Stringification and the design of debug string facilities

[soc]

Plan

Stringable#toString() focuses on general-purpose output:

• A textual representation that provides useful information about a type for general consumption.
• It avoids exposing innards or implementation details (capacity for collections, the structure of binary trees, the occupancy ratio of hash maps etc.).
• The output remains relatively stable across version and implementation changes.
• It requires implementing the Stringable traits, and putting that constraint on parameters/values when calling the method.
• It can be implemented manually by users or derived by the compiler via an annotation
• It has no access to runtime-internal facilities and has to follow the general rules of the language regarding visibility and accessibility.

runtime.debugString(value) provides debugging-oriented output:

• A textual representation that provides insights into implementation details and helps investigating bugs in the type that is being printed.
• It potentially exposes private fields, which would generally elided from general-purpose output.
• The output is not stable and makes no guarantees.
• It is an @internal static method implemented by the runtime.
• It uses runtime-internal facilities, which may ignore language rules to inspect innards of instances and retrieve information not accessible from user code, e. g. pointer addresses.
• Supported by these facilities, it makes reasonable efforts to avoid non-termination due to circular references, disambiguation of equal but not identical references etc.
• Usage does not require any kind of trait.
• Output of debugString is "" except in release builds.