Compiler intrinsics

This is an alternate proposal for the compiler intrinsicts feature:

#191
dotnet/roslyn#11475

This alternate design proposal comes after reviewing a prototype implementation
of the original proposal by @mjsabby as well as the use throughout a significant
code base.

This design was done with significant input from @mjsabby, @tmat and @jkotas.

intrinsics.md

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+# Compiler Intrinsics
+
+* [x] Proposed
+* [ ] Prototype: Not Started
+* [ ] Implementation: Not Started
+* [ ] Specification: Below
+
+## Summary
+
+This proposal provides language constructs that expose low level IL opcodes that cannot currently
+be accessed efficiently, or at all: `ldftn`, `ldvirtftn`, `ldtoken` and `calli`. These low level op 
+codes can be important in high performance code and developers need an efficient way to access 
+them.
+
+## Motivation
+
+The motivations and background for this feature are described in the following isssue (as is a 
+potential implementation of the feature): 
+
+https://github.com/dotnet/csharplang/issues/191
+
+This alternate design proposal comes after reviewing a prototype implementation of the original
+proposal by @msjabby as well as the use throughout a significant code base. This design was done 
+with significant input from @mjsabby, @tmat and @jkotas.
+
+## Detailed Design 
+
+### Allow address of to target methods
+
+Method groups will now be allowed as arguments to an address-of expression. The type of such an 
+expression will be `void*`. 
+
+``` csharp
+class Util { 
+    public static void Log() { } 
+}
+
+// ldftn Util.Log
+void* ptr = &Util.Log; 
+```
+
+Given there is no delegate conversion here the only mechanism for filtering members in the method
+group is by static / instance access. If that cannot distinguish the members then a compile time
+error will occur.
+
+``` csharp
+class Util { 
+    public void Log() { } 
+    public void Log(string p1) { } 
+    public static void Log(int i) { };
+}
+
+unsafe {
+    // Error: Method group Log has more than one applicable candidate.
+    void* ptr1 = &Log; 
+
+    // Okay: only one static member to consider here.
+    void* ptr2 = &Util.Log;
+}
+```
+
+The addressof expression in this context will be implemented in the following manner:
+
+- ldftn: when the method is static 
+- ldvirtftn: when the method is an instance method.
+
+Restrictions of this feature:
+
+- Instance methods can only be specified when using an invocation expression on a value
+- Local functions cannot be used in `&`. The implementation details of these methods are
+deliberately not specified by the language. This includes whether they are static vs. instance or
+exactly what signature they are emitted with.
+
+### handleof
+
+The `handleof` contextual keyword will translate a field, member or type into their equivalent 
+`RuntimeHandle` using the `ldtoken` instruction.
+
+The arguments to `handleof` are identical to `nameof`. It must be a simple name, qualified name, 
+member access, base access with a specified member, or this access with a specified member. The 
+argument expression identifies a code definition, but it is never evaluated.
+
+The `handleof` expression is evaluated at runtime and has a return type of `RuntimeHandle`. This 
+can be executed in safe code as well as unsafe. 
+
+``` 
+RuntimeHandle stringHandle = handleof(string);
+```
+
+Restrictions of this feature:
+
+- Properties cannot be used in a `handleof` expression.
+- The `handleof` expression cannot be used when there is an existing `handelof` name in scope. For 
+example a type, namespace, etc ...
+
+### calli
+
+The compiler will add support for a new type of `extern` function that efficients translates into
+a `.calli` instruction. The exten attribute will be marked with an attribute of the following
+shape:
+
+``` csharp
+[AttributeUsage(AttributeTargets.Method)]
+public sealed class CallIndirectAttribute : Attribute
+{
+    public CallingConvention CallingConvention { get; set; }
+    public CallIndirectAttribute(CallingConvention callingConvention)
+    {
+        CallingConvention = callingConvention;
+    }
+}
+```
+
+This allows developers to define methods in the following form:
+
+``` csharp
+[CallIndirect(CallingConvention.Cdecl)]
+static extern int MapValue(string s, void *ptr);
+
+unsafe {
+    var i = MapValue("42", &int.Parse);
+    Console.WriteLine(i);
+}
+```
+
+## Considerations
+
+### Disambiguating method groups
+
+There was some discussion around features that would make it easier to disambiguate method groups
+passed to an addressof expression. For instance potentially adding signature elements to the 
+syntax:
+
+``` csharp
+class Util {
+    public static void Log() { ... }
+    public static void Log(string) { ... }
+}
+
+unsafe {
+    // Error: ambiguous Log
+    void *ptr1 = &Util.Log;
+
+    // Use Util.Log();
+    void *ptr2 = &Util.Log();
+}
+```
+
+This was rejected because a compelling case could not be made nor could a simple syntax be 
+envisioned here. Also there is a fairly straight forward work around: simple define another 
+method, possible local function, that is unambiguous and uses C# code to call into the 
+desired function. 
+
+``` csharp
+unsafe { 
+    static void LocalLog() => Util.Log();
+    void* ptr = &LocalLog;
+}
+```
+
+### LoadTypeTokenInt32
+
+The original proposal allowed for metadata tokens to be loaded as `int` values at compile time. 
+Essentially have `tokenof` that has the same arguments as `handleof` but is evaluated at 
+compile time to an `int` constant. 
+
+This was rejected as it causes significant problem for IL rewrites (of which .NET has many). Such 
+rewriters often manipulate the metadata tables in a way that could invalidate these values. There 
+is no reasonable way for such rewriters to update these values when they are stored as simple 
+`int` values.
+
+The underlying idea of having an opaque handle for metadata entries will continue to be explored 
+by the runtime team. 
+
+## Future Considerations
+
+### static local functions
+
+This refers to [the proposal](https://github.com/dotnet/csharplang/issues/1565) to allow the 
+`static` modifier on local functions. Such a function would be guaranteed to be emitted as 
+`static` and with the exact signature specified in source code. Such a function should be a valid
+argument to `&` as it contains none of the problems local functions have today.
+