compilation.EmitDifference produces no updated methods when "#line hidden" is present

[SteveSandersonMS]

Version Used: 3.8.0-5.final

Steps to Reproduce:

1. Call compilation.EmitDifference, passing a single SemanticEdit that corresponds to a C# line that is preceded by #line hidden
2. Observe that the updatedMethods collection is not populated with anything, whereas it would be if #line hidden was not present

Expected Behavior:

I definitely did not expect #line hidden to influence this. It took a long time to track down that this was causing the failure to produce deltas :)

Scenario:

I'm prototyping .NET 6 hot reload for Blazor. The Razor compiler prefixes pretty much all generated user code with #line hidden, so this behavior prevents changes in .razor files from being applied to the running app.

To show this is definitely what's interfering, I tried amending my hot reload prototype to contain newSourceText = newSourceText.Replace("#line hidden", "//#line hidden"); when reading the generated Razor source code, and it does make the problem go away, so user changes in .razor files do flow into the running app.

Related issue:

I expect this is related to #40655 and probably ties in with some historical quirks.

Request:

Is there some option to disable this special-casing of #line hidden? It's not something we'd want when it comes to processing the Razor compiler's generated code. If there's no option for that yet, would you consider adding one?

[jaredpar]

ENC level issue moving to IDE

[jinujoseph]

cc @tmat

[tmat]

EnC now works in Razor files (with #52537), but function remapping is still broken due to #line mapping issues.