Fix ref safe context of assignments to be their RHS

[jjonescz]

Fixes #79054.
Closes #73549.

[AlekseyTs]

Briefly glanced over changes (commit 2), not a complete review pass #Closed

[jjonescz]

@AlekseyTs @dotnet/roslyn-compiler for reviews, thanks

[AlekseyTs]

LGTM (commit 4), but I recommend getting a sign-off from ref-safety experts.

[jjonescz]

@dotnet/roslyn-compiler for reviews, thanks

[RikkiGibson]

I think I agree that the change being made here is correct, in the sense that it doesn't open any ref safety holes, and removes ref safety errors which were "unnecessary".

It also looks like this decision is a reversal of ref safety changes made in #24904, which were themselves a reversal of a prior state where the ref-safe-context of an assignment was the RHS.

In my view this emphasizes that this area needs to be well-specified. Basically I am OK with letting this PR in as it is now, but, let's make sure that before too long we get the behavior we actually want recorded in the standard. I would be happy to work together on this.

[jjonescz]

It also looks like this decision is a reversal of ref safety changes made in #24904, which were themselves a reversal of a prior state where the ref-safe-context of an assignment was the RHS.

Interesting find, thanks.

Note that there are already some places where RHS is used, it's just not very consistent. For example:

roslyn/src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs

Line 4623 in 58047ea

return GetValEscape(((BoundAssignmentOperator)expr).Right, localScopeDepth);

In my view this emphasizes that this area needs to be well-specified. Basically I am OK with letting this PR in as it is now, but, let's make sure that before too long we get the behavior we actually want recorded in the standard. I would be happy to work together on this.

The standard has this section, but it doesn't seem to answer what the safe-context should be for assignment operators I think:

16.4.15.5 Operators

The application of a user-defined operator is treated as a method invocation (§16.4.15.6).

For an operator that yields a value, such as e1 + e2 or c ? e1 : e2, the safe-context of the result is the narrowest context among the safe-contexts of the operands of the operator. As a consequence, for a unary operator that yields a value, such as +e, the safe-context of the result is the safe-context of the operand.

Note: The first operand of a conditional operator is a bool, so its safe-context is caller-context. It follows that the resulting safe-context is the narrowest safe-context of the second and third operand. end note

Filed dotnet/csharpstandard#1444