strings: add a test case of growLen is negative

Before committing,  the test coverage of strings/builder.go is 97.4%
After committing,  the test coverage of strings/builder.go is 100%

Change-Id: I22643b1c4632b5ca7ef98362f32bb85faae80bad
GitHub-Last-Rev: 2a55ca3e33d3aabd2ccc047de580abcfe05d6bb4
GitHub-Pull-Request: golang/go#55004
Reviewed-on: https://go-review.googlesource.com/c/go/+/430156
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>

diff --git a/src/strings/builder_test.go b/src/strings/builder_test.go
index e3d239266fd959aa9e4a4f38ec8cec7d43a0e506..dbc2c1943b5940dba10a2e7041881b40867bc8b5 100644 (file)
--- a/src/strings/builder_test.go
+++ b/src/strings/builder_test.go
@@ -109,6 +109,15 @@ func TestBuilderGrow(t *testing.T) {
                        t.Errorf("growLen=%d: got %d allocs during Write; want %v", growLen, g, w)
                }
        }
+       // when growLen < 0, should panic
+       var a Builder
+       n := -1
+       defer func() {
+               if r := recover(); r == nil {
+                       t.Errorf("a.Grow(%d) should panic()", n)
+               }
+       }()
+       a.Grow(n)
 }
 
 func TestBuilderWrite2(t *testing.T) {