cmd/compile: disallow converting string to notinheap slice

Unlikely to happen in practice, but easy enough to prevent and might
as well do so for completeness.

Fixes #28243.

Change-Id: I848c3af49cb923f088e9490c6a79373e182fad08
Reviewed-on: https://go-review.googlesource.com/c/142719
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/test/notinheap.go b/test/notinheap.go
index 44b79646ef..16c3f8faf0 100644
--- a/test/notinheap.go
+++ b/test/notinheap.go
@@ -46,10 +46,18 @@ type t1 struct{ x int }
 //go:notinheap
 type t2 t1
 
+//go:notinheap
+type t3 byte
+
+//go:notinheap
+type t4 rune
+
 var sink interface{}
 
 func i() {
 	sink = new(t1)                     // no error
 	sink = (*t2)(new(t1))              // ERROR "cannot convert(.|\n)*t2 is go:notinheap"
 	sink = (*t2)(new(struct{ x int })) // ERROR "cannot convert(.|\n)*t2 is go:notinheap"
+	sink = []t3("foo")                 // ERROR "cannot convert(.|\n)*t3 is go:notinheap"
+	sink = []t4("bar")                 // ERROR "cannot convert(.|\n)*t4 is go:notinheap"
 }