Scala 3 slowed us down?

In Scala 3, the inline keyword is part of the macro system.

When inline is used on a parameter, it instructs the compiler to inline the expression at the call site. If the expression is substantial, this creates considerable work for the JIT compiler.

Requesting inlining at the compiler level (as opposed to letting the JIT handle it) is risky unless you can guarantee that a later compiler phase will simplify the inlined code.

There's an important behavioral difference between Scala 2 and 3: in 2, @inline was merely a suggestion to the compiler, whereas in 3, the compiler unconditionally applies the inline keyword. Consequently, directly replacing @inline with inline when migrating from 2 to 3 is a mistake.

> There's an important behavioral difference between Scala 2 and 3: in 2, @inline was merely a suggestion to the compiler, whereas in 3, the compiler unconditionally applies the inline keyword. Consequently, directly replacing @inline with inline when migrating from 2 to 3 is a mistake.

This reminds me of a similar lesson C/C++ compilers had to learn with the "register" keyword. Early versions treated the keyword as a mandate. As compiler optimizers became more refined, "register" was first a recommendation and then ultimately ignored.

The C++ inline keyword is treated similarly as well, with different metrics used of course.

EDIT:

Corrected reference to early C/C++ keyword from "auto" to "register".