Logging source generator generates uncompilable code for new .NET Framework project

[stephentoub]

Same issue as #83392, but for the logging source generator rather than for the JSON source generator.

Repro:

1. Create a new .NET Framework 4.8 console app.
2. Add a nuget reference to the latest stable Microsoft.Extensions.Logging.Abstractions nuget package.
3. Paste this code into your Program.cs, which is based on the docs for [LoggerMessage]:

using Microsoft.Extensions.Logging;

internal partial class Program
{
    static void Main() { }

    [LoggerMessage(
        EventId = 0,
        Level = LogLevel.Critical,
        Message = "Could not open socket to `{hostName}`")]
    static partial void CouldNotOpenSocket(ILogger logger, string hostName);
}

4. Compile. You'll get errors like this:

1>ConsoleApp7\Microsoft.Extensions.Logging.Generators\Microsoft.Extensions.Logging.Generators.LoggerMessageGenerator\LoggerMessage.g.cs(7,148,7,149): error CS8370: Feature 'nullable reference types' is not available in C# 7.3. Please use language version 8.0 or greater.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

To my knowledge, the only fix is to change your project's language version to something >= 8.0, which is officially an unsupported configuration.

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Issue Details

---

Same issue as #83392, but for the logging source generator rather than for the JSON source generator.

Repro:

1. Create a new .NET Framework 4.8 console app.
2. Add a nuget reference to the latest stable Microsoft.Extensions.Logging.Abstractions nuget package.
3. Paste this code into your Program.cs, which is based on the docs for [LoggerMessage]:

using Microsoft.Extensions.Logging;

internal partial class Program
{
    static void Main() { }

    [LoggerMessage(
        EventId = 0,
        Level = LogLevel.Critical,
        Message = "Could not open socket to `{hostName}`")]
    static partial void CouldNotOpenSocket(ILogger logger, string hostName);
}

4. Compile. You'll get errors like this:
   1>ConsoleApp7\Microsoft.Extensions.Logging.Generators\Microsoft.Extensions.Logging.Generators.LoggerMessageGenerator\LoggerMessage.g.cs(7,148,7,149): error CS8370: Feature 'nullable reference types' is not available in C# 7.3. Please use language version 8.0 or greater.
   ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

To my knowledge, the only fix is to change your project's language version to something >= 8.0, which is officially an unsupported configuration.

<table>
  <tr>
    <th align="left">Author:</th>
    <td>stephentoub</td>
  </tr>
  <tr>
    <th align="left">Assignees:</th>
    <td>-</td>
  </tr>
  <tr>
    <th align="left">Labels:</th>
    <td>

`untriaged`, `area-Extensions-Logging`

</td>
  </tr>
  <tr>
    <th align="left">Milestone:</th>
    <td>-</td>
  </tr>
</table>
</details>