[dev.typeparams] cmd/compile: use nil instead of syntax.ImplicitOne

Represent x++/-- as x +=/-= with the RHS of the assignment being nil
rather than syntax.ImplicitOne.

Dependent code already had to check for syntax.ImplicitOne, but
then shared some existing code for regular assignment operations.
Now always handle this case fully explicit, which simplifies the
code.

Change-Id: I28c7918153c27cbbf97b041d0c85ff027c58687c
Reviewed-on: https://go-review.googlesource.com/c/go/+/285172
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

10 files changed, 48 insertions, 53 deletions

diff --git a/src/cmd/compile/internal/noder/expr.go b/src/cmd/compile/internal/noder/expr.go
index be592003e1..76db774229 100644
--- a/src/cmd/compile/internal/noder/expr.go
+++ b/src/cmd/compile/internal/noder/expr.go
@@ -19,10 +19,6 @@ func (g *irgen) expr(expr syntax.Expr) ir.Node {
 		return nil
 	}
 
-	if expr == syntax.ImplicitOne {
-		base.Fatalf("expr of ImplicitOne")
-	}
-
 	if expr, ok := expr.(*syntax.Name); ok && expr.Value == "_" {
 		return ir.BlankNode
 	}
diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 0c7d015977..e1ae2569e0 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -677,11 +677,7 @@ func (p *noder) expr(expr syntax.Expr) ir.Node {
 	case *syntax.Name:
 		return p.mkname(expr)
 	case *syntax.BasicLit:
-		pos := base.Pos
-		if expr != syntax.ImplicitOne { // ImplicitOne doesn't have a unique position
-			pos = p.pos(expr)
-		}
-		n := ir.NewBasicLit(pos, p.basicLit(expr))
+		n := ir.NewBasicLit(p.pos(expr), p.basicLit(expr))
 		if expr.Kind == syntax.RuneLit {
 			n.SetType(types.UntypedRune)
 		}
@@ -1039,9 +1035,15 @@ func (p *noder) stmtFall(stmt syntax.Stmt, fallOK bool) ir.Node {
 	case *syntax.DeclStmt:
 		return ir.NewBlockStmt(src.NoXPos, p.decls(stmt.DeclList))
 	case *syntax.AssignStmt:
+		if stmt.Rhs == nil {
+			pos := p.pos(stmt)
+			n := ir.NewAssignOpStmt(pos, p.binOp(stmt.Op), p.expr(stmt.Lhs), ir.NewBasicLit(pos, one))
+			n.IncDec = true
+			return n
+		}
+
 		if stmt.Op != 0 && stmt.Op != syntax.Def {
 			n := ir.NewAssignOpStmt(p.pos(stmt), p.binOp(stmt.Op), p.expr(stmt.Lhs), p.expr(stmt.Rhs))
-			n.IncDec = stmt.Rhs == syntax.ImplicitOne
 			return n
 		}
 
@@ -1502,7 +1504,7 @@ func (p *noder) wrapname(n syntax.Node, x ir.Node) ir.Node {
 }
 
 func (p *noder) setlineno(n syntax.Node) {
-	if n != nil && n != syntax.ImplicitOne {
+	if n != nil {
 		base.Pos = p.pos(n)
 	}
 }
diff --git a/src/cmd/compile/internal/noder/stmt.go b/src/cmd/compile/internal/noder/stmt.go
index 7d79595a04..267a34dbc8 100644
--- a/src/cmd/compile/internal/noder/stmt.go
+++ b/src/cmd/compile/internal/noder/stmt.go
@@ -53,7 +53,7 @@ func (g *irgen) stmt0(stmt syntax.Stmt) ir.Node {
 	case *syntax.AssignStmt:
 		if stmt.Op != 0 && stmt.Op != syntax.Def {
 			op := g.op(stmt.Op, binOps[:])
-			if stmt.Rhs == syntax.ImplicitOne {
+			if stmt.Rhs == nil {
 				return IncDec(g.pos(stmt), op, g.expr(stmt.Lhs))
 			}
 			return ir.NewAssignOpStmt(g.pos(stmt), op, g.expr(stmt.Lhs), g.expr(stmt.Rhs))
diff --git a/src/cmd/compile/internal/syntax/nodes.go b/src/cmd/compile/internal/syntax/nodes.go
index a06d6e85b1..fb9786daa3 100644
--- a/src/cmd/compile/internal/syntax/nodes.go
+++ b/src/cmd/compile/internal/syntax/nodes.go
@@ -367,7 +367,7 @@ type (
 
 	AssignStmt struct {
 		Op       Operator // 0 means no operation
-		Lhs, Rhs Expr     // Rhs == ImplicitOne means Lhs++ (Op == Add) or Lhs-- (Op == Sub)
+		Lhs, Rhs Expr     // Rhs == nil means Lhs++ (Op == Add) or Lhs-- (Op == Sub)
 		simpleStmt
 	}
 
diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index e3fb1003a2..c4ccbb82cb 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -1874,10 +1874,6 @@ func (p *parser) badExpr() *BadExpr {
 // ----------------------------------------------------------------------------
 // Statements
 
-// We represent x++, x-- as assignments x += ImplicitOne, x -= ImplicitOne.
-// ImplicitOne should not be used elsewhere.
-var ImplicitOne = &BasicLit{Value: "1"}
-
 // SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
 func (p *parser) simpleStmt(lhs Expr, keyword token) SimpleStmt {
 	if trace {
@@ -1910,7 +1906,7 @@ func (p *parser) simpleStmt(lhs Expr, keyword token) SimpleStmt {
 			// lhs++ or lhs--
 			op := p.op
 			p.next()
-			return p.newAssignStmt(pos, op, lhs, ImplicitOne)
+			return p.newAssignStmt(pos, op, lhs, nil)
 
 		case _Arrow:
 			// lhs <- rhs
diff --git a/src/cmd/compile/internal/syntax/printer.go b/src/cmd/compile/internal/syntax/printer.go
index 161eb0d092..9109ce2363 100644
--- a/src/cmd/compile/internal/syntax/printer.go
+++ b/src/cmd/compile/internal/syntax/printer.go
@@ -549,7 +549,7 @@ func (p *printer) printRawNode(n Node) {
 
 	case *AssignStmt:
 		p.print(n.Lhs)
-		if n.Rhs == ImplicitOne {
+		if n.Rhs == nil {
 			// TODO(gri) This is going to break the mayCombine
 			//           check once we enable that again.
 			p.print(n.Op, n.Op) // ++ or --
diff --git a/src/cmd/compile/internal/syntax/walk.go b/src/cmd/compile/internal/syntax/walk.go
index 418b26d674..c26e97a0d8 100644
--- a/src/cmd/compile/internal/syntax/walk.go
+++ b/src/cmd/compile/internal/syntax/walk.go
@@ -207,7 +207,9 @@ func (w *walker) node(n Node) {
 
 	case *AssignStmt:
 		w.node(n.Lhs)
-		w.node(n.Rhs)
+		if n.Rhs != nil {
+			w.node(n.Rhs)
+		}
 
 	case *BranchStmt:
 		if n.Label != nil {
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index b728238d9f..22dc47b1e7 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -889,6 +889,7 @@ var binaryOpPredicates = opPredicates{
 }
 
 // The binary expression e may be nil. It's passed in for better error messages only.
+// TODO(gri) revisit use of e and opPos
 func (check *Checker) binary(x *operand, e *syntax.Operation, lhs, rhs syntax.Expr, op syntax.Operator, opPos syntax.Pos) {
 	var y operand
 
diff --git a/src/cmd/compile/internal/types2/pos.go b/src/cmd/compile/internal/types2/pos.go
index 0a19cd1a23..955bb2ad08 100644
--- a/src/cmd/compile/internal/types2/pos.go
+++ b/src/cmd/compile/internal/types2/pos.go
@@ -286,7 +286,7 @@ func endPos(n syntax.Node) syntax.Pos {
 			return n.Pos()
 		case *syntax.AssignStmt:
 			m = n.Rhs
-			if m == syntax.ImplicitOne {
+			if m == nil {
 				p := endPos(n.Lhs)
 				return syntax.MakePos(p.Base(), p.Line(), p.Col()+2)
 			}
diff --git a/src/cmd/compile/internal/types2/stmt.go b/src/cmd/compile/internal/types2/stmt.go
index 52b9794c10..cbfe97b03c 100644
--- a/src/cmd/compile/internal/types2/stmt.go
+++ b/src/cmd/compile/internal/types2/stmt.go
@@ -367,47 +367,45 @@ func (check *Checker) stmt(ctxt stmtContext, s syntax.Stmt) {
 
 	case *syntax.AssignStmt:
 		lhs := unpackExpr(s.Lhs)
-		rhs := unpackExpr(s.Rhs)
-		if s.Op == 0 || s.Op == syntax.Def {
-			// regular assignment or short variable declaration
-			if len(lhs) == 0 {
-				check.invalidASTf(s, "missing lhs in assignment")
-				return
-			}
-			if s.Op == syntax.Def {
-				check.shortVarDecl(s.Pos(), lhs, rhs)
-			} else {
-				// regular assignment
-				check.assignVars(lhs, rhs)
-			}
-		} else {
-			// assignment operations
-			if len(lhs) != 1 || len(rhs) != 1 {
-				check.errorf(s, "assignment operation %s requires single-valued expressions", s.Op)
+		if s.Rhs == nil {
+			// x++ or x--
+			if len(lhs) != 1 {
+				check.invalidASTf(s, "%s%s requires one operand", s.Op, s.Op)
 				return
 			}
-
-			// provide better error messages for x++ and x--
-			if rhs[0] == syntax.ImplicitOne {
-				var x operand
-				check.expr(&x, lhs[0])
-				if x.mode == invalid {
-					return
-				}
-				if !isNumeric(x.typ) {
-					check.invalidOpf(lhs[0], "%s%s%s (non-numeric type %s)", lhs[0], s.Op, s.Op, x.typ)
-					return
-				}
-			}
-
 			var x operand
-			check.binary(&x, nil, lhs[0], rhs[0], s.Op, rhs[0].Pos()) // TODO(gri) should have TokPos here (like in go/types)
+			check.expr(&x, lhs[0])
 			if x.mode == invalid {
 				return
 			}
+			if !isNumeric(x.typ) {
+				check.invalidOpf(lhs[0], "%s%s%s (non-numeric type %s)", lhs[0], s.Op, s.Op, x.typ)
+				return
+			}
 			check.assignVar(lhs[0], &x)
+			return
+		}
+
+		rhs := unpackExpr(s.Rhs)
+		switch s.Op {
+		case 0:
+			check.assignVars(lhs, rhs)
+			return
+		case syntax.Def:
+			check.shortVarDecl(s.Pos(), lhs, rhs)
+			return
+		}
+
+		// assignment operations
+		if len(lhs) != 1 || len(rhs) != 1 {
+			check.errorf(s, "assignment operation %s requires single-valued expressions", s.Op)
+			return
 		}
 
+		var x operand
+		check.binary(&x, nil, lhs[0], rhs[0], s.Op, s.Pos())
+		check.assignVar(lhs[0], &x)
+
 	// case *syntax.GoStmt:
 	// 	check.suspendedCall("go", s.Call)