shader: Check at least one variable on left side of :=

Updates #1330

internal/shader/expr.go

@@ -369,6 +369,13 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr) ([]shaderir.Expr,
 				},
 			}, []shaderir.Type{{Main: shaderir.Bool}}, nil, true
 		}
+		if e.Name == "_" {
+			return []shaderir.Expr{
+				{
+					Type: shaderir.Blank,
+				},
+			}, nil, nil, true
+		}
 		if i, t, ok := block.findLocalVariable(e.Name); ok {
 			return []shaderir.Expr{
 				{

internal/shader/shader.go

@@ -104,6 +104,10 @@ func (b *block) totalLocalVariableNum() int {
 }
 
 func (b *block) findLocalVariable(name string) (int, shaderir.Type, bool) {
+	if name == "" || name == "_" {
+		panic("shader: variable name must be non-empty and non-underscore")
+	}
+
 	idx := 0
 	for outer := b.outer; outer != nil; outer = outer.outer {
 		idx += len(outer.vars)

internal/shader/stmt.go

@@ -444,6 +444,7 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 	var stmts []shaderir.Stmt
 	var rhsExprs []shaderir.Expr
 	var rhsTypes []shaderir.Type
+	allblank := true
 
 	for i, e := range lhs {
 		if len(lhs) == len(rhs) {
@@ -462,9 +463,6 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 						return nil, false
 					}
 				}
-				v := variable{
-					name: name,
-				}
 				ts, ok := cs.functionReturnTypes(block, rhs[i])
 				if !ok {
 					ts = origts
@@ -473,6 +471,10 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 					cs.addError(pos, fmt.Sprintf("single-value context and multiple-value context cannot be mixed"))
 					return nil, false
 				}
+
+				v := variable{
+					name: name,
+				}
 				if len(ts) == 1 {
 					v.typ = ts[0]
 				}
@@ -490,6 +492,11 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 			}
 			stmts = append(stmts, ss...)
 
+			if l[0].Type == shaderir.Blank {
+				continue
+			}
+			allblank = false
+
 			if r[0].Type == shaderir.NumberExpr {
 				t, ok := block.findLocalVariableByIndex(l[0].Index)
 				if !ok {
@@ -572,11 +579,22 @@ func (cs *compileState) assign(block *block, pos token.Pos, lhs, rhs []ast.Expr,
 			}
 			stmts = append(stmts, ss...)
 
+			if l[0].Type == shaderir.Blank {
+				continue
+			}
+			allblank = false
+
 			stmts = append(stmts, shaderir.Stmt{
 				Type:  shaderir.Assign,
 				Exprs: []shaderir.Expr{l[0], rhsExprs[i]},
 			})
 		}
 	}
+
+	if define && allblank {
+		cs.addError(pos, fmt.Sprintf("no new variables on left side of :="))
+		return nil, false
+	}
+
 	return stmts, true
 }

internal/shaderir/program.go

@@ -111,7 +111,8 @@ type Expr struct {
 type ExprType int
 
 const (
-	NumberExpr ExprType = iota
+	Blank ExprType = iota
+	NumberExpr
 	UniformVariable
 	TextureVariable
 	LocalVariable

shader_test.go

@@ -181,3 +181,56 @@ func TestShaderNoMain(t *testing.T) {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+func TestShaderNoNewVariables(t *testing.T) {
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	_ := 1
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	_, _ := 1, 1
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Foo() (int, int) {
+	return 1, 1
+}
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	_, _ := Foo()
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	a, _ := 1, 1
+	_ = a
+}
+`)); err != nil {
+		t.Errorf("error must be nil but non-nil: %v", err)
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	_, a := 1, 1
+	_ = a
+}
+`)); err != nil {
+		t.Errorf("error must be nil but non-nil: %v", err)
+	}
+}