[dev.regabi] all: merge master (ff0e93e) into dev.regabi

Conflicts:

- src/cmd/compile/internal/gc/walk.go

  gc/walk.go is changed in CL 290950 on the master branch but
  deleted in the dev.regabi branch and moved over to the walk
  package. This CL includes porting CL 290950 over to the new
  walk.

Merge List:

+ 2021-02-12 ff0e93ea31 doc/go1.16: note that package path elements beginning with '.' are disallowed
+ 2021-02-11 249da7ec02 CONTRIBUTORS: update for the Go 1.16 release
+ 2021-02-11 864d4f1c6b cmd/go: multiple small 'go help' fixes
+ 2021-02-11 26ceae85a8 spec: More precise wording in section on function calls.
+ 2021-02-11 930c2c9a68 cmd/go: reject embedded files that can't be packed into modules
+ 2021-02-11 e5b08e6d5c io/fs: allow backslash in ValidPath, reject in os.DirFS.Open
+ 2021-02-10 ed8079096f cmd/compile: mark concrete call of reflect.(*rtype).Method as REFLECTMETHOD
+ 2021-02-09 e9c9683597 cmd/go: suppress errors from 'go get -d' for packages that only conditionally exist
+ 2021-02-09 e0ac989cf3 archive/tar: detect out of bounds accesses in PAX records resulting from padded lengths
+ 2021-02-09 c9d6f45fec runtime/metrics: fix a couple of documentation typpos
+ 2021-02-09 cea4e21b52 io/fs: backslash is always a glob meta character
+ 2021-02-08 dc725bfb3c doc/go1.16: mention new vet check for asn1.Unmarshal

Change-Id: Ib28fffa7dfbff7f6cdbfaf4a304757fead7bbf19

src/cmd/compile/internal/walk/expr.go

@@ -480,8 +480,12 @@ func walkAddString(n *ir.AddStringExpr, init *ir.Nodes) ir.Node {
 
 // walkCall walks an OCALLFUNC, OCALLINTER, or OCALLMETH node.
 func walkCall(n *ir.CallExpr, init *ir.Nodes) ir.Node {
-	if n.Op() == ir.OCALLINTER {
+	if n.Op() == ir.OCALLINTER || n.Op() == ir.OCALLMETH {
+		// We expect both interface call reflect.Type.Method and concrete
+		// call reflect.(*rtype).Method.
 		usemethod(n)
+	}
+	if n.Op() == ir.OCALLINTER {
 		reflectdata.MarkUsedIfaceMethod(n)
 	}
 
@@ -899,6 +903,16 @@ func usemethod(n *ir.CallExpr) {
 		}
 	}
 
+	// Don't mark reflect.(*rtype).Method, etc. themselves in the reflect package.
+	// Those functions may be alive via the itab, which should not cause all methods
+	// alive. We only want to mark their callers.
+	if base.Ctxt.Pkgpath == "reflect" {
+		switch ir.CurFunc.Nname.Sym().Name { // TODO: is there a better way than hardcoding the names?
+		case "(*rtype).Method", "(*rtype).MethodByName", "(*interfaceType).Method", "(*interfaceType).MethodByName":
+			return
+		}
+	}
+
 	// Note: Don't rely on res0.Type.String() since its formatting depends on multiple factors
 	//       (including global variables such as numImports - was issue #19028).
 	// Also need to check for reflect package itself (see Issue #38515).