go/parser: add a SkipObjectResolution mode to bypass object resolution

Parser object resolution is an auxiliary feature in which the parser
attempts to resolve identifiers to their declarations. In functionality,
it significantly overlaps with go/types and in fact cannot be correctly
computed at parse-time without type information (for example, it is
generally not possible to resolve k in the composite lit c{k: v}). Due
to these limitations, it is of limited utility and rarely used.

Now that object resolution is isolated as a post-processing pass, it is
trivial to offer a parser mode that skips it entirely. This CL adds that
mode.

Fixes #45104

Change-Id: I5a2c05437e298964ad2039e1ff98e63d6efbd1af
Reviewed-on: https://go-review.googlesource.com/c/go/+/306149
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 14 insertions, 9 deletions

diff --git a/src/go/parser/interface.go b/src/go/parser/interface.go
index dcc5fa6616..85486d2f4b 100644
--- a/src/go/parser/interface.go
+++ b/src/go/parser/interface.go
@@ -49,13 +49,14 @@ func readSource(filename string, src interface{}) ([]byte, error) {
 type Mode uint
 
 const (
-	PackageClauseOnly Mode             = 1 << iota // stop parsing after package clause
-	ImportsOnly                                    // stop parsing after import declarations
-	ParseComments                                  // parse comments and add them to AST
-	Trace                                          // print a trace of parsed productions
-	DeclarationErrors                              // report declaration errors
-	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
-	AllErrors         = SpuriousErrors             // report all errors (not just the first 10 on different lines)
+	PackageClauseOnly    Mode             = 1 << iota // stop parsing after package clause
+	ImportsOnly                                       // stop parsing after import declarations
+	ParseComments                                     // parse comments and add them to AST
+	Trace                                             // print a trace of parsed productions
+	DeclarationErrors                                 // report declaration errors
+	SpuriousErrors                                    // same as AllErrors, for backward-compatibility
+	SkipObjectResolution                              // don't resolve identifiers to objects - see ParseFile
+	AllErrors            = SpuriousErrors             // report all errors (not just the first 10 on different lines)
 )
 
 // ParseFile parses the source code of a single Go source file and returns
@@ -68,8 +69,12 @@ const (
 // If src == nil, ParseFile parses the file specified by filename.
 //
 // The mode parameter controls the amount of source text parsed and other
-// optional parser functionality. Position information is recorded in the
-// file set fset, which must not be nil.
+// optional parser functionality. If the SkipObjectResolution mode bit is set,
+// the object resolution phase of parsing will be skipped, causing File.Scope,
+// File.Unresolved, and all Ident.Obj fields to be nil.
+//
+// Position information is recorded in the file set fset, which 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