slices: improve Insert panic message for index out of range

The panic message of the current implementation for index out of range is not ideal.
This PR tries to improve it.

Fixes #63913 and #64152

Change-Id: Ibcf6c9c0f555c8b8bf46b7d6f20f0ccc5698acd4
GitHub-Last-Rev: 1bbec230f4b0ec547d21c8d38dd46007bd282855
GitHub-Pull-Request: golang/go#64163
Reviewed-on: https://go-review.googlesource.com/c/go/+/542455
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>

diff --git a/src/slices/slices.go b/src/slices/slices.go
index 38b0fc14ad406a7a1c54345e5932cf97dcb4e001..b0f048a6566014875edd80606d12cb4b82fce2ce 100644 (file)
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -130,14 +130,13 @@ func ContainsFunc[S ~[]E, E any](s S, f func(E) bool) bool {
 // Insert panics if i is out of range.
 // This function is O(len(s) + len(v)).
 func Insert[S ~[]E, E any](s S, i int, v ...E) S {
-       n := len(s)
+       _ = s[i:] // bounds check
+
        m := len(v)
        if m == 0 {
-               // Panic if i is not in the range [0:n] inclusive.
-               // See issue 63913.
-               _ = s[:n:n][i:]
                return s
        }
+       n := len(s)
        if i == n {
                return append(s, v...)
        }