go/doc: linkify interface methods

Create doc links for references to interface methods,
such as "[Context.Err]".

This does not attempt to handle embedded interfaces:
The linked-to method must be declared within the named type.

Fixes #54033

Change-Id: I4d7a132affe682c85958ca785bcc96571404e6c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/687395
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

3 files changed, 39 insertions, 7 deletions

diff --git a/src/go/doc/comment_test.go b/src/go/doc/comment_test.go
index 004ae9d13d..0e7de3eb78 100644
--- a/src/go/doc/comment_test.go
+++ b/src/go/doc/comment_test.go
@@ -24,12 +24,12 @@ func TestComment(t *testing.T) {
 	pkg := New(pkgs["pkgdoc"], "testdata/pkgdoc", 0)
 
 	var (
-		input           = "[T] and [U] are types, and [T.M] is a method, but [V] is a broken link. [rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods.\n"
-		wantHTML        = `<p><a href="#T">T</a> and <a href="#U">U</a> are types, and <a href="#T.M">T.M</a> is a method, but [V] is a broken link. <a href="/math/rand#Int">rand.Int</a> and <a href="/crypto/rand#Reader">crand.Reader</a> are things. <a href="#G.M1">G.M1</a> and <a href="#G.M2">G.M2</a> are generic methods.` + "\n"
-		wantOldHTML     = "<p>[T] and [U] are <i>types</i>, and [T.M] is a method, but [V] is a broken link. [rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods.\n"
-		wantMarkdown    = "[T](#T) and [U](#U) are types, and [T.M](#T.M) is a method, but \\[V] is a broken link. [rand.Int](/math/rand#Int) and [crand.Reader](/crypto/rand#Reader) are things. [G.M1](#G.M1) and [G.M2](#G.M2) are generic methods.\n"
-		wantText        = "T and U are types, and T.M is a method, but [V] is a broken link. rand.Int and\ncrand.Reader are things. G.M1 and G.M2 are generic methods.\n"
-		wantOldText     = "[T] and [U] are types, and [T.M] is a method, but [V] is a broken link.\n[rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods.\n"
+		input           = "[T] and [U] are types, and [T.M] is a method, but [V] is a broken link. [rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods. [I.F] is an interface method and [I.V] is a broken link.\n"
+		wantHTML        = `<p><a href="#T">T</a> and <a href="#U">U</a> are types, and <a href="#T.M">T.M</a> is a method, but [V] is a broken link. <a href="/math/rand#Int">rand.Int</a> and <a href="/crypto/rand#Reader">crand.Reader</a> are things. <a href="#G.M1">G.M1</a> and <a href="#G.M2">G.M2</a> are generic methods. <a href="#I.F">I.F</a> is an interface method and [I.V] is a broken link.` + "\n"
+		wantOldHTML     = "<p>[T] and [U] are <i>types</i>, and [T.M] is a method, but [V] is a broken link. [rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods. [I.F] is an interface method and [I.V] is a broken link.\n"
+		wantMarkdown    = "[T](#T) and [U](#U) are types, and [T.M](#T.M) is a method, but \\[V] is a broken link. [rand.Int](/math/rand#Int) and [crand.Reader](/crypto/rand#Reader) are things. [G.M1](#G.M1) and [G.M2](#G.M2) are generic methods. [I.F](#I.F) is an interface method and \\[I.V] is a broken link.\n"
+		wantText        = "T and U are types, and T.M is a method, but [V] is a broken link. rand.Int and\ncrand.Reader are things. G.M1 and G.M2 are generic methods. I.F is an interface\nmethod and [I.V] is a broken link.\n"
+		wantOldText     = "[T] and [U] are types, and [T.M] is a method, but [V] is a broken link.\n[rand.Int] and [crand.Reader] are things. [G.M1] and [G.M2] are generic methods.\n[I.F] is an interface method and [I.V] is a broken link.\n"
 		wantSynopsis    = "T and U are types, and T.M is a method, but [V] is a broken link."
 		wantOldSynopsis = "[T] and [U] are types, and [T.M] is a method, but [V] is a broken link."
 	)
diff --git a/src/go/doc/doc.go b/src/go/doc/doc.go
index f7e3c1bad8..0c23f1a46c 100644
--- a/src/go/doc/doc.go
+++ b/src/go/doc/doc.go
@@ -167,6 +167,7 @@ func (p *Package) collectTypes(types []*Type) {
 		p.collectValues(t.Vars)
 		p.collectFuncs(t.Funcs)
 		p.collectFuncs(t.Methods)
+		p.collectInterfaceMethods(t)
 	}
 }
 
@@ -184,6 +185,33 @@ func (p *Package) collectFuncs(funcs []*Func) {
 	}
 }
 
+// collectInterfaceMethods adds methods of interface types within t to p.syms.
+// Note that t.Methods will contain methods of non-interface types, but not interface types.
+// Adding interface methods to t.Methods might make sense, but would cause us to
+// include those methods in the documentation index. Adding interface methods to p.syms
+// here allows us to linkify references like [io.Reader.Read] without making any other
+// changes to the documentation formatting at this time.
+//
+// If we do start adding interface methods to t.Methods in the future,
+// collectInterfaceMethods can be dropped as redundant with collectFuncs(t.Methods).
+func (p *Package) collectInterfaceMethods(t *Type) {
+	for _, s := range t.Decl.Specs {
+		spec, ok := s.(*ast.TypeSpec)
+		if !ok {
+			continue
+		}
+		list, isStruct := fields(spec.Type)
+		if isStruct {
+			continue
+		}
+		for _, field := range list {
+			for _, name := range field.Names {
+				p.syms[t.Name+"."+name.Name] = true
+			}
+		}
+	}
+}
+
 // NewFromFiles computes documentation for a package.
 //
 // The package is specified by a list of *ast.Files and corresponding
diff --git a/src/go/doc/testdata/pkgdoc/doc.go b/src/go/doc/testdata/pkgdoc/doc.go
index 3f822c7554..d542dc2cdd 100644
--- a/src/go/doc/testdata/pkgdoc/doc.go
+++ b/src/go/doc/testdata/pkgdoc/doc.go
@@ -20,5 +20,9 @@ var _ = crand.Reader
 
 type G[T any] struct{ x T }
 
-func (g G[T]) M1() {}
+func (g G[T]) M1()  {}
 func (g *G[T]) M2() {}
+
+type I interface {
+	F()
+}