cmd/compile: allow conversion from slice to array ptr

Panic if the slice is too short.

Updates #395

Change-Id: I90f4bff2da5d8f3148ba06d2482084f32b25c29a
Reviewed-on: https://go-review.googlesource.com/c/go/+/301650
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

3 files changed, 104 insertions, 0 deletions

diff --git a/test/convert2.go b/test/convert2.go
index e7044b2453..8e43967aaa 100644
--- a/test/convert2.go
+++ b/test/convert2.go
@@ -313,3 +313,17 @@ func _() {
 	t = u       // ERROR "cannot use .* in assignment|incompatible type"
 	t = (*T)(u) // ERROR "cannot convert"
 }
+
+func _() {
+	var s []byte
+	_ = ([4]byte)(s) // ERROR "cannot convert"
+	_ = (*[4]byte)(s)
+
+	type A [4]byte
+	_ = (A)(s) // ERROR "cannot convert"
+	_ = (*A)(s)
+
+	type P *[4]byte
+	_ = (P)(s)
+	_ = (*P)(s) // ERROR "cannot convert"
+}
diff --git a/test/convert4.go b/test/convert4.go
new file mode 100644
index 0000000000..2bc9c96a52
--- /dev/null
+++ b/test/convert4.go
@@ -0,0 +1,86 @@
+// run
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test conversion from slice to array pointer.
+
+package main
+
+func wantPanic(fn func(), s string) {
+	defer func() {
+		err := recover()
+		if err == nil {
+			panic("expected panic")
+		}
+		if got := err.(error).Error(); got != s {
+			panic("expected panic " + s + " got " + got)
+		}
+	}()
+	fn()
+}
+
+func main() {
+	s := make([]byte, 8, 10)
+	if p := (*[8]byte)(s); &p[0] != &s[0] {
+		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",
+	)
+
+	var n []byte
+	if p := (*[0]byte)(n); p != nil {
+		panic("nil slice converted to *[0]byte should be nil")
+	}
+
+	z := make([]byte, 0)
+	if p := (*[0]byte)(z); p == nil {
+		panic("empty slice converted to *[0]byte should be non-nil")
+	}
+
+	// Test with named types
+	type Slice []int
+	type Int4 [4]int
+	type PInt4 *[4]int
+	ii := make(Slice, 4)
+	if p := (*Int4)(ii); &p[0] != &ii[0] {
+		panic("*Int4 conversion failed")
+	}
+	if p := PInt4(ii); &p[0] != &ii[0] {
+		panic("PInt4 conversion failed")
+	}
+}
+
+// test static variable conversion
+
+var (
+	ss  = make([]string, 10)
+	s5  = (*[5]string)(ss)
+	s10 = (*[10]string)(ss)
+
+	ns  []string
+	ns0 = (*[0]string)(ns)
+
+	zs  = make([]string, 0)
+	zs0 = (*[0]string)(zs)
+)
+
+func init() {
+	if &ss[0] != &s5[0] {
+		panic("s5 conversion failed")
+	}
+	if &ss[0] != &s10[0] {
+		panic("s5 conversion failed")
+	}
+	if ns0 != nil {
+		panic("ns0 should be nil")
+	}
+	if zs0 == nil {
+		panic("zs0 should not be nil")
+	}
+}
diff --git a/test/escape_slice.go b/test/escape_slice.go
index 6ce852e9c5..d60414736c 100644
--- a/test/escape_slice.go
+++ b/test/escape_slice.go
@@ -101,6 +101,10 @@ func slice11() {
 	_ = s
 }
 
+func slice12(x []int) *[1]int { // ERROR "leaking param: x to result ~r1 level=0$"
+	return (*[1]int)(x)
+}
+
 func envForDir(dir string) []string { // ERROR "dir does not escape"
 	env := os.Environ()
 	return mergeEnvLists([]string{"PWD=" + dir}, env) // ERROR ".PWD=. \+ dir escapes to heap" "\[\]string{...} does not escape"