Compiler doesn't terminate on file with syntax errors

[tamasvajk]

Version Used:
C# Compiler version 4.8.0-7.23572.1 (7b75981)

Steps to Reproduce:

1. Get this file from microsoft/fhir-codegen.
2. Run dotnet /usr/local/share/dotnet/sdk/8.0.101/Roslyn/bincore/csc.dll generated/CSharpFirely2_R5/Generated/Bundle.cs

Another option is copying the content of that file into https://sharplab.io/.

Expected Behavior:
The compiler should terminate in reasonable time.

Actual Behavior:
The compiler doesn't terminate in reasonable time.

Additional Note:
C# Compiler version 3.9.0-6.21124.20 (db94f4c) can parse the file, and it reports the syntax errors.

[cston]

@CyrusNajmabadi, it looks like parsing ends up in LanguageParser.ParseCollectionExpression() and unable to make progress.

[CyrusNajmabadi]

Relates to this error construct:

      [EnumLiteral("archives", "http://hl7.org/fhir/CodeSystem/iana-link-relations"), Description("Refers to a collection of records, documents, or other
      materials of historical interest.")]
      Archives,

Note the wrapping over multiple lines.

[CyrusNajmabadi]

Ick. this is challenging/problematic. So the string is unterminated on the first line. So we terminate it (with an error saying it is missing). But then we get to the next line and we are just in extreme error mode. We see "materials", "of", "historical" "interest" all as continued arguments. We then hit the quote and that stats ")] another argument to the attribute. And, at that point, all the remaining enum members just appear to be more and more arguments. And, of course, as arguments, we think we might be seeing collection-exprs when we run into attributes on the enum members. THis starts a cascading of speculative parsing, which explodes given how many elements there are. Will have to noodle on what we can do here.

[CyrusNajmabadi]

Note: it's not an infinite loop. More just really expensive with backtracking and retrying things.