internal/shader: bug fix: variables in an index should be marked as used

Closes #2848

internal/shader/expr.go

@@ -913,7 +913,7 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 		var stmts []shaderir.Stmt
 
 		// Parse the index first
-		exprs, _, ss, ok := cs.parseExpr(block, fname, e.Index, markLocalVariableUsed)
+		exprs, _, ss, ok := cs.parseExpr(block, fname, e.Index, true)
 		if !ok {
 			return nil, nil, nil, false
 		}

internal/shader/syntax_test.go

@@ -514,6 +514,21 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 `)); err == nil {
 		t.Errorf("error must be non-nil but was nil")
 	}
+
+	// Issue #2848
+	if _, err := compileToIR([]byte(`package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	var floats [4]float
+	for i := 0; i < 3; i++ {
+		j := i + 1
+		floats[j] = float(i)
+	}
+	return vec4(floats[0], floats[1], floats[2], floats[3])
+}
+`)); err != nil {
+		t.Error(err)
+	}
 }
 
 func TestSyntaxBlankLhs(t *testing.T) {