cmd/compile: implement slice-to-array conversions

The conversion T(x) is implemented as *(*T)(x). Accordingly, runtime
panic messages for (*T)(x) are made more general.

Fixes #46505.

Change-Id: I76317c0878b6a5908299506d392eed50d7ef6523
Reviewed-on: https://go-review.googlesource.com/c/go/+/430415
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 23 insertions, 1 deletions

diff --git a/test/convert4.go b/test/convert4.go
index 2bc9c96a52..3cc0aea7be 100644
--- a/test/convert4.go
+++ b/test/convert4.go
@@ -23,25 +23,47 @@ func wantPanic(fn func(), s string) {
 
 func main() {
 	s := make([]byte, 8, 10)
+	for i := range s {
+		s[i] = byte(i)
+	}
 	if p := (*[8]byte)(s); &p[0] != &s[0] {
 		panic("*[8]byte conversion failed")
 	}
+	if [8]byte(s) != *(*[8]byte)(s) {
+		panic("[8]byte conversion failed")
+	}
 	wantPanic(
 		func() {
 			_ = (*[9]byte)(s)
 		},
-		"runtime error: cannot convert slice with length 8 to pointer to array with length 9",
+		"runtime error: cannot convert slice with length 8 to array or pointer to array with length 9",
+	)
+	wantPanic(
+		func() {
+			_ = [9]byte(s)
+		},
+		"runtime error: cannot convert slice with length 8 to array or pointer to array with length 9",
 	)
 
 	var n []byte
 	if p := (*[0]byte)(n); p != nil {
 		panic("nil slice converted to *[0]byte should be nil")
 	}
+	_ = [0]byte(n)
 
 	z := make([]byte, 0)
 	if p := (*[0]byte)(z); p == nil {
 		panic("empty slice converted to *[0]byte should be non-nil")
 	}
+	_ = [0]byte(z)
+
+	var p *[]byte
+	wantPanic(
+		func() {
+			_ = [0]byte(*p) // evaluating *p should still panic
+		},
+		"runtime error: invalid memory address or nil pointer dereference",
+	)
 
 	// Test with named types
 	type Slice []int