Sending output to Nim’s standard error stream?

You can't redirect echo, though calling writeLine with the stderr handle gives the equivalent result (you don't need any special imports):

stderr.writeLine(
  "Failure: ", 57)

Relevant documentation:

• The streams module's writeLine (https://nim-lang.org/docs/streams.html#writeLn,Stream,varargs[string,])

• The system module's stderr (https://nim-lang.org/docs/system.html#stderr)

You can alternatively invoke writeLine (https://nim-lang.org/docs/io.html#writeLine%2CFile%2Cvarargs%5BTy%2C%5D). You'll find it in the Exports of system (https://nim-lang.org/docs/system.html#19). The echo documentation (https://nim-lang.org/docs/system.html#echo%2Cvarargs%5Btyped%2C%5D) also recommends using flushFile (https://nim-lang.org/docs/io.html#flushFile,File), as shown here.

writeLine(stderr,
          "my failure")
flushFile(stderr)

I tested this using Nim 1.4.2.