I was basically referring to the usual technique in today's low and medium power boost (e. g. PFC) and flyback (e. g. isolating off-mains) SMPS. As a matter of fact, they mainly use current sense resistors.
Obviously other techniques are available as well. Current sensing by MOS Rds,on is an interesting alternative particularly for low voltage synchronous buck regulators. It has to consider parameter variations and Rds,on TC, however.
Hall sensors are good for power electronics beyond several 10 A.
I don't yet understand, how a current mirror could have a meaningful application in this regard. Can anyone suggest a principle circuit?