go/doc/comment: add data structures

[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]

Implement just the data structures of the new API for
parsing and printing doc comments, as well as a syntax tree
form for inspecting and manipulating them.

The API itself was discussed and accepted as part of the
proposal process in #51082.

For #51082.

Change-Id: Iae7fbc85705964585273b970c5c62e394feb1288
Reviewed-on: https://go-review.googlesource.com/c/go/+/397276
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

api/next/51082.txt

@@ -0,0 +1,35 @@
+pkg go/doc/comment, method (*List) BlankBefore() bool #51082
+pkg go/doc/comment, method (*List) BlankBetween() bool #51082
+pkg go/doc/comment, type Block interface, unexported methods #51082
+pkg go/doc/comment, type Code struct #51082
+pkg go/doc/comment, type Code struct, Text string #51082
+pkg go/doc/comment, type Doc struct #51082
+pkg go/doc/comment, type Doc struct, Content []Block #51082
+pkg go/doc/comment, type Doc struct, Links []*LinkDef #51082
+pkg go/doc/comment, type DocLink struct #51082
+pkg go/doc/comment, type DocLink struct, ImportPath string #51082
+pkg go/doc/comment, type DocLink struct, Name string #51082
+pkg go/doc/comment, type DocLink struct, Recv string #51082
+pkg go/doc/comment, type DocLink struct, Text []Text #51082
+pkg go/doc/comment, type Heading struct #51082
+pkg go/doc/comment, type Heading struct, Text []Text #51082
+pkg go/doc/comment, type Italic string #51082
+pkg go/doc/comment, type Link struct #51082
+pkg go/doc/comment, type Link struct, Auto bool #51082
+pkg go/doc/comment, type Link struct, Text []Text #51082
+pkg go/doc/comment, type Link struct, URL string #51082
+pkg go/doc/comment, type LinkDef struct #51082
+pkg go/doc/comment, type LinkDef struct, Text string #51082
+pkg go/doc/comment, type LinkDef struct, URL string #51082
+pkg go/doc/comment, type LinkDef struct, Used bool #51082
+pkg go/doc/comment, type List struct #51082
+pkg go/doc/comment, type List struct, ForceBlankBefore bool #51082
+pkg go/doc/comment, type List struct, ForceBlankBetween bool #51082
+pkg go/doc/comment, type List struct, Items []*ListItem #51082
+pkg go/doc/comment, type ListItem struct #51082
+pkg go/doc/comment, type ListItem struct, Content []Block #51082
+pkg go/doc/comment, type ListItem struct, Number string #51082
+pkg go/doc/comment, type Paragraph struct #51082
+pkg go/doc/comment, type Paragraph struct, Text []Text #51082
+pkg go/doc/comment, type Plain string #51082
+pkg go/doc/comment, type Text interface, unexported methods #51082

src/go/build/deps_test.go

@@ -288,9 +288,9 @@ var depsRules = `
 	< go/parser;
 
 	FMT
-	< go/build/constraint;
+	< go/build/constraint, go/doc/comment;
 
-	go/build/constraint, go/parser, text/tabwriter
+	go/build/constraint, go/doc/comment, go/parser, text/tabwriter
 	< go/printer
 	< go/format;
 

src/go/doc/comment/parse.go

@@ -0,0 +1,169 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package comment
+
+// A Doc is a parsed Go doc comment.
+type Doc struct {
+	// Content is the sequence of content blocks in the comment.
+	Content []Block
+
+	// Links is the link definitions in the comment.
+	Links []*LinkDef
+}
+
+// A LinkDef is a single link definition.
+type LinkDef struct {
+	Text string // the link text
+	URL  string // the link URL
+	Used bool   // whether the comment uses the definition
+}
+
+// A Block is block-level content in a doc comment,
+// one of *[Code], *[Heading], *[List], or *[Paragraph].
+type Block interface {
+	block()
+}
+
+// A Heading is a doc comment heading.
+type Heading struct {
+	Text []Text // the heading text
+}
+
+func (*Heading) block() {}
+
+// A List is a numbered or bullet list.
+// Lists are always non-empty: len(Items) > 0.
+// In a numbered list, every Items[i].Number is a non-empty string.
+// In a bullet list, every Items[i].Number is an empty string.
+type List struct {
+	// Items is the list items.
+	Items []*ListItem
+
+	// ForceBlankBefore indicates that the list must be
+	// preceded by a blank line when reformatting the comment,
+	// overriding the usual conditions. See the BlankBefore method.
+	//
+	// The comment parser sets ForceBlankBefore for any list
+	// that is preceded by a blank line, to make sure
+	// the blank line is preserved when printing.
+	ForceBlankBefore bool
+
+	// ForceBlankBetween indicates that list items must be
+	// separated by blank lines when reformatting the comment,
+	// overriding the usual conditions. See the BlankBetween method.
+	//
+	// The comment parser sets ForceBlankBetween for any list
+	// that has a blank line between any two of its items, to make sure
+	// the blank lines are preserved when printing.
+	ForceBlankBetween bool
+}
+
+func (*List) block() {}
+
+// BlankBefore reports whether a reformatting of the comment
+// should include a blank line before the list.
+// The default rule is the same as for [BlankBetween]:
+// if the list item content contains any blank lines
+// (meaning at least one item has multiple paragraphs)
+// then the list itself must be preceded by a blank line.
+// A preceding blank line can be forced by setting [List].ForceBlankBefore.
+func (l *List) BlankBefore() bool {
+	return l.ForceBlankBefore || l.BlankBetween()
+}
+
+// BlankBetween reports whether a reformatting of the comment
+// should include a blank line between each pair of list items.
+// The default rule is that if the list item content contains any blank lines
+// (meaning at least one item has multiple paragraphs)
+// then list items must themselves be separated by blank lines.
+// Blank line separators can be forced by setting [List].ForceBlankBetween.
+func (l *List) BlankBetween() bool {
+	if l.ForceBlankBetween {
+		return true
+	}
+	for _, item := range l.Items {
+		if len(item.Content) != 1 {
+			// Unreachable for parsed comments today,
+			// since the only way to get multiple item.Content
+			// is multiple paragraphs, which must have been
+			// separated by a blank line.
+			return true
+		}
+	}
+	return false
+}
+
+// A ListItem is a single item in a numbered or bullet list.
+type ListItem struct {
+	// Number is a decimal string in a numbered list
+	// or an empty string in a bullet list.
+	Number string // "1", "2", ...; "" for bullet list
+
+	// Content is the list content.
+	// Currently, restrictions in the parser and printer
+	// require every element of Content to be a *Paragraph.
+	Content []Block // Content of this item.
+}
+
+// A Paragraph is a paragraph of text.
+type Paragraph struct {
+	Text []Text
+}
+
+func (*Paragraph) block() {}
+
+// A Code is a preformatted code block.
+type Code struct {
+	// Text is the preformatted text, ending with a newline character.
+	// It may be multiple lines, each of which ends with a newline character.
+	// It is never empty, nor does it start or end with a blank line.
+	Text string
+}
+
+func (*Code) block() {}
+
+// A Text is text-level content in a doc comment,
+// one of [Plain], [Italic], *[Link], or *[DocLink].
+type Text interface {
+	text()
+}
+
+// A Plain is a string rendered as plain text (not italicized).
+type Plain string
+
+func (Plain) text() {}
+
+// An Italic is a string rendered as italicized text.
+type Italic string
+
+func (Italic) text() {}
+
+// A Link is a link to a specific URL.
+type Link struct {
+	Auto bool   // is this an automatic (implicit) link of a literal URL?
+	Text []Text // text of link
+	URL  string // target URL of link
+}
+
+func (*Link) text() {}
+
+// A DocLink is a link to documentation for a Go package or symbol.
+type DocLink struct {
+	Text []Text // text of link
+
+	// ImportPath, Recv, and Name identify the Go package or symbol
+	// that is the link target. The potential combinations of
+	// non-empty fields are:
+	//  - ImportPath: a link to another package
+	//  - ImportPath, Name: a link to a const, func, type, or var in another package
+	//  - ImportPath, Recv, Name: a link to a method in another package
+	//  - Name: a link to a const, func, type, or var in this package
+	//  - Recv, Name: a link to a method in this package
+	ImportPath string // import path
+	Recv       string // receiver type, without any pointer star, for methods
+	Name       string // const, func, type, var, or method name
+}
+
+func (*DocLink) text() {}