Add be.AssignedAs and deprecate be.Panic

I don't think the current panic assertion is very good, because
asserting _that_ something panics is a lot less effective than asserting
the _value_ of a panic. So this refactor makes it so you have to
orchestrate the panic recovery yourself, but also makes it easier to
assert something of type `any` with a more specific assertion.

And generally I think `be.AssignedAs` is a smart assertion to have. It
roughly resembles the shape of `be.ErrorAs`, so it shouldn't be very
surprising.

Author: rliebz

README.md

@@ -109,8 +109,6 @@ g.Should(be.DeepEqual([]string{"a", "b"}, []string{"a", "b"}))
 g.Should(be.SliceContaining([]int{1, 2, 3}, 2))
 g.Should(be.StringContaining("foobar", "foo"))
 
-g.Should(be.Panic(func() { panic("oh no") }))
-
 var err error
 g.NoError(err)
 g.Must(be.Nil(err))
@@ -203,6 +201,24 @@ g.Should(BeThirteen(myInt)) // "myInt is 0"
 g.Should(BeThirteen(5 + 6)) // "5 + 6 is 11"
 ```
 
+#### Handling Panics
+
+If you expect your code to panic, it is better to assert that the value passed
+to `panic` has the properties you expect, rather than to make an assumption
+that the panic you encountered is the panic you were expecting. Ghost can be
+combined with `defer`/`recover` to access the full expressiveness of test
+assertions:
+
+```go
+defer func() {
+	var err error
+	g.Must(be.AssignedAs(recover(), &err))
+	g.Should(be.ErrorEqual(err, "a specific error occurred"))
+}()
+
+doStuff()
+```
+
 ## Philosophy
 
 ### Ghost Does Assertions

be/assertions.go

@@ -14,6 +14,52 @@ import (
 	"github.com/rliebz/ghost/internal/constraints"
 )
 
+// AssignedAs assigns a value to a target of an arbitrary type.
+//
+// The target must be a non-nil pointer.
+func AssignedAs[T any](value any, target *T) ghost.Result {
+	args := ghostlib.ArgsFromAST(value, target)
+	argValue, argTarget := args[0], args[1]
+
+	if target == nil {
+		return ghost.Result{
+			Ok:      false,
+			Message: fmt.Sprintf("target %s cannot be nil", argTarget),
+		}
+	}
+
+	typedValue, ok := value.(T)
+	if !ok {
+		return ghost.Result{
+			Ok: false,
+			Message: fmt.Sprintf(
+				`%s (%T) could not be assigned to %s (%T)
+value: %v`,
+				argValue,
+				value,
+				argTarget,
+				target,
+				value,
+			),
+		}
+	}
+
+	*target = typedValue
+
+	return ghost.Result{
+		Ok: true,
+		Message: fmt.Sprintf(
+			`%s (%T) was assigned to %s (%T)
+value: %v`,
+			argValue,
+			value,
+			argTarget,
+			target,
+			value,
+		),
+	}
+}
+
 // Close asserts that a value is within a delta of another.
 func Close[T constraints.Integer | constraints.Float](got, want, delta T) ghost.Result {
 	args := ghostlib.ArgsFromAST(got, want, delta)
@@ -287,6 +333,10 @@ func isNil(v any) bool {
 }
 
 // Panic asserts that the given function panics when invoked.
+//
+// Deprecated: It is better to defer a call to recover with a more specific
+// assertion about the value of a panic, rather than only asserting that a
+// function panicked.
 func Panic(f func()) (result ghost.Result) {
 	args := ghostlib.ArgsFromAST(f)
 	argF := args[0]

be/assertions_test.go

@@ -1,14 +1,110 @@
 package be_test
 
 import (
+	"bytes"
 	"errors"
+	"io"
 	"strings"
 	"testing"
 
 	"github.com/rliebz/ghost"
 	"github.com/rliebz/ghost/be"
 )
 
+func TestAssignedAs(t *testing.T) {
+	t.Run("primitive valid", func(t *testing.T) {
+		g := ghost.New(t)
+
+		var got any = "some-value"
+		var want string
+
+		result := be.AssignedAs(got, &want)
+		g.Should(be.True(result.Ok))
+		g.Should(be.Equal(result.Message, `got (string) was assigned to &want (*string)
+value: some-value`))
+
+		result = be.AssignedAs("some-value", new(string))
+		g.Should(be.True(result.Ok))
+		g.Should(be.Equal(result.Message, `"some-value" (string) was assigned to new(string) (*string)
+value: some-value`))
+	})
+
+	t.Run("primitive invalid", func(t *testing.T) {
+		g := ghost.New(t)
+
+		var got any = 15
+		var want string
+
+		result := be.AssignedAs(got, &want)
+		g.Should(be.False(result.Ok))
+		g.Should(be.Equal(result.Message, `got (int) could not be assigned to &want (*string)
+value: 15`))
+
+		result = be.AssignedAs(15, new(string))
+		g.Should(be.False(result.Ok))
+		g.Should(be.Equal(result.Message, `15 (int) could not be assigned to new(string) (*string)
+value: 15`,
+		))
+	})
+
+	t.Run("interface valid", func(t *testing.T) {
+		g := ghost.New(t)
+
+		var got any = new(bytes.Buffer)
+		var want io.Reader
+
+		result := be.AssignedAs(got, &want)
+		g.Should(be.True(result.Ok))
+		g.Should(be.Equal(result.Message, `got (*bytes.Buffer) was assigned to &want (*io.Reader)
+value: `))
+
+		result = be.AssignedAs(new(bytes.Buffer), new(io.Reader))
+		g.Should(be.True(result.Ok))
+		g.Should(be.Equal(result.Message, `new(bytes.Buffer) (*bytes.Buffer) was assigned to new(io.Reader) (*io.Reader)
+value: `))
+	})
+
+	t.Run("interface invalid", func(t *testing.T) {
+		g := ghost.New(t)
+
+		var got any = 15
+		var want io.Reader
+
+		result := be.AssignedAs(got, &want)
+		g.Should(be.False(result.Ok))
+		g.Should(be.Equal(result.Message, `got (int) could not be assigned to &want (*io.Reader)
+value: 15`))
+
+		result = be.AssignedAs(15, new(io.Reader))
+		g.Should(be.False(result.Ok))
+		g.Should(be.Equal(result.Message, `15 (int) could not be assigned to new(io.Reader) (*io.Reader)
+value: 15`))
+	})
+
+	t.Run("nil target", func(t *testing.T) {
+		g := ghost.New(t)
+
+		var got any = 15
+		var want *int
+
+		result := be.AssignedAs(got, want)
+		g.Should(be.False(result.Ok))
+		g.Should(be.Equal(result.Message, "target want cannot be nil"))
+	})
+
+	t.Run("panic", func(t *testing.T) {
+		g := ghost.New(t)
+
+		defer func() {
+			var err error
+			g.Must(be.AssignedAs(recover(), &err))
+			g.Should(be.ErrorEqual(err, "oops"))
+		}()
+
+		panic(errors.New("oops"))
+	})
+}
+
 func TestClose(t *testing.T) {
 	t.Run("in delta", func(t *testing.T) {
 		g := ghost.New(t)

example_test.go

@@ -27,9 +27,6 @@ func TestExample(t *testing.T) {
 
 	g.Should(be.StringContaining("foobar", "foo"))
 
-	g.Should(be.Panic(func() { panic("oh no") }))
-	g.ShouldNot(be.Panic(func() {}))
-
 	var err error
 	g.NoError(err)
 	g.Must(be.Nil(err))
@@ -44,6 +41,19 @@ func TestExample(t *testing.T) {
 	g.ShouldNot(be.JSONEqual(`{"a":1}`, `{"a":2}`))
 }
 
+func ExampleAssignedAs() {
+	t := new(testing.T) // from the test
+	g := ghost.New(t)
+
+	defer func() {
+		var err error
+		g.Must(be.AssignedAs(recover(), &err))
+		g.Should(be.ErrorEqual(err, "oops"))
+	}()
+
+	panic(errors.New("oops"))
+}
+
 func ExampleEventually() {
 	t := new(testing.T) // from the test
 	g := ghost.New(t)