3.1.11: Disable NRTs in scaffolding

[roji]

Original PR merged for 5.0: #21190

Note for Tactics

This issue was previously approved for 3.1.11. However, our ever astute MVP @ErikEJ pointed out that since 3.1 targets .NET Standard 2.0, we will potentially be generating

#nullable disable

in projects where that won't compile.

Therefore, we could either not do this at all, or instead generate something like:

// Code scaffolded by EF Core assumes nullable reference types (NRTs) are disabled.
// If you have enabled NRTs for your project, then un-comment the following line:
// #nullable disable

---

This backports the fix for #23016 to 3.1.

Description

This adds #nullable disable at the top of all C# files generated by EF Core's scaffolding.

Customer Impact

EF Core scaffolding is currently unaware of C# 8.0 Nullable Reference Types (NRT), and generates a code model without annotations. If the project has NRTs turned on, the model get incorrectly interpreted, i.e. nullable database text columns are represented by string SomeProperty, which with NRTs is interpreted as non-nullable.

How found

Reported by users (e.g. #23016). The issue for adding fully supporting NRTs, i.e. correctly scaffolding code with annotations (#15520) also has 28 votes (making it the 29th most requested feature), giving reason to believe many users are running into this.

Test coverage

This PR includes testing that the #nullable disable actually gets scaffolded.

Regression?

No.

Risk

Low. This touches scaffolding code only - which does not affect EF Core runtime behavior - and in a minor way.

(replaces #23053 which had a persistent CI issue)

[ajcvickers]

@roji Approved for 3.1.11 by Tactics. Please wait until the branch is open for 3.1.11 before merging.

[ErikEJ]

Will this affect usage of the scaffolded code with .NET Standard 2.0 projects in a bad way?

[ErikEJ]

See ErikEJ/EFCorePowerTools#557 (comment) @roji @ajcvickers

[roji]

Good point @ErikEJ. 5.0 targets netstandard2.1 so this makes sense there, but depending on the of netstandard2.0 users this could be disruptive.

@ajcvickers let's cancel backporting this?

[ajcvickers]

@roji @ErikEJ It depends on the compiler version, right? Which compiler versions don't understand this?

[ErikEJ]

As I understand, .NET Standard 2.0 does not support C# 8 and 7.3 does not understand this (see linked comment)

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version#defaults

[roji]

Yeah, it's possible (but unsupported) to use C# 8 with netstandard2.0, but for many people out there the language version is 7.3 (the default), and if #nullable disable starts to appear that won't compile.

[ajcvickers]

@roji Let's discuss this in triage tomorrow.

[ajcvickers]

@dotnet/efteam Should we still do this on 3.1, but change it to a comment?

[ajcvickers]

Approved by Tactics for 3.1 with the #nullable disable in a comment. PR updated.