cmd/compile: increase nesting depth limit for type descriptors

The formatting routines for types use a depth limit as primitive
mechanism to detect cycles. For now, increase the limit from 100
to 250 and file #29312 so we don't drop this on the floor.

Also, adjust some fatal error messages elsewhere to use
better formatting.

Fixes #29264.
Updates #29312.

Change-Id: Idd529f6682d478e0dcd2d469cb802192190602f6
Reviewed-on: https://go-review.googlesource.com/c/154583Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1749,7 +1749,11 @@ func tconv(t *types.Type, flag FmtFlag, mode fmtMode, depth int) string {
 		return t.FieldType(0).String() + "," + t.FieldType(1).String()
 	}
 
-	if depth > 100 {
+	// Avoid endless recursion by setting an upper limit. This also
+	// limits the depths of valid composite types, but they are likely
+	// artificially created.
+	// TODO(gri) should have proper cycle detection here, eventually (issue #29312)
+	if depth > 250 {
 		return "<...>"
 	}
 

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

@@ -1325,7 +1325,7 @@ func (w *exportWriter) expr(n *Node) {
 
 	default:
 		Fatalf("cannot export %v (%d) node\n"+
-			"==> please file an issue and assign to gri@\n", n.Op, int(n.Op))
+			"\t==> please file an issue and assign to gri@", n.Op, int(n.Op))
 	}
 }
 

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

@@ -1053,7 +1053,7 @@ func (r *importReader) node() *Node {
 
 	default:
 		Fatalf("cannot import %v (%d) node\n"+
-			"==> please file an issue and assign to gri@\n", op, int(op))
+			"\t==> please file an issue and assign to gri@", op, int(op))
 		panic("unreachable") // satisfy compiler
 	}
 }

test/fixedbugs/issue29264.go

+// run
+
+// Copyright 2018 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.
+
+// Verify that we emit a valid type descriptor for
+// a fairly deeply nested type.
+
+package main
+
+import "fmt"
+import "strings"
+
+func main() {
+	a := [][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][][]int{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{42}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
+	got := fmt.Sprint(a)
+	want := strings.Repeat("[", 100) + "42" + strings.Repeat("]", 100)
+	if got != want {
+		fmt.Printf("got  %q\nwant %q\n", got, want)
+	}
+}