I was essentially doing something like the following:

e <- rlang::expr(x + 1)
rlang::expr(lapply(X, function(x) { !!e }))
# lapply(X, function(x) {
#    x + 1
# })

However, when running covr, the function inside lapply gets an extra set of curly braces, so expr ends up seeing:

rlang::expr(lapply(X, function(x) {{ !!e }}))
# Error: `arg` must be a symbol

Causing an error because rlang 0.4.0 now treats {{ as an interpolation marker.