Propose more relaxtions to closure catpures

lang-changes.md

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+# Language Changes
+
+This document lists changes that will be made to the 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 pieces of code to be accepted by RS21
+
+```rust
+let x;
+let c = || {
+      let _ = x;
+};
+
+let f : fn() = c; 
+
+f();
+```
+
+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`.
+
+```rust
+fn main() {
+    let mut x = String::from("1");
+
+    let c = || {
+        let _ = x;
+    };
+
+
+    let mx = &mut x;
+    *mx = String::from("2");
+
+    c();
+
+    println!("{}", x);
+}
+```
+RS20 would immutably borrow x within the closure (even thought it's discarded), preventing a mutable borrow to exist outside the closure. 
+The above code works with RS21 because `x` isn't captured within the closure allowing the mutable borrow outside the closure to exist.