cmd/compile: avoid assignment conversion in append(a, b...)

There's no need for a and b to match types. The typechecker already
ensured that a and b are both slices with the same base type, or
a and b are (possibly named) []byte and string.

The optimization to treat append(b, make([], ...)) as a zeroing
slice extension doesn't fire when there's a OCONVNOP wrapping the make.
Fixes #53888

Change-Id: Ied871ed0bbb8e4a4b35d280c71acbab8103691bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/418475
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index ddbccf4ff4..50254e694a 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -730,11 +730,11 @@ func transformAppend(n *ir.CallExpr) ir.Node {
 	assert(t.IsSlice())
 
 	if n.IsDDD {
-		if t.Elem().IsKind(types.TUINT8) && args[1].Type().IsString() {
-			return n
-		}
-
-		args[1] = assignconvfn(args[1], t.Underlying())
+		// assignconvfn is of args[1] not required here, as the
+		// types of args[0] and args[1] don't need to match
+		// (They will both have an underlying type which are
+		// slices of indentical base types, or be []byte and string.)
+		// See issue 53888.
 		return n
 	}