internal/lsp: de-duplicate logic for canExtractVariable

We'd like to call canExtractVariable in extractVariable without
duplicating logic. The same needs to be done for canExtractFunction.

Change-Id: Ia99befabbafffcf13dd3bc12355f9ddb81a71002
Reviewed-on: https://go-review.googlesource.com/c/tools/+/245135
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Josh Baum <joshbaum@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/source/command.go

@@ -105,7 +105,10 @@ var (
 		Name:           "extract_variable",
 		Title:          "Extract to variable",
 		suggestedFixFn: extractVariable,
-		appliesFn:      canExtractVariable,
+		appliesFn: func(fset *token.FileSet, rng span.Range, src []byte, file *ast.File, pkg *types.Package, info *types.Info) bool {
+			_, _, ok, _ := canExtractVariable(fset, rng, src, file, pkg, info)
+			return ok
+		},
 	}
 
 	// CommandExtractFunction extracts statements to a function.

internal/lsp/source/extract.go

@@ -22,22 +22,12 @@ import (
 )
 
 func extractVariable(fset *token.FileSet, rng span.Range, src []byte, file *ast.File, pkg *types.Package, info *types.Info) (*analysis.SuggestedFix, error) {
-	if rng.Start == rng.End {
+	expr, path, ok, err := canExtractVariable(fset, rng, src, file, pkg, info)
-		return nil, fmt.Errorf("extractVariable: start and end are equal (%v)", fset.Position(rng.Start))
-	}
-	path, _ := astutil.PathEnclosingInterval(file, rng.Start, rng.End)
-	if len(path) == 0 {
-		return nil, fmt.Errorf("extractVariable: no path enclosing interval")
-	}
-	node := path[0]
-	if rng.Start != node.Pos() || rng.End != node.End() {
-		return nil, fmt.Errorf("extractVariable: node doesn't perfectly enclose range")
-	}
-	expr, ok := node.(ast.Expr)
 	if !ok {
-		return nil, fmt.Errorf("extractVariable: node is not an expression")
+		return nil, fmt.Errorf("extractVariable: cannot extract %s: %v", fset.Position(rng.Start), err)
 	}
-	name := generateAvailableIdentifier(node.Pos(), file, path, info, "x", 0)
+
+	name := generateAvailableIdentifier(expr.Pos(), file, path, info, "x", 0)
 
 	// Create new AST node for extracted code.
 	var assignment string
@@ -65,7 +55,7 @@ func extractVariable(fset *token.FileSet, rng span.Range, src []byte, file *ast.
 		return nil, nil
 	}
 
-	tok := fset.File(node.Pos())
+	tok := fset.File(expr.Pos())
 	if tok == nil {
 		return nil, nil
 	}
@@ -88,22 +78,23 @@ func extractVariable(fset *token.FileSet, rng span.Range, src []byte, file *ast.
 
 // canExtractVariable reports whether the code in the given range can be
 // extracted to a variable.
-// TODO(rstambler): De-duplicate the logic between extractVariable and
+func canExtractVariable(fset *token.FileSet, rng span.Range, src []byte, file *ast.File, pkg *types.Package, info *types.Info) (ast.Expr, []ast.Node, bool, error) {
-// canExtractVariable.
-func canExtractVariable(fset *token.FileSet, rng span.Range, src []byte, file *ast.File, pkg *types.Package, info *types.Info) bool {
 	if rng.Start == rng.End {
-		return false
+		return nil, nil, false, fmt.Errorf("start and end are equal")
 	}
 	path, _ := astutil.PathEnclosingInterval(file, rng.Start, rng.End)
 	if len(path) == 0 {
-		return false
+		return nil, nil, false, fmt.Errorf("no path enclosing interval")
 	}
 	node := path[0]
 	if rng.Start != node.Pos() || rng.End != node.End() {
-		return false
+		return nil, nil, false, fmt.Errorf("range does not map to an AST node")
 	}
-	_, ok := node.(ast.Expr)
+	expr, ok := node.(ast.Expr)
-	return ok
+	if !ok {
+		return nil, nil, false, fmt.Errorf("node is not an expression")
+	}
+	return expr, path, true, nil
 }
 
 // Calculate indentation for insertion.
@@ -415,7 +406,8 @@ func extractFunction(fset *token.FileSet, rng span.Range, src []byte, file *ast.
 	hasReturnValues := len(returns)+len(retVars) > 0
 	if hasReturnValues {
 		extractedBlock.List = append(extractedBlock.List, &ast.ReturnStmt{
-			Results: append(returns, getZeroVals(retVars)...)})
+			Results: append(returns, getZeroVals(retVars)...),
+		})
 	}
 
 	// Construct the appropriate call to the extracted function.