go/parser: return partial result from ParseExpr in case of error

Remove redundant code and improve documentation in the process.

Fixes #34211.

Change-Id: I9a6d1467f1a2c98a163f41f9df147fc6500c6fad
Reviewed-on: https://go-review.googlesource.com/c/go/+/196077Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/parser/interface.go

@@ -75,7 +75,7 @@ const (
 // indicates the specific failure. If the source was read but syntax
 // errors were found, the result is a partial AST (with ast.Bad* nodes
 // representing the fragments of erroneous source code). Multiple errors
-// are returned via a scanner.ErrorList which is sorted by file position.
+// are returned via a scanner.ErrorList which is sorted by source position.
 //
 func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (f *ast.File, err error) {
 	if fset == nil {
@@ -173,6 +173,12 @@ func ParseDir(fset *token.FileSet, path string, filter func(os.FileInfo) bool, m
 // be a valid Go (type or value) expression. Specifically, fset must not
 // be nil.
 //
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by source position.
+//
 func ParseExprFrom(fset *token.FileSet, filename string, src interface{}, mode Mode) (expr ast.Expr, err error) {
 	if fset == nil {
 		panic("parser.ParseExprFrom: no token.FileSet provided (fset == nil)")
@@ -204,7 +210,7 @@ func ParseExprFrom(fset *token.FileSet, filename string, src interface{}, mode M
 	// in case of an erroneous x.
 	p.openScope()
 	p.pkgScope = p.topScope
-	e := p.parseRhsOrType()
+	expr = p.parseRhsOrType()
 	p.closeScope()
 	assert(p.topScope == nil, "unbalanced scopes")
 
@@ -215,18 +221,17 @@ func ParseExprFrom(fset *token.FileSet, filename string, src interface{}, mode M
 	}
 	p.expect(token.EOF)
 
-	if p.errors.Len() > 0 {
-		p.errors.Sort()
-		return nil, p.errors.Err()
-	}
-
-	return e, nil
+	return
 }
 
 // ParseExpr is a convenience function for obtaining the AST of an expression x.
 // The position information recorded in the AST is undefined. The filename used
 // in error messages is the empty string.
 //
+// If syntax errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors are
+// returned via a scanner.ErrorList which is sorted by source position.
+//
 func ParseExpr(x string) (ast.Expr, error) {
 	return ParseExprFrom(token.NewFileSet(), "", []byte(x), 0)
 }

src/go/parser/parser_test.go

@@ -108,9 +108,16 @@ func TestParseExpr(t *testing.T) {
 
 	// an invalid expression
 	src = "a + *"
-	if _, err := ParseExpr(src); err == nil {
+	x, err = ParseExpr(src)
+	if err == nil {
 		t.Errorf("ParseExpr(%q): got no error", src)
 	}
+	if x == nil {
+		t.Errorf("ParseExpr(%q): got no (partial) result", src)
+	}
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
 
 	// a valid expression followed by extra tokens is invalid
 	src = "a[i] := x"