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Guide to Scala 3 Compiler Contribution

Compiler Phases


As described in the compiler overview, dotc is divided into a list of phases, specified in the Compiler class.

Printing the phases of the Compiler

a flattened list of all the phases can be displayed by invoking the compiler with the -Xshow-phases flag:

$ scalac -Xshow-phases

Phase Groups

In class Compiler you can access the list of phases with the method phases:

def phases: List[List[Phase]] =
  frontendPhases ::: picklerPhases ::: transformPhases ::: backendPhases

You can see that phases are actually grouped into sublists, given by the signature List[List[Phase]]; that is, each sublist forms a phase group that is then fused into a single tree traversal when a Run is executed.

Phase fusion allows each phase of a group to be small and modular, (each performing a single function), while reducing the number of tree traversals and increasing performance.

Phases are able to be grouped together if they inherit from MiniPhase.

Phase Categories

Phases fall into four categories, allowing customisation by sub-classes of Compiler:


In the main compiler these include parser, typer, posttyper, prepjsinterop and phases for producing SemanticDB and communicating with the incremental compiler Zinc. The parser reads source programs and generates untyped abstract syntax trees, which in typer are then typechecked and transformed into typed abstract syntax trees. Following is posttyper, performing checks and cleanups that require a fully typed program. In particular, it

  • creates super accessors representing super calls in traits
  • creates implementations of compiler-implemented methods, such as equals and hashCode for case classes.
  • marks compilation units that require inline expansion, or quote pickling
  • simplifies trees of erased definitions
  • checks variance of type parameters
  • mark parameters passed unchanged from subclass to superclass for later pruning.


These phases start with pickler, which serializes typed trees produced by the frontendPhases into TASTy format. Following is inlining, which expand calls to inline methods, and postInlining providing implementations of the Mirror framework for inlined calls. Finally are staging, which ensures that quotes conform to the Phase Consistency Principle (PCP), and pickleQuotes which converts quoted trees to embedded TASTy strings.


These phases are concerned with tranformation into lower-level forms suitable for the runtime system, with two sub-groupings:

  • High-level transformations: All phases from firstTransform to erasure. Most of these phases transform syntax trees, expanding high-level constructs to more primitive ones.
    • An important transform phase is patternMatcher, which converts match trees and patterns into lower level forms, as well as checking the exhaustivity of sealed types, and unreachability of pattern cases.
    • Some phases perform further checks on more primitive trees, e.g. refchecks verifies that no abstract methods exist in concrete classes, and initChecker checks that fields are not used before initialisation.
    • The last phase in the group, erasure translates all types into types supported directly by the JVM. To do this, it performs another type checking pass, but using the rules of the JVM’s type system instead of Scala’s.
  • Low-level transformations: All phases from ElimErasedValueType to CollectSuperCalls. These further transform trees until they are essentially a structured version of Java bytecode.


These map the transformed trees to Java classfiles or SJSIR files.

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