Fixes #10635. Improves warning and error messages shown when parsing project files and their imports.

Warning Messages

To trigger these warning messages, the examples use badly formed comments that have a single dash instead of two as is required of a line comment in .cabal and .project files (and imported .config files).

• Before the fix:

  The cabal.project file name is repeated. Warnings are misattributed to having been in the project rather than from a configuration file imported by the project. Warnings are shown in reverse line number order.

  $ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
  ...
  Warning:
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 123
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 3
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 2
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 1
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 123
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 3
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 2
  /.../ParseWarningProvenance/cabal.project,
  cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
  section '-' on line 1
  

• After the fix:

  The warnings are shown in a list. For warnings within the same .project or imported .config file, warnings are sorted by line number. The file that is the source of the warning is shown.

  The warnings associated with configuration files are shown in the order these files were imported by the project:

  $ cat cabal.project
  packages: no-pkg-dir
  import: dir-x/a.config
  import: dir-y/a.config
  import: x.config
  import: y.config
  

  $ cabal build all --dry-run
  ...
  Warnings found while parsing the project file, cabal.project:
  - dir-x/a.config: Unrecognized section '-' on line 1
  - dir-x/a.config: Unrecognized section '-' on line 2
  - dir-x/a.config: Unrecognized section '-' on line 3
  - dir-y/a.config: Unrecognized section '-' on line 123
  - x.config: Unrecognized section '-' on line 1
  - x.config: Unrecognized section '-' on line 2
  - x.config: Unrecognized section '-' on line 3
  - y.config: Unrecognized section '-' on line 123
  

Error Messages from Project

To trigger these error messages, the examples use badly formed conditions:

$ cat cabal.project
-- The following failing condition is not on the first line so we can check the
-- line number:
if _

• Before the fix:

  The parse error is shown with hard line breaks.

  $ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
  ...
  Error: [Cabal-7090]
  Error parsing project file /.../ParseErrorProvenance/cabal.project:3:
  "<condition>" (line 1, column 1):
  unexpected SecArgName (Position 1 4) "_"
  

• After the fix:

  The snippet that failed to parse may be shown and the parse error is shown as one line, with no hard line breaks.

  $ cabal build all --dry-run
  ...
  Error: [Cabal-7090]
  Error parsing project file cabal.project:3:
  - Failed to parse 'if(_)' with error:
      "<condition>" (line 1, column 1): unexpected SecArgName (Position 1 4) "_"
  

Error Messages from Imported Config

With the same setup but now with the error in an imported file:

$ cat elif.project 
import: dir-elif/elif.config

$ cat dir-elif/elif.config 
-- The following failing condition is not on the first line so we can check the
-- line number:
if false
elif _

• Before the fix:

  The project rather than the imported configuration file is shown as the source file.

  $ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
  ...
  Error: [Cabal-7090]
  Error parsing project file /.../ParseErrorProvenance/elif.project:4:
  "<condition>" (line 1, column 1):
  unexpected SecArgName (Position 1 6) "_"
  

• After the fix:

  The imported configuration file is shown as the source with a snippet of the error.

  $ cabal build all --dry-run
  ...
  Error: [Cabal-7090]
  Error parsing project file dir-elif/elif.config:4:
    - dir-elif/elif.config
        imported by: elif.project
    - Failed to parse 'elif(_)' with error:
      "<condition>" (line 1, column 1): unexpected SecArgName (Position 1 6) "_"
  

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)