opam admin add-constraint does too much reformatting

[kit-ty-kate]

I tried to use opam admin add-constraint and I found out that apart from adding the correct constraint it also reformat unrelated lines in the depends field.
This makes it a bit impractical to use.

For example opam admin add-constraint dune>=1.11 changes hardcaml.v0.12.0, lwt.4.2.1 and other similar files in the following manner:

diff --git a/packages/hardcaml/hardcaml.v0.12.0/opam b/packages/hardcaml/hardcaml.v0.12.0/opam
index ca3c8171bf..87f513d55c 100644
--- a/packages/hardcaml/hardcaml.v0.12.0/opam
+++ b/packages/hardcaml/hardcaml.v0.12.0/opam
@@ -10,13 +10,13 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml"            {>= "4.07.0"}
-  "base"             {>= "v0.12" & < "v0.13"}
-  "ppx_jane"         {>= "v0.12" & < "v0.13"}
-  "stdio"            {>= "v0.12" & < "v0.13"}
+  "ocaml" {>= "4.07.0"}
+  "base" {>= "v0.12" & < "v0.13"}
+  "ppx_jane" {>= "v0.12" & < "v0.13"}
+  "stdio" {>= "v0.12" & < "v0.13"}
   "topological_sort" {>= "v0.12" & < "v0.13"}
-  "dune"             {>= "1.5.1"}
-  "zarith"           {>= "1.5"}
+  "dune" {>= "1.11"}
+  "zarith" {>= "1.5"}
 ]
 synopsis: "RTL Hardware Design in OCaml"
 description: "

diff --git a/packages/lwt/lwt.4.2.1/opam b/packages/lwt/lwt.4.2.1/opam
index 6c4fb5e040..431af99ad7 100644
--- a/packages/lwt/lwt.4.2.1/opam
+++ b/packages/lwt/lwt.4.2.1/opam
@@ -20,12 +20,11 @@ dev-repo: "git+https://github.com/ocsigen/lwt.git"
 
 depends: [
   "cppo" {build & >= "1.1.0"}
-  "dune"
-  "mmap"  # mmap is needed as long as Lwt supports OCaml < 4.06.0.
+  "dune" {>= "1.11"}
+  "mmap"
   "ocaml" {>= "4.02.0"}
-  "result" # result is needed as long as Lwt supports OCaml 4.02.
-  "seq" # seq is needed as long as Lwt supports OCaml < 4.07.0.
-
+  "result"
+  "seq"
   "bisect_ppx" {dev & >= "1.3.0"}
   "ocamlfind" {dev & >= "1.7.3-1"}
 ]

This would require to modify OpamFile.Syntax.to_string_with_preserved_format by detecting which elements changed and only modifying the constraints bloc for those elements only, leaving the rest of the line untouched.

Tested with opam 2.0.5

[rjbou]

Handling it is not so trivial, as we work on an unformatted input, and not having the end position of values & fields lead to try to determine it via the next one.
Thanks for the report!