unhelpful failed-to-parse error from get_source_expressions

[MichaelChirico]

I had a stray = in the middle of a large file, the linting error was pretty arcane. MRE:

s <- '
foo(
  x,
  key = y["key"],
  = l$key
)'
writeLines(s, tmp <- tempfile())

Compare:

parse(tmp)
# Error in parse(tmp) : 
#   /tmp/Rtmpag3DPy/file33c5731c590bf5:5:3: unexpected '='
# 4:   key = y["key"],
# 5:   =
#      ^
lintr::get_source_expressions(tmp)
# Error in vapply(eq_assign_locs, next_with_parent, pc = pc, integer(1)) : 
#   values must be length 1,
#  but FUN(X[[1]]) result is length 0

[MichaelChirico]

IINM #560 closed this as well, thanks @renkun-ken!

lintr::lint(tmp)
# /tmp/Rtmp94vinS/file50cc4454e041:2:4: style: Place a space before left parenthesis, except in a function call.
# foo(
#    ^
# /tmp/Rtmp94vinS/file50cc4454e041:5:3: style: Use <-, not =, for assignment.
#   = l$key
#   ^
# /tmp/Rtmp94vinS/file50cc4454e041:5:3: error: unexpected '='
#   = l$key
#   ^

@AshesITR please feel free to open if you'd like to add a regression test, I would be happy to file a PR if so