doc: document change in nil-ptr checking behavior

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 <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: randall77

doc/next/5-toolchain.md

@@ -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}