cmd/compile/internal/syntax: remove need for missing_statement (fixed TODO)

Now that we have consistent use of xOrNil parse methods, we don't
need a special missing_statement singleton to distinguish between
missing actually statements and other errors (which have returned
nil before).

For #19663.

Change-Id: I8364f1441bdf8dd966bcd6d8219b2a42d6b88abd
Reviewed-on: https://go-review.googlesource.com/38656
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index 840648683a190725d825c5142ef14fcba716d00e..e55a2219d6d9cf9fd2201f607891f805f048eb74 100644 (file)
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -1494,8 +1494,6 @@ func (p *parser) bad() *BadExpr {
 var ImplicitOne = &BasicLit{Value: "1"}
 
 // SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
-//
-// simpleStmt may return missing_stmt if labelOk is set.
 func (p *parser) simpleStmt(lhs Expr, rangeOk bool) SimpleStmt {
        if trace {
                defer p.trace("simpleStmt")()
@@ -1627,7 +1625,7 @@ func (p *parser) newAssignStmt(pos src.Pos, op Operator, lhs, rhs Expr) *AssignS
        return a
 }
 
-func (p *parser) labeledStmt(label *Name) Stmt {
+func (p *parser) labeledStmtOrNil(label *Name) Stmt {
        if trace {
                defer p.trace("labeledStmt")()
        }
@@ -1638,17 +1636,25 @@ func (p *parser) labeledStmt(label *Name) Stmt {
 
        p.want(_Colon)
 
-       if p.tok != _Rbrace && p.tok != _EOF {
-               s.Stmt = p.stmt()
-               if s.Stmt == missing_stmt {
-                       // report error at line of ':' token
-                       p.syntax_error_at(label.Pos(), "missing statement after label")
-                       // we are already at the end of the labeled statement - no need to advance
-                       return missing_stmt
-               }
+       if p.tok == _Rbrace {
+               // We expect a statement (incl. an empty statement), which must be
+               // terminated by a semicolon. Because semicolons may be omitted before
+               // an _Rbrace, seeing an _Rbrace implies an empty statement.
+               e := new(EmptyStmt)
+               e.pos = p.pos()
+               s.Stmt = e
+               return s
        }
 
-       return s
+       s.Stmt = p.stmtOrNil()
+       if s.Stmt != nil {
+               return s
+       }
+
+       // report error at line of ':' token
+       p.syntax_error_at(s.pos, "missing statement after label")
+       // we are already at the end of the labeled statement - no need to advance
+       return nil // avoids follow-on errors (see e.g., fixedbugs/bug274.go)
 }
 
 func (p *parser) blockStmt(context string) *BlockStmt {
@@ -1922,17 +1928,12 @@ func (p *parser) commClause() *CommClause {
        return c
 }
 
-// TODO(gri) find a better solution
-var missing_stmt Stmt = new(EmptyStmt) // = nod(OXXX, nil, nil)
-
 // Statement =
 //     Declaration | LabeledStmt | SimpleStmt |
 //     GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
 //     FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
 //     DeferStmt .
-//
-// stmt may return missing_stmt.
-func (p *parser) stmt() Stmt {
+func (p *parser) stmtOrNil() Stmt {
        if trace {
                defer p.trace("stmt " + p.tok.String())()
        }
@@ -1942,7 +1943,7 @@ func (p *parser) stmt() Stmt {
        if p.tok == _Name {
                lhs := p.exprList()
                if label, ok := lhs.(*Name); ok && p.tok == _Colon {
-                       return p.labeledStmt(label)
+                       return p.labeledStmtOrNil(label)
                }
                return p.simpleStmt(lhs, false)
        }
@@ -2026,7 +2027,7 @@ func (p *parser) stmt() Stmt {
                return s
        }
 
-       return missing_stmt
+       return nil
 }
 
 // StatementList = { Statement ";" } .
@@ -2036,8 +2037,8 @@ func (p *parser) stmtList() (l []Stmt) {
        }
 
        for p.tok != _EOF && p.tok != _Rbrace && p.tok != _Case && p.tok != _Default {
-               s := p.stmt()
-               if s == missing_stmt {
+               s := p.stmtOrNil()
+               if s == nil {
                        break
                }
                l = append(l, s)