runtime: during panic, print value instead of address, if kind is printable

Make panics more useful by printing values, if their
underlying kind is printable, instead of just their memory address.

Thus now given any custom type derived from any of:
    float*, int*, string, uint*

if we have panic with such a result, its value will be printed.

Thus given any of:
    type MyComplex128 complex128
    type MyFloat64 float64
    type MyString string
    type MyUintptr uintptr

    panic(MyComplex128(32.1 + 10i))
    panic(MyFloat64(-93.7))
    panic(MyString("This one"))
    panic(MyUintptr(93))

They will now print in the panic:

    panic: main.MyComplex64(+1.100000e-001+3.000000e+000i)
    panic: main.MyFloat64(-9.370000e+001)
    panic: main.MyString("This one")
    panic: main.MyUintptr(93)

instead of:

    panic: (main.MyComplex128) (0xe0100,0x138cc0)
    panic: (main.MyFloat64) (0xe0100,0x138068)
    panic: (main.MyString) (0x48aa00,0x4c0840)
    panic: (main.MyUintptr) (0xe0100,0x137e58)

and anything else will be printed as in the past with:

    panic: (main.MyStruct) (0xe4ee0,0x40a0e0)

Also while here, updated the Go1.15 release notes.

Fixes #37531

Change-Id: Ia486424344a386014f2869ab3483e42a9ef48ac4
Reviewed-on: https://go-review.googlesource.com/c/go/+/221779
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.15.html

@@ -92,6 +92,18 @@ TODO
 TODO
 </p>
 
+<dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
+  <dd>
+    <p><!-- CL 221779 -->
+      If <code>panic</code> is invoked with a value whose type is derived from any
+      of: <code>bool</code>, <code>complex64</code>, <code>complex128</code>, <code>float32</code>, <code>float64</code>,
+      <code>int</code>, <code>int8</code>, <code>int16</code>, <code>int32</code>, <code>int64</code>, <code>string</code>,
+      <code>uint</code>, <code>uint8</code>, <code>uint16</code>, <code>uint32</code>, <code>uint64</code>, <code>uintptr</code>,
+      then the value will be printed, instead of just its address.
+    </p>
+  </dd>
+</dl>
+
 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
   <dd>
     <p><!-- golang.org/issue/33762 -->

src/runtime/error.go

@@ -185,11 +185,6 @@ type stringer interface {
 	String() string
 }
 
-func typestring(x interface{}) string {
-	e := efaceOf(&x)
-	return e._type.string()
-}
-
 // printany prints an argument passed to panic.
 // If panic is called with a value that has a String or Error method,
 // it has already been converted into a string by preprintpanics.
@@ -232,7 +227,51 @@ func printany(i interface{}) {
 	case string:
 		print(v)
 	default:
-		print("(", typestring(i), ") ", i)
+		printanycustomtype(i)
+	}
+}
+
+func printanycustomtype(i interface{}) {
+	eface := efaceOf(&i)
+	typestring := eface._type.string()
+
+	switch eface._type.kind {
+	case kindString:
+		print(typestring, `("`, *(*string)(eface.data), `")`)
+	case kindBool:
+		print(typestring, "(", *(*bool)(eface.data), ")")
+	case kindInt:
+		print(typestring, "(", *(*int)(eface.data), ")")
+	case kindInt8:
+		print(typestring, "(", *(*int8)(eface.data), ")")
+	case kindInt16:
+		print(typestring, "(", *(*int16)(eface.data), ")")
+	case kindInt32:
+		print(typestring, "(", *(*int32)(eface.data), ")")
+	case kindInt64:
+		print(typestring, "(", *(*int64)(eface.data), ")")
+	case kindUint:
+		print(typestring, "(", *(*uint)(eface.data), ")")
+	case kindUint8:
+		print(typestring, "(", *(*uint8)(eface.data), ")")
+	case kindUint16:
+		print(typestring, "(", *(*uint16)(eface.data), ")")
+	case kindUint32:
+		print(typestring, "(", *(*uint32)(eface.data), ")")
+	case kindUint64:
+		print(typestring, "(", *(*uint64)(eface.data), ")")
+	case kindUintptr:
+		print(typestring, "(", *(*uintptr)(eface.data), ")")
+	case kindFloat32:
+		print(typestring, "(", *(*float32)(eface.data), ")")
+	case kindFloat64:
+		print(typestring, "(", *(*float64)(eface.data), ")")
+	case kindComplex64:
+		print(typestring, *(*complex64)(eface.data))
+	case kindComplex128:
+		print(typestring, *(*complex128)(eface.data))
+	default:
+		print("(", typestring, ") ", eface.data)
 	}
 }
 

src/runtime/panic_test.go

@@ -0,0 +1,48 @@
+// 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.
+
+package runtime_test
+
+import (
+	"strings"
+	"testing"
+)
+
+// Test that panics print out the underlying value
+// when the underlying kind is directly printable.
+// Issue: https://golang/go/issues/37531
+func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
+	tests := []struct {
+		name            string
+		wantPanicPrefix string
+	}{
+		{"panicCustomBool", `panic: main.MyBool(true)`},
+		{"panicCustomComplex128", `panic: main.MyComplex128(+3.210000e+001+1.000000e+001i)`},
+		{"panicCustomComplex64", `panic: main.MyComplex64(+1.100000e-001+3.000000e+000i)`},
+		{"panicCustomFloat32", `panic: main.MyFloat32(-9.370000e+001)`},
+		{"panicCustomFloat64", `panic: main.MyFloat64(-9.370000e+001)`},
+		{"panicCustomInt", `panic: main.MyInt(93)`},
+		{"panicCustomInt8", `panic: main.MyInt8(93)`},
+		{"panicCustomInt16", `panic: main.MyInt16(93)`},
+		{"panicCustomInt32", `panic: main.MyInt32(93)`},
+		{"panicCustomInt64", `panic: main.MyInt64(93)`},
+		{"panicCustomString", `panic: main.MyString("Panic")`},
+		{"panicCustomUint", `panic: main.MyUint(93)`},
+		{"panicCustomUint8", `panic: main.MyUint8(93)`},
+		{"panicCustomUint16", `panic: main.MyUint16(93)`},
+		{"panicCustomUint32", `panic: main.MyUint32(93)`},
+		{"panicCustomUint64", `panic: main.MyUint64(93)`},
+		{"panicCustomUintptr", `panic: main.MyUintptr(93)`},
+	}
+
+	for _, tt := range tests {
+		t := t
+		t.Run(tt.name, func(t *testing.T) {
+			output := runTestProg(t, "testprog", tt.name)
+			if !strings.HasPrefix(output, tt.wantPanicPrefix) {
+				t.Fatalf("%q\nis not present in\n%s", tt.wantPanicPrefix, output)
+			}
+		})
+	}
+}

src/runtime/testdata/testprog/panicprint.go

@@ -0,0 +1,111 @@
+// 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.
+
+package main
+
+type MyBool bool
+type MyComplex128 complex128
+type MyComplex64 complex64
+type MyFloat32 float32
+type MyFloat64 float64
+type MyInt int
+type MyInt8 int8
+type MyInt16 int16
+type MyInt32 int32
+type MyInt64 int64
+type MyString string
+type MyUint uint
+type MyUint8 uint8
+type MyUint16 uint16
+type MyUint32 uint32
+type MyUint64 uint64
+type MyUintptr uintptr
+
+func panicCustomComplex64() {
+	panic(MyComplex64(0.11 + 3i))
+}
+
+func panicCustomComplex128() {
+	panic(MyComplex128(32.1 + 10i))
+}
+
+func panicCustomString() {
+	panic(MyString("Panic"))
+}
+
+func panicCustomBool() {
+	panic(MyBool(true))
+}
+
+func panicCustomInt() {
+	panic(MyInt(93))
+}
+
+func panicCustomInt8() {
+	panic(MyInt8(93))
+}
+
+func panicCustomInt16() {
+	panic(MyInt16(93))
+}
+
+func panicCustomInt32() {
+	panic(MyInt32(93))
+}
+
+func panicCustomInt64() {
+	panic(MyInt64(93))
+}
+
+func panicCustomUint() {
+	panic(MyUint(93))
+}
+
+func panicCustomUint8() {
+	panic(MyUint8(93))
+}
+
+func panicCustomUint16() {
+	panic(MyUint16(93))
+}
+
+func panicCustomUint32() {
+	panic(MyUint32(93))
+}
+
+func panicCustomUint64() {
+	panic(MyUint64(93))
+}
+
+func panicCustomUintptr() {
+	panic(MyUintptr(93))
+}
+
+func panicCustomFloat64() {
+	panic(MyFloat64(-93.70))
+}
+
+func panicCustomFloat32() {
+	panic(MyFloat32(-93.70))
+}
+
+func init() {
+	register("panicCustomComplex64", panicCustomComplex64)
+	register("panicCustomComplex128", panicCustomComplex128)
+	register("panicCustomBool", panicCustomBool)
+	register("panicCustomFloat32", panicCustomFloat32)
+	register("panicCustomFloat64", panicCustomFloat64)
+	register("panicCustomInt", panicCustomInt)
+	register("panicCustomInt8", panicCustomInt8)
+	register("panicCustomInt16", panicCustomInt16)
+	register("panicCustomInt32", panicCustomInt32)
+	register("panicCustomInt64", panicCustomInt64)
+	register("panicCustomString", panicCustomString)
+	register("panicCustomUint", panicCustomUint)
+	register("panicCustomUint8", panicCustomUint8)
+	register("panicCustomUint16", panicCustomUint16)
+	register("panicCustomUint32", panicCustomUint32)
+	register("panicCustomUint64", panicCustomUint64)
+	register("panicCustomUintptr", panicCustomUintptr)
+}