27fb36f
commit 3309658d39
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29
doc/next/5-toolchain.md
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@ -9,6 +9,35 @@ information in Go binaries.
DWARF 5 generation is gated by the "dwarf5" GOEXPERIMENT; this
functionality can be disabled (for now) using GOEXPERIMENT=nodwarf5.
<!-- https://go.dev/issue/72860, CL 657715 -->
The compiler [has been fixed](/cl/657715)
to ensure that nil pointer checks are performed promptly. Programs like the following,
which used to execute successfully, will now panic with a nil-pointer exception:
```
package main
import "os"
func main() {
f, err := os.Open("nonExistentFile")
name := f.Name()
if err != nil {
return
}
println(name)
}
```
This program is incorrect in that it uses the result of `os.Open` before checking
the error. The main result of `os.Open` can be a nil pointer if the error result is non-nil.
But because of [a compiler bug](/issue/72860), this program ran successfully under
Go versions 1.21 through 1.24 (in violation of the Go spec). It will no longer run
successfully in Go 1.25. If this change is affecting your code, the solution is to put
the non-nil error check earlier in your code, preferrably immediately after
the error-generating statement.
## Assembler {#assembler}
## Linker {#linker}