Unexpected behaviour when prepending/appending to initially empty variables multiple times

[gridbugs]

When a variable is initially unset or empty, appending to it and then prepending to it using the colon operators (:= and =:) causes an unexpected leading colon in the result. Prepending to an unset variable followed by appending to it produces the same unexpected result.

Here is a package reproducing the problem:

opam-version: "2.0"

build-env: [

 [ X := "a" ]
# X should be "a:"
 [ X =: "b" ]
# X shoulde be "a:b"
# (assuming opam doesn't add a separator when appending to a variable which ends with a separator)
# X unexpectedly has the value ":a:b"

 [ Y =: "a" ]
# Y should be ":a"
 [ Y := "b" ]
# Y should be "b:a"
# (assuming opam doesn't add a separator when prepending to a variable which starts with a separator)
# Y unexpectedly has the value ":b:a"

]

install: [
    [ "bash" "-c" "echo X=$X >> %{lib}%/foo" ]
    [ "bash" "-c" "echo Y=$Y >> %{lib}%/foo" ]
]

Reproduce by running opam install ./foo.opam && cat _opam/lib/foo which will produce the output:

X=:a:b
Y=:b:a

But according to the documentation the output should be:

X=a:b
Y=b:a

I'm on linux. Reproduced on both 2.1.5 and opam-2.2.0-beta2.

[dra27]

As in #5935, cross-referencing:

• fish: add colon for fish manpath #3886 (fixing opam 2.0.4 clobbers MANPATH on fish #3878, opam2 2.0.0~beta2 destroyed my MANPATH env var (I use bash) #2870 and opam init breaks fish shell with configuration #2255)
• Fixes: Env var & delimiters, unversionned opam file pin, doc #3404 (fixing opam breaks man #3391)
• Don't set MANPATH on any BSD-like (incl. macos) #3325 (fixing opam config env appears to blow away MANPATH #3319)

The documentation for =: and := I think is not correct since #3404. I think the semantics of := and =: should be as given for empty/unset values but also that they both preserve trailing and leading : in the current value.

Given those semantics:

• [ X := "a" ] with X empty gives a: (current behaviour is correct)
• [ X =: "b" ] with X set to a: gives a:b:, adding b at the end, but preserving the trailing colon (current behaviour is incorrect as X=:a:b)
• [ Y =: "a" ] with Y empty gives :a (current behaviour is correct)
• [ Y := "b" ] with Y set to :a gives :b:a, adding b at the start, but preserving the leading colon (current behaviour is correct)

So I think that the treatment of X here is incorrect, but the treatment of Y is correct (but the documentation should be updated to reflect this).