Give me a smarter compiler! How? Here's how!

In dark ages writing code and compiling code was two separate processes. As we get better languages and faster hardware and compilers, writing code and compiling code becomes more and more integrated. For instance, when programming Java in Eclipse the source is compiled in the background every time the file is saved. Very nice because it gives fast feedback!

This means that since the code is compiled so often there usually only a small difference between the previous compiled version of the code and the current. This got me thinking: would it be possible to improve the error messages from the compiler by comparing the current code with the previously compiled code?

Let's take an example. Assume that there is an interface I with a method void m(int) that is implemented by a class C. The code compiles without errors. Then you change the signature of m in C to void m(long), which breaks the code of course. In this case the compiler could give an error message something like "Changing C.m(int) to C.m(long) makes non-abstract class C abstract, because I.m(int) is no longer implemented" instead of the error message given by today's compilers which is something like "Non-abstract class C is missing implementation for I.m(int)".

For this small example it may seem like a small improvement, but I think that if the compiler has access to the latest working code, then it can help in several way. Better error messages is only one improvement. Other improvements could be to give warnings when the semantics of the code has changed in an uncommon way. Let's take an example of this.

Let's say an instance of the java.lang.Thread class is created and its void start() method is called. During refactoring, the call to start is removed by mistake. The compiles knows that its a common pattern to instantiate a java.lang.Thread and then call start on it, thus, the compiler gives a warning indicating the (potential) problem. For instance something like: "Thread is created but not started because the call to Thread.start() has be removed between the two latest version of the code." It's almost like having a conversation with the compiler.

Another nice thing with this kind of warning is that it only appears when the code has changed. In other words, it is the difference between two consecutive versions of the code that triggers the warning, not the individual versions.

If the programmers intent is that the thread should not be started, then the warning can be ignored and the next time the code is compiled the compiler will not warn about the "missing" call to Thread.start(), because from the compiler's perspective the call is not missing anymore.

This idea can of course be useful for static code analyzers, such as FindBugs, as well.