Extending 15.28 to include toString() on an enum constant

[Daniel Trebbien]

One small idea I have for enhancing the Java language is to allow
toString() on enum constants to be used in constant expressions, as long as
toString()
<https://docs.oracle.com/javase/10/docs/api/java/lang/Enum.html#toString()>
is not overidden, or if it is, the implementation returns a constant
expression.

There are two use cases I have in mind for this:

1. When using the SLF4J logging library, if you want to embed the name of
an enum constant in the log message, you should use parameterized logging
to avoid incurring a wasted runtime string concatenation if logging at the
specified level is disabled.  See SLF4J's FAQ for "What is the fastest way
of (not) logging?":  https://www.slf4j.org/faq.html#logging_performance
As an example of this in the wild, I submitted a pull request in which I
converted a log statement involving an enum constant to parameterized
logging:
https://github.com/Netflix/eureka/commit/ad81a1140f351ec19165cb94d5aa668cfc08a932
There are other cases where an SLF4J logger call involving an enum constant
correctly used parameterized logging, but I am not able to find an example
right now.

Although using parameterized logging solves the performance issue, it is
desirable to keep down the number of parameters to an SLF4J logger call
because the SLF4J Logger interface
<https://www.slf4j.org/apidocs/org/slf4j/Logger.html> has more efficient
variants of the trace(), debug(), info(), warn(), and error() calls that
take zero, exactly one, and exactly two parameters.  If a logger call
passes at most two parameters, then the construction of an Object array to
package the parameters into varargs is avoided.  Needing to "waste" a
parameter slot on an enum constant is undesirable.

2.  When using Spring Security, it is possible to define an enum that lists
the roles and other categories of permissions that are used by a Spring
application.  For example:

package com.myapp.security;

import org.springframework.security.core.GrantedAuthority;

public enum AppAuthority implements GrantedAuthority {

    ROLE_USER,
    ROLE_ADMINISTRATOR;

    @Override
    public String getAuthority() {
        return toString();
    }
}

When using the org.springframework.security.access.annotation.Secured
<https://docs.spring.io/spring-security/site/docs/5.0.6.RELEASE/api/org/springframework/security/access/annotation/Secured.html>
annotation to secure controllers or actions, it would be nice to be able to
use an enum constant in the annotation's value, as in @Secured({
ROLE_ADMINISTRATOR.toString() }); however, this is currently not valid Java
code because ROLE_ADMINISTRATOR.toString() is not a constant expression.
Instead, the name of the role must be duplicated:  @Secured({ "
ROLE_ADMINISTRATER" }).  This is undesirable because it is error-prone and
makes refactoring more difficult.  It is also more difficult to find usages
of a particular authority.  If toString() on an enum constant were allowed
within a constant expression, then finding usages of a particular authority
would be a "Find Usages" operation in an IDE.