shader: Implement swapping variables

Fixes #1248

internal/shader/stmt.go

@@ -42,7 +42,6 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 			}
 			stmts = append(stmts, ss...)
 		case token.ASSIGN:
-			// TODO: What about the statement `a,b = b,a?`
 			if len(stmt.Lhs) != len(stmt.Rhs) && len(stmt.Rhs) != 1 {
 				cs.addError(stmt.Pos(), fmt.Sprintf("single-value context and multiple-value context cannot be mixed"))
 				return nil, false
@@ -498,10 +497,39 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 				}
 			}
 
+			if len(lhs) == 1 {
 				stmts = append(stmts, shaderir.Stmt{
 					Type:  shaderir.Assign,
 					Exprs: []shaderir.Expr{l[0], r[0]},
 				})
+			} else {
+				// For variable swapping, use temporary variables.
+				block.vars = append(block.vars, variable{
+					typ: origts[0],
+				})
+				idx := len(block.vars) - 1
+				stmts = append(stmts,
+					shaderir.Stmt{
+						Type: shaderir.Assign,
+						Exprs: []shaderir.Expr{
+							{
+								Type:  shaderir.LocalVariable,
+								Index: idx,
+							},
+							r[0],
+						},
+					},
+					shaderir.Stmt{
+						Type: shaderir.Assign,
+						Exprs: []shaderir.Expr{
+							l[0],
+							{
+								Type:  shaderir.LocalVariable,
+								Index: idx,
+							},
+						},
+					})
+			}
 		} else {
 			if i == 0 {
 				var ss []shaderir.Stmt

internal/shader/testdata/issue1248.expected.vs

@@ -0,0 +1,26 @@
+void F0(out vec2 l0);
+
+void F0(out vec2 l0) {
+	vec2 l1 = vec2(0);
+	vec2 l2 = vec2(0);
+	vec2 l3 = vec2(0);
+	vec2 l4 = vec2(0);
+	vec2 l5 = vec2(0);
+	vec2 l6 = vec2(0);
+	vec2 l7 = vec2(0);
+	vec2 l8 = vec2(0);
+	vec2 l9 = vec2(0);
+	vec2 l10 = vec2(0);
+	l3 = l2;
+	l1 = l3;
+	l4 = l1;
+	l2 = l4;
+	l8 = l6;
+	l5 = l8;
+	l9 = l7;
+	l6 = l9;
+	l10 = l5;
+	l7 = l10;
+	l0 = l1;
+	return;
+}

internal/shader/testdata/issue1248.go

@@ -0,0 +1,9 @@
+package main
+
+func Foo() vec2 {
+	var a, b vec2
+	a, b = b, a
+	var c, d, e vec2
+	c, d, e = d, e, c
+	return a
+}