Incorrect error message when keywords are used on the LHS of comprehensions

[charlesdaniels]

Short description

In some circumstances, use of keywords on the left hand side of comprehensions can cause the error message to be unexpected identifier token: non-terminated object rather than unexpected ___ keyword. Although OPA is correct to throw an error in this situation, it makes the problem much harder to debug.

Steps To Reproduce

$ opa eval -f pretty '{{"b": bt, "a": as} | as := [1,2,3][_]; bt := ["a","b","c"][_]}'
1 error occurred: 1:8: rego_parse_error: unexpected identifier token: non-terminated object
	{{"b": bt, "a": as} | as := [1,2,3][_]; bt := ["a","b","c"][_]}
	      ^
$ opa eval -f pretty '[{"b": bt, "a": as} | as := [1,2,3][_]; bt := ["a","b","c"][_]]'
1 error occurred: 1:8: rego_parse_error: unexpected identifier token: non-terminated object
	[{"b": bt, "a": as} | as := [1,2,3][_]; bt := ["a","b","c"][_]]
	      ^
$ opa eval -f pretty '{{"a": as, "b": bt} | as := [1,2,3][_]; bt := ["a","b","c"][_]}'
1 error occurred: 1:8: rego_parse_error: unexpected as keyword
	{{"a": as, "b": bt} | as := [1,2,3][_]; bt := ["a","b","c"][_]}
	      ^
$ opa eval -f pretty '[{"a": as, "b": bt} | as := [1,2,3][_]; bt := ["a","b","c"][_]]'
1 error occurred: 1:8: rego_parse_error: unexpected as keyword
	[{"a": as, "b": bt} | as := [1,2,3][_]; bt := ["a","b","c"][_]]

Expected behavior

All of the above examples should have emitted rego_parse_error: unexpected as keyword errors, and pointed to the as, not to the bt, which is an allowed variable name.

Additional context

$ uname -a
Linux eos 6.6.10-76060610-generic #202401051437~1709085277~22.04~31d73d8 SMP PREEMPT_DYNAMIC Wed F x86_64 x86_64 x86_64 GNU/Linux
$ cat /etc/lsb-release
DISTRIB_ID=Pop
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Pop!_OS 22.04 LTS"
$ opa version 
Version: 0.63.0
Build Commit: bb30b153692aa77738b982e29424c8b24a1ad14a
Build Timestamp: 2024-03-28T15:50:15Z
Build Hostname: 
Go Version: go1.22.2
Platform: linux/amd64
WebAssembly: unavailable