Formatter removes first option in alternation rules

[samir-abis]

Langium version: 3.1.1 (extension)
Package name: langium

Steps To Reproduce

1. Create a .langium file with an alternation rule like:

NumberTypeSpecialization:
    {infer Int} 'Int' |
    {infer Float} 'Float' |
    {infer Decimal} 'Decimal';

2. Activate the format on Langium files

The current behavior

The formatter removes the first option in alternation rules, resulting in:

NumberTypeSpecialization:
    
     |
    {infer Float} 'Float' |
    {infer Decimal} 'Decimal';

This occurs for any rule that starts with an alternation (|).

The expected behavior

The formatter should preserve all options in alternation rules, maintaining the original structure:

NumberTypeSpecialization:
    {infer Int} 'Int' |
    {infer Float} 'Float' |
    {infer Decimal} 'Decimal';

[ydaveluy]

Hello,

I found where is the issue in the formatter code:
Sometime the between range has a start position after the end position that lead to incorrect edits :

langium/packages/langium/src/lsp/formatter.ts

Line 362 in b2c40e3

const betweenRange: Range = {

Returning an empty edit in such case fix the issue.

// Ignore the edit if the previous node ends after the current node starts
if (a && (a.range.end.line > b.range.start.line ||
    (a.range.end.line === b.range.start.line && a.range.end.character > b.range.start.character))) {
    return [];
}
const betweenRange: Range = {
    start: a?.range.end ?? {
        character: 0,
        line: 0
    },
    end: b.range.start
};

@msujew I can provide a PR if you are interested. It can be included in #1628 or in a separate PR if you prefer.

[msujew]

@ydaveluy Thanks for looking into this! A separate PR would be appreciated :)

[ydaveluy]

ok see PR #1629