Containing{Type/Assembly/Symbol/etc} are marked nullable oblivious

[agocke]

These are some of the most common places to receive null from the Roslyn type system, but they're annotated as non-nullable, so it's easy to forget to check for null. See

roslyn/src/Compilers/Core/Portable/Symbols/ISymbol.cs

Lines 66 to 97 in 0b2c30d

	#nullable disable // Skipped for now https://github.com/dotnet/roslyn/issues/39166
	#pragma warning disable RS0041 // uses oblivious reference types
	/// <summary>
	/// Gets the <see cref="ISymbol"/> for the immediately containing symbol.
	/// </summary>
	ISymbol ContainingSymbol { get; }
	/// <summary>
	/// Gets the <see cref="IAssemblySymbol"/> for the containing assembly. Returns null if the
	/// symbol is shared across multiple assemblies.
	/// </summary>
	IAssemblySymbol ContainingAssembly { get; }
	/// <summary>
	/// Gets the <see cref="IModuleSymbol"/> for the containing module. Returns null if the
	/// symbol is shared across multiple modules.
	/// </summary>
	IModuleSymbol ContainingModule { get; }
	/// <summary>
	/// Gets the <see cref="INamedTypeSymbol"/> for the containing type. Returns null if the
	/// symbol is not contained within a type.
	/// </summary>
	INamedTypeSymbol ContainingType { get; }
	/// <summary>
	/// Gets the <see cref="INamespaceSymbol"/> for the nearest enclosing namespace. Returns null if the
	/// symbol isn't contained in a namespace.
	/// </summary>
	INamespaceSymbol ContainingNamespace { get; }
	#nullable enable
	#pragma warning restore RS0041 // uses oblivious reference types

• ContainingSymbol
• ContainingAssembly
• ContainingModule
• ContainingType
• ContainingNamespace

[agocke]

Another bug due to null oblivious: dotnet/linker#2534