panic when marshalling a map with the key in a custom string type

[falau]

Env

• Go version: 1.22.3
• BurntSushi/toml@v1.4.0

Description

When marshalling a map with keys in a custom string type like:

type CustomString string
type MyMap map[CustomString]int

func main() {
	var myKey CustomString = "MyKey"
	m := MyMap{myKey: 10}
	toml.NewEncoder(os.Stdout).Encode(m)
}

it turns out that the encoder simply uses a primitive string to set the map key instead of the CustomString type here, and causes panic.
As both encoding/json and goyaml can handle this case without any problem, I expect this toml package should also work in the same way?
If not, is there any way to do this properly?

Thanks

Stacktrace

panic: reflect.Value.MapIndex: value of type string is not assignable to type main.CustomString [recovered]
	panic: reflect.Value.MapIndex: value of type string is not assignable to type main.CustomString

goroutine 1 [running]:
github.com/BurntSushi/toml.(*Encoder).safeEncode.func1()
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:160 +0x73
panic({0xfb7b680?, 0xc0000140d0?})
	/usr/local/Cellar/go/1.22.3/libexec/src/runtime/panic.go:770 +0x132
reflect.Value.assignTo({0xfb7b680?, 0xc0000140c0?, 0xfc1ba80?}, {0xfb4ecf7, 0x16}, 0xfb7b640, 0x0)
	/usr/local/Cellar/go/1.22.3/libexec/src/reflect/value.go:3356 +0x299
reflect.Value.MapIndex({0xfb80360?, 0xc0000760c0?, 0x0?}, {0xfb7b680, 0xc0000140c0, 0x98})
	/usr/local/Cellar/go/1.22.3/libexec/src/reflect/value.go:1821 +0xe8
github.com/BurntSushi/toml.(*Encoder).eMap.func1({0xc0000140b0, 0x1, 0x2?}, 0x0)
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:418 +0x154
github.com/BurntSushi/toml.(*Encoder).eMap(0xc00010df20, {0xfc7a980, 0x0, 0x0}, {0xfb80360?, 0xc0000760c0?, 0xfb712f0?}, 0x0)
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:437 +0x42c
github.com/BurntSushi/toml.(*Encoder).eMapOrStruct(0xfb06f32?, {0xfc7a980?, 0xfacba9b?, 0xc00010dce0?}, {0xfb80360?, 0xc0000760c0?, 0xfb80360?}, 0x20?)
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:388 +0x4c
github.com/BurntSushi/toml.(*Encoder).eTable(0xc00010df20, {0xfc7a980, 0x0, 0x0}, {0xfb80360?, 0xc0000760c0?, 0x0?})
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:382 +0x19c
github.com/BurntSushi/toml.(*Encoder).encode(0xc00010df20, {0xfc7a980, 0x0, 0x0}, {0xfb80360?, 0xc0000760c0?, 0xc000100680?})
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:203 +0x1b5
github.com/BurntSushi/toml.(*Encoder).safeEncode(0xfb80360?, {0xfc7a980?, 0x0?, 0xfc1c100?}, {0xfb80360?, 0xc0000760c0?, 0x40?})
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:163 +0x6c
github.com/BurntSushi/toml.(*Encoder).Encode(0xc00010df20, {0xfb80360?, 0xc0000760c0?})
	/go/pkg/mod/github.com/!burnt!sushi/toml@v1.4.0/encode.go:146 +0x9c

main.main()
	/myrepo/mygo/cmd/mytest/main.go:16 +0x10f
exit status 2