go/doc: add new mode bit PreserveAST to control clearing of data in AST

To save memory in godoc, this package routinely clears fields of
the AST to avoid keeping data that godoc no longer needs. For other
programs, such as cmd/doc, this behavior is unfortunate. Also, one
should be able to tell any package like this, "don't change my
data".

Add a Mode bit, defaulting to off to preserve existing behavior,
that allows a client to specify that the AST is inviolate.

This is necessary to address some of the outstanding issues
in cmd/doc that require, for example, looking at function bodies.

Fixes #26835

Change-Id: I01cc97c6addc5ab6abff885fff4bd53454a03bbc
Reviewed-on: https://go-review.googlesource.com/c/140958Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/doc.go

@@ -79,13 +79,18 @@ type Note struct {
 type Mode int
 
 const (
-	// extract documentation for all package-level declarations,
-	// not just exported ones
+	// AllDecls says to extract documentation for all package-level
+	// declarations, not just exported ones.
 	AllDecls Mode = 1 << iota
 
-	// show all embedded methods, not just the ones of
-	// invisible (unexported) anonymous fields
+	// AllMethods says to show all embedded methods, not just the ones of
+	// invisible (unexported) anonymous fields.
 	AllMethods
+
+	// PreserveAST says to leave the AST unmodified. Originally, pieces of
+	// the AST such as function bodies were nil-ed out to save memory in
+	// godoc, but not all programs want that behavior.
+	PreserveAST
 )
 
 // New computes the package documentation for the given package AST.

src/go/doc/reader.go

@@ -36,9 +36,10 @@ func recvString(recv ast.Expr) string {
 
 // set creates the corresponding Func for f and adds it to mset.
 // If there are multiple f's with the same name, set keeps the first
-// one with documentation; conflicts are ignored.
+// one with documentation; conflicts are ignored. The boolean
+// specifies whether to leave the AST untouched.
 //
-func (mset methodSet) set(f *ast.FuncDecl) {
+func (mset methodSet) set(f *ast.FuncDecl, preserveAST bool) {
 	name := f.Name.Name
 	if g := mset[name]; g != nil && g.Doc != "" {
 		// A function with the same name has already been registered;
@@ -65,7 +66,9 @@ func (mset methodSet) set(f *ast.FuncDecl) {
 		Recv: recv,
 		Orig: recv,
 	}
-	f.Doc = nil // doc consumed - remove from AST
+	if !preserveAST {
+		f.Doc = nil // doc consumed - remove from AST
+	}
 }
 
 // add adds method m to the method set; m is ignored if the method set
@@ -299,8 +302,9 @@ func (r *reader) readValue(decl *ast.GenDecl) {
 		Decl:  decl,
 		order: r.order,
 	})
-	decl.Doc = nil // doc consumed - remove from AST
-
+	if r.mode&PreserveAST == 0 {
+		decl.Doc = nil // doc consumed - remove from AST
+	}
 	// Note: It's important that the order used here is global because the cleanupTypes
 	// methods may move values associated with types back into the global list. If the
 	// order is list-specific, sorting is not deterministic because the same order value
@@ -339,12 +343,14 @@ func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
 
 	// compute documentation
 	doc := spec.Doc
-	spec.Doc = nil // doc consumed - remove from AST
 	if doc == nil {
 		// no doc associated with the spec, use the declaration doc, if any
 		doc = decl.Doc
 	}
-	decl.Doc = nil // doc consumed - remove from AST
+	if r.mode&PreserveAST == 0 {
+		spec.Doc = nil // doc consumed - remove from AST
+		decl.Doc = nil // doc consumed - remove from AST
+	}
 	typ.doc = doc.Text()
 
 	// record anonymous fields (they may contribute methods)
@@ -362,8 +368,10 @@ func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
 // readFunc processes a func or method declaration.
 //
 func (r *reader) readFunc(fun *ast.FuncDecl) {
-	// strip function body
-	fun.Body = nil
+	// strip function body if requested.
+	if r.mode&PreserveAST == 0 {
+		fun.Body = nil
+	}
 
 	// associate methods with the receiver type, if any
 	if fun.Recv != nil {
@@ -380,7 +388,7 @@ func (r *reader) readFunc(fun *ast.FuncDecl) {
 			return
 		}
 		if typ := r.lookupType(recvTypeName); typ != nil {
-			typ.methods.set(fun)
+			typ.methods.set(fun, r.mode&PreserveAST != 0)
 		}
 		// otherwise ignore the method
 		// TODO(gri): There may be exported methods of non-exported types
@@ -414,13 +422,13 @@ func (r *reader) readFunc(fun *ast.FuncDecl) {
 		}
 		// If there is exactly one result type, associate the function with that type.
 		if numResultTypes == 1 {
-			typ.funcs.set(fun)
+			typ.funcs.set(fun, r.mode&PreserveAST != 0)
 			return
 		}
 	}
 
 	// just an ordinary function
-	r.funcs.set(fun)
+	r.funcs.set(fun, r.mode&PreserveAST != 0)
 }
 
 var (
@@ -481,7 +489,9 @@ func (r *reader) readFile(src *ast.File) {
 	// add package documentation
 	if src.Doc != nil {
 		r.readDoc(src.Doc)
-		src.Doc = nil // doc consumed - remove from AST
+		if r.mode&PreserveAST == 0 {
+			src.Doc = nil // doc consumed - remove from AST
+		}
 	}
 
 	// add all declarations
@@ -545,7 +555,9 @@ func (r *reader) readFile(src *ast.File) {
 
 	// collect MARKER(...): annotations
 	r.readNotes(src.Comments)
-	src.Comments = nil // consumed unassociated comments - remove from AST
+	if r.mode&PreserveAST == 0 {
+		src.Comments = nil // consumed unassociated comments - remove from AST
+	}
 }
 
 func (r *reader) readPackage(pkg *ast.Package, mode Mode) {