cmd/compile: add Type.SetNumElem

This removes all access to Type.Bound
from outside type.go.

Update sinit to make a new type rather than
copy and mutate.

Update bimport to create a new slice type
instead of mutating TDDDFIELD.
These are rare, so the extra allocs are nominal.

I’m not happy about having a setter,
but it appears the most practical route
forward at the moment, and it only has a few uses.

Passes toolstash -cmp.

Change-Id: I174f07c8f336afc656904bde4bdbde4f3ef0db96
Reviewed-on: https://go-review.googlesource.com/21423
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/bimport.go

@@ -277,15 +277,15 @@ func (p *importer) typ() *Type {
 
 	case arrayTag, sliceTag:
 		t = p.newtyp(TARRAY)
-		t.Bound = sliceBound
 		if i == arrayTag {
-			t.Bound = p.int64()
+			t.SetNumElem(p.int64())
+		} else {
+			t.SetNumElem(sliceBound)
 		}
 		t.Type = p.typ()
 
 	case dddTag:
 		t = p.newtyp(TDDDFIELD)
-		t.Bound = sliceBound
 		t.Type = p.typ()
 
 	case structTag:
@@ -448,9 +448,8 @@ func (p *importer) param(named bool) *Node {
 
 	isddd := false
 	if typ.Etype == TDDDFIELD {
-		// TDDDFIELD indicates ... type
+		// TDDDFIELD indicates wrapped ... slice type
-		// TODO(mdempsky): Fix Type rekinding.
+		typ = typSlice(typ.Wrapped())
-		typ.Etype = TARRAY
 		isddd = true
 	}
 

src/cmd/compile/internal/gc/sinit.go

@@ -688,12 +688,8 @@ func arraylit(ctxt int, pass int, n *Node, var_ *Node, init *Nodes) {
 }
 
 func slicelit(ctxt int, n *Node, var_ *Node, init *Nodes) {
-	// make an array type
+	// make an array type corresponding the number of elements we have
-	t := n.Type.Copy()
+	t := typArray(n.Type.Elem(), n.Right.Int())
-	t.Bound = n.Right.Int()
-	t.Width = 0
-	t.Sym = nil
-	t.Haspointers = 0
 	dowidth(t)
 
 	if ctxt != 0 {

src/cmd/compile/internal/gc/type.go

@@ -936,6 +936,15 @@ func (t *Type) NumElem() int64 {
 	return t.Bound
 }
 
+// SetNumElem sets the number of elements in an array type.
+// It should not be used if at all possible.
+// Create a new array/slice/dddArray with typX instead.
+// TODO(josharian): figure out how to get rid of this.
+func (t *Type) SetNumElem(n int64) {
+	t.wantEtype(TARRAY)
+	t.Bound = n
+}
+
 func (t *Type) IsMemory() bool { return false }
 func (t *Type) IsFlags() bool  { return false }
 func (t *Type) IsVoid() bool   { return false }

src/cmd/compile/internal/gc/typecheck.go

@@ -2977,7 +2977,7 @@ func typecheckcomplit(n *Node) *Node {
 					setlineno(l)
 					Yyerror("array index %d out of bounds [0:%d]", length-1, t.NumElem())
 					// suppress any further errors out of bounds errors for the same type by pretending it is a slice
-					t.Bound = sliceBound
+					t.SetNumElem(sliceBound)
 				}
 			}
 
@@ -2989,7 +2989,7 @@ func typecheckcomplit(n *Node) *Node {
 		}
 
 		if t.isDDDArray() {
-			t.Bound = length
+			t.SetNumElem(length)
 		}
 		if t.IsSlice() {
 			n.Right = Nodintconst(length)