Do not use [Out] string for P/Invokes

[elinor-fung]

Add a rule to flag P/Invoke declarations passing string parameters by value with the [Out] attribute, which can destabilize the runtime if the string is an interned string.

 // Flag [Out] string
[DllImport("MyLibrary")]
private static extern void Foo([Out] string s);

// OK
[DllImport("MyLibrary")]
private static extern void Bar(out string s1, [Out] ref string s2);

Category: Interoperability
Default: Enabled

cc @terrajobst @stephentoub @AaronRobinsonMSFT @jkoritzinsky

[stephentoub]

Is there any situation in which this is considered valid? It's akin to pinning a string and then writing into it, right? I'm wondering if in addition to an analyzer (which I agree is warranted; I didn't realize we allowed this), .NET 5 should simply throw an exception at runtime if you try to do this?

[AaronRobinsonMSFT]

.NET 5 should simply throw an exception at runtime if you try to do this?

I believe @jkoritzinsky added this.

[elinor-fung]

@terrajobst @stephentoub Any concerns with us going ahead with this? Is there a specific/official approval we should ask for?

[terrajobst]

@terrajobst @stephentoub Any concerns with us going ahead with this? Is there a specific/official approval we should ask for?

Approval for which part? Throwing the exception, the analyzer or both? I thinking doing both seems great to me.