checker: move more checks from parser

vlib/v/checker/checker.v

@@ -670,8 +670,13 @@ pub fn (c mut Checker) assign_stmt(assign_stmt mut ast.AssignStmt) {
 	c.expected_type = table.none_type	// TODO a hack to make `x := if ... work`
 	// check variablename for beginning with capital letter 'Abc'
 	for ident in assign_stmt.left {
-		if assign_stmt.op == .decl_assign && scanner.contains_capital(ident.name) {
+		is_decl := assign_stmt.op == .decl_assign
+		if is_decl && scanner.contains_capital(ident.name) {
 			c.error('variable names cannot contain uppercase letters, use snake_case instead', ident.pos)
+		} else if is_decl && ident.kind != .blank_ident {
+			if ident.name.starts_with('__') {
+				c.error('variable names cannot start with `__`', ident.pos)
+			}
 		}
 	}
 	if assign_stmt.left.len > assign_stmt.right.len {
@@ -998,6 +1003,9 @@ fn (c mut Checker) stmt(node ast.Stmt) {
 			c.in_for_count--
 		}
 		ast.GoStmt {
+			if !is_call_expr(it.call_expr) {
+				c.error('expression in `go` must be a function call', expr_pos(it.call_expr))1
+			}
 			c.expr(it.call_expr)
 		}
 		// ast.HashStmt {}
@@ -1020,6 +1028,13 @@ fn (c mut Checker) stmt(node ast.Stmt) {
 	}
 }
 
+fn is_call_expr(expr ast.Expr) bool {
+	return match expr {
+		ast.CallExpr { true }
+		else { false }
+	}
+}
+
 fn (c mut Checker) stmts(stmts []ast.Stmt) {
 	c.expected_type = table.void_type
 	for stmt in stmts {

vlib/v/checker/tests/inout/decl_underscore.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/inout/decl_underscore.v:2:2: error: variable names cannot start with `__`
+    1| fn main() {
+    2|     __abc := 1
+           ~~~~~
+    3| }

vlib/v/checker/tests/inout/decl_underscore.vv

@@ -0,0 +1,3 @@
+fn main() {
+	__abc := 1
+}

vlib/v/checker/tests/inout/go_expr.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/inout/go_expr.v:2:5: error: expression in `go` must be a function call
+    1| fn main() {
+    2|     go 1
+              ^
+    3| }

vlib/v/checker/tests/inout/go_expr.vv

@@ -0,0 +1,3 @@
+fn main() {
+	go 1
+}

vlib/v/parser/parser.v

@@ -242,8 +242,7 @@ fn (p mut Parser) check(expected token.Kind) {
 	// p.next()
 	// }
 	if p.tok.kind != expected {
-		s := 'unexpected `${p.tok.kind.str()}`, expecting `${expected.str()}`'
-		p.error(s)
+		p.error('unexpected `${p.tok.kind.str()}`, expecting `${expected.str()}`')
 	}
 	p.next()
 }
@@ -420,9 +419,7 @@ pub fn (p mut Parser) stmt() ast.Stmt {
 				ast.CallExpr {
 					// call_expr = it
 				}
-				else {
-					p.error('expression in `go` must be a function call')
-				}
+				else {}
 			}
 			return ast.GoStmt{
 				call_expr: expr
@@ -1808,9 +1805,6 @@ fn (p mut Parser) assign_stmt() ast.Stmt {
 			p.error('unknown variable `$ident.name`')
 		}
 		if is_decl && ident.kind != .blank_ident {
-			if ident.name.starts_with('__') {
-				p.error('variable names cannot start with `__`')
-			}
 			if p.scope.known_var(ident.name) {
 				p.error('redefinition of `$ident.name`')
 			}