Matthew Poole and continuators on James 5:3

3 Your gold and silver is cankred, and the rust of them shall be a witness against you , and shall eat your flesh as it were fire : Ye have heaped treasure together for the last daies .

The most precious and lasting Metals; yet even they with long disuse, canker, and go to decay. Under these, other Metals in esteem among them, may be understood.

By a Prosopopoeia, that which properly belongs to living Persons, is ascribed to dead things, as Hab. 2.11. Luke 19.40. it is as much as if he had said, The Rust shall be a certain Evidence against you, and which will as effectually convict you, as any living Witness could do, of your Folly in putting your trust in perishing things, your Greediness in hoarding them up, your Unmercifulness in not supplying the Wants of others, and your Unreasonableness in denying the Use of them to your selves, when you had rather let them ly by and perish, than enjoy the Comfort of them, or do good with them. The like Expression we have, Mark 6.11.

The Rust (the Witness of your Covetousness and Cruelty) which now eats your Money, shall hereafter devour your selves, Soul and Body (which he means by Flesh) viz. by procuring, and kindling the Wrath of God upon you, (compared to Fire) and likewise by galling your Consciences with a vexatious Remembrance of your Sin, and Folly; and so what in the Judgment is a Witness against you, in Hell will be a Tormentor to you.

As if you had reserved Fire in your Treasure, as well as Treasure in your Chests.

Either this may be understood metaphorically, ye have heaped a Treasure of Wrath for the last days, Rom. 2.5. or literally, ye have hoarded up your Wealth against the last, and fatal days, in which God is bringing those Judgments upon you, which will consume all.