[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>

3 files changed, 10 insertions, 12 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))