Propose language changes

Breif details about implementation:
- Instead of pre-initializing `closure_captured` and `upvars_capture_map` using `upvars_mentioned` we will be rely on use within the closure. This will be handled as part of #7 
- For coercing a closure to FnPtr, instead of relying on `upvars_mentioned` in typechk we will accept such code in typechk while setting a no-capture obligation that will be validated in trait selection post capture analysis.

Author: arora-aman

lang-changes.md

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+# Language Changes
+
+This document lists changes that will be made to language along with RFC-2229.
+
+For this document let RS20 be the pre-RFC compiler and RS21 be the post-RFC compiler.
+
+## `_` pattern in closures
+
+Consider the following code 
+```rust 
+let x;
+let c = || {
+      let _ = x;
+}; 
+```
+
+RS20 will result in the capture of x within the closure, but `_` is just an ignore/discard and that means x can't/won't be used within the closure and should not result in a capture.
+
+For RS21 we propose to ignore such captures allowing the following code to be accepted by RS21
+```
+let x;
+let c = || {
+      let _ = x;
+};
+
+let f : fn() = c; 
+```
+
+The above code works now because `x` isn't captured by `c` anymore making `c` a non-capturing closure, which can be coerced to a `FnPtr`
+
+