[dev.typeparams] cmd/compile: enable parsing of generic code with new -G flag

Providing the -G flag instructs the compiler to accept type parameters.
For now, the compiler only parses such files and then exits.

Added a new test directory (test/typeparam) and initial test case.

Port from dev.go2go branch.

Change-Id: Ic11e33a9d5f012f8def0bdae205043659562ac73
Reviewed-on: https://go-review.googlesource.com/c/go/+/261660
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/main.go

@@ -216,6 +216,7 @@ func Main(archInit func(*Arch)) {
 	objabi.Flagcount("C", "disable printing of columns in error messages", &Debug['C']) // TODO(gri) remove eventually
 	flag.StringVar(&localimport, "D", "", "set relative `path` for local imports")
 	objabi.Flagcount("E", "debug symbol export", &Debug['E'])
+	objabi.Flagcount("G", "accept generic code", &Debug['G'])
 	objabi.Flagfn1("I", "add `directory` to import search path", addidir)
 	objabi.Flagcount("K", "debug missing line numbers", &Debug['K'])
 	objabi.Flagcount("L", "show full file names in error messages", &Debug['L'])
@@ -571,9 +572,16 @@ func Main(archInit func(*Arch)) {
 	loadsys()
 
 	timings.Start("fe", "parse")
-	lines := parseFiles(flag.Args())
+	lines := parseFiles(flag.Args(), Debug['G'] != 0)
 	timings.Stop()
 	timings.AddEvent(int64(lines), "lines")
+	if Debug['G'] != 0 {
+		// can only parse generic code for now
+		if nerrors+nsavederrors != 0 {
+			errorexit()
+		}
+		return
+	}
 
 	finishUniverse()
 

src/cmd/compile/internal/gc/noder.go

@@ -24,7 +24,7 @@ import (
 // Each declaration in every *syntax.File is converted to a syntax tree
 // and its root represented by *Node is appended to xtop.
 // Returns the total count of parsed lines.
-func parseFiles(filenames []string) uint {
+func parseFiles(filenames []string, allowGenerics bool) uint {
 	noders := make([]*noder, 0, len(filenames))
 	// Limit the number of simultaneously open files.
 	sem := make(chan struct{}, runtime.GOMAXPROCS(0)+10)
@@ -49,7 +49,11 @@ func parseFiles(filenames []string) uint {
 			}
 			defer f.Close()
 
-			p.file, _ = syntax.Parse(base, f, p.error, p.pragma, syntax.CheckBranches) // errors are tracked via p.error
+			mode := syntax.CheckBranches
+			if allowGenerics {
+				mode |= syntax.AllowGenerics
+			}
+			p.file, _ = syntax.Parse(base, f, p.error, p.pragma, mode) // errors are tracked via p.error
 		}(filename)
 	}
 
@@ -59,7 +63,10 @@ func parseFiles(filenames []string) uint {
 			p.yyerrorpos(e.Pos, "%s", e.Msg)
 		}
 
-		p.node()
+		// noder cannot handle generic code yet
+		if !allowGenerics {
+			p.node()
+		}
 		lines += p.file.EOF.Line()
 		p.file = nil // release memory
 

test/run.go

@@ -58,7 +58,7 @@ var (
 
 	// dirs are the directories to look for *.go files in.
 	// TODO(bradfitz): just use all directories?
-	dirs = []string{".", "ken", "chan", "interface", "syntax", "dwarf", "fixedbugs", "codegen", "runtime"}
+	dirs = []string{".", "ken", "chan", "interface", "syntax", "dwarf", "fixedbugs", "codegen", "runtime", "typeparam"}
 
 	// ratec controls the max number of tests running at a time.
 	ratec chan bool

test/typeparam/smoketest.go

@@ -0,0 +1,57 @@
+// compile -G
+
+// Copyright 2020 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.
+
+// This file checks simple code using type parameters.
+
+package smoketest
+
+// type parameters for functions
+func f1[P any]()
+func f2[P1, P2 any, P3 any]()
+func f3[P interface{}](x P, y T1[int])
+
+// function instantiations
+var _ = f1[int]
+var _ = f2[int, string, struct{}]
+var _ = f3[bool]
+
+// type parameters for types
+type T1[P any] struct{}
+type T2[P1, P2 any, P3 any] struct{}
+type T3[P interface{}] interface{}
+
+// type instantiations
+type _ T1[int]
+type _ T2[int, string, struct{}]
+type _ T3[bool]
+
+// methods
+func (T1[P]) m1() {}
+func (x T2[P1, P2, P3]) m1() {}
+func (_ T3[_]) m1() {}
+
+// type lists
+type _ interface {
+	m1()
+	m2()
+	type int, float32, string
+	m3()
+	type bool
+}
+
+// embedded instantiated types
+type _ struct {
+	f1, f2 int
+	T1[int]
+	T2[int, string, struct{}]
+	T3[bool]
+}
+
+type _ interface {
+	m1()
+	m2()
+	T3[bool]
+}