Jamieson, Fausset and Brown on James 5:3

3. is cankered--"rusted through" [Alford].

rust ... witness against you--in the day of judgment; namely, that your riches were of no profit to any, lying unemployed and so contracting rust.

shall eat your flesh--The rust which once ate your riches, shall then gnaw your conscience, accompanied with punishment which shall prey upon your bodies for ever.

as ... fire--not with the slow process of rusting, but with the swiftness of consuming fire.

for the last days--Ye have heaped together, not treasures as ye suppose (compare Lu 12:19), but wrath against the last days, namely, the coming judgment of the Lord. Alford translates more literally, "In these last days (before the coming judgment) ye laid up (worldly) treasure" to no profit, instead of repenting and seeking salvation (see on Jas 5:5).