assignment or shadowing drops a live linear value

Why this fires¶

An assignment or a let overwrites or shadows a linear value that is still owed, so nothing could ever consume the old value.

let f = open("a")
f = open("b")        // TE176: the first file is lost

A defer over f does not make the assignment safe: it reads the variable when it fires, so it would close the second file and the first would still leak. A shadowing let f = … is accepted when a defer already covers the old f, because the shadow is a new variable and the defer keeps the old one.

Fix it¶

Consume the old value first, or give the new one another name.