Interpolated string handler lowering isn't unescaping {{ and }}

[stephentoub]

Version Used:
1119ab9

Steps to Reproduce:
https://sharplab.io/#v2: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

public class C
{
    public string M(int i) => $"{{ {i} }}";
}

Expected Behavior:
This should produce the equivalent of "{ " + i + " }".

Actual Behavior:
Instead it's producing the equivalent of "{{ " + i + " }}".

This is a behavioral regression.

cc: @333fred

[333fred]

It is not the job of the C# compiler to do escaping here. This is the equivalent of string.Format("{{ {0} }}", i). The AppendLiteral call should be the thing handling the escape of the {{, just like string.Format would do. If we need interpolated strings to be doing the same escape rules in the compiler that string.Format does, I'm extremely concerned by this as it is not part of the specification for the feature and goes well beyond the standard handling we have in the compiler here.

[stephentoub]

I completely disagree. A key tenet of this feature is the compiler is already parsing the strings at compile-time and emitting the literals to be appended so no parsing is required at run-time. The C# compiler needs to do the escaping.

[333fred]

I'll send an email to LDM to get approval for a spec change here and get a PR out to change this while we wait for approval.