cmd/gc: fix escape analysis for &x inside switch x := v.(type)

The analysis for &x was using the loop depth on x set
during x's declaration. A type switch creates a list of
implicit declarations that were not getting initialized
with loop depths.

Fixes #8176.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/108860043

src/cmd/gc/esc.c

@@ -442,6 +442,18 @@ esc(EscState *e, Node *n, Node *up)
 	if(n->op == OFOR || n->op == ORANGE)
 		e->loopdepth++;
 
+	// type switch variables have no ODCL.
+	// process type switch as declaration.
+	// must happen before processing of switch body,
+	// so before recursion.
+	if(n->op == OSWITCH && n->ntest && n->ntest->op == OTYPESW) {
+		for(ll=n->list; ll; ll=ll->next) {  // cases
+			// ll->n->nname is the variable per case
+			if(ll->n->nname)
+				ll->n->nname->escloopdepth = e->loopdepth;
+		}
+	}
+
 	esc(e, n->left, n);
 	esc(e, n->right, n);
 	esc(e, n->ntest, n);
@@ -658,8 +670,10 @@ esc(EscState *e, Node *n, Node *up)
 		// current loop depth is an upper bound on actual loop depth
 		// of addressed value.
 		n->escloopdepth = e->loopdepth;
-		// for &x, use loop depth of x.
-		if(n->left->op == ONAME) {
+		// for &x, use loop depth of x if known.
+		// it should always be known, but if not, be conservative
+		// and keep the current loop depth.
+		if(n->left->op == ONAME && (n->left->escloopdepth != 0 || n->left->class == PPARAMOUT)) {
 			switch(n->left->class) {
 			case PAUTO:
 			case PPARAM:

test/escape2.go

@@ -1468,3 +1468,13 @@ func foo152() {
 	v := NewV(u)
 	println(v)
 }
+
+// issue 8176 - &x in type switch body not marked as escaping
+
+func foo153(v interface{}) *int { // ERROR "leaking param: v"
+	switch x := v.(type) {
+	case int: // ERROR "moved to heap: x"
+		return &x // ERROR "&x escapes to heap"
+	}
+	panic(0)
+}