Regardless of the implementation details (which can be bikeshed

separately afterwards if the idea is considered worthy), there seem to
be 2 different suggestions in my reading:

1. Pluggable string prefixes allowing custom parsing of string literals
   and their resulting in arbitrary types, e.g. a `d` prefix on a string
   literal would yield a decimal.Decimal (one can also imagine an
   `h`-string for markup-safe strings literals[1] for instance)

2. Some sort of codegen/environment hook allowing (amongst other
   things?) the implementation of shell/ruby/perl string interpolation
   (Scala uses prefixes for interpolator specifications[2])