Propose updates to closure pretty print

Author: arora-aman

update_closure_printing.md

@@ -0,0 +1,84 @@
+# Update closure pretty printing
+
+Currently the compiler prints the capture information (name and type) of the root variable being captured.  For example, current pretty print:
+
+```rust
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+let p = Point { x: 10, y: 20 };
+
+// pretty print: [closure@src/main.rs:8:14: 10:6 p:_]
+let c1 = || {
+    println!("{}", p.x);
+};
+
+// pretty print: [closure@src/main.rs:11:14: 13:6 p:_]
+let c2 = || {
+    println!("{}", p.y);
+};
+```
+(`p:_`, means p is captured and its type hasn't been inferred yet)
+
+
+## Motivation for change
+- This information isn't really diagnostically useful to the end user.
+- Once [RFC 2229](https://github.com/rust-lang/rfcs/blob/master/text/2229-capture-disjoint-fields.md) is implemented, the current closure pretty print won't be completely accurate. In the above example, after RFC-2229 `c1` only captures `p.x` and c2 only captures `p.y`.
+- Closures don't print information about generics they inherit from their parent scope.
+
+## Proposed changes
+- Don't print capture information for closures unless `-Zverbose` is specified.
+- When the compiler is invoked with `-Zverbose`  only print the type of the capture and not the name of or path to the variable.
+
+Applying these two changes to example above we get
+```rust
+// pretty print: [closure@src/main.rs:8:14: 10:6] 
+let c1 = || {
+    println!("{}", p.x);
+};
+
+// compile-flags: -Zverbose 
+// pretty print: [closure@src/main.rs:8:14: 10:6 (&i32)]
+let c1 = || {
+    println!("{}", p.x);
+};
+```
+(Note this example is missing changes explained below)
+
+- If type information about captures isn't available or can't be verified as available then the information won't be printed out.
+
+```rust
+// compile-flags: -Zverbose 
+// pretty print: [closure@src/main.rs:8:14: 10:6 (unavailable)]
+let c1 = || {
+    println!("{}", p.x);
+};
+
+let c :() = c1; // Error here prevents us from doing capture analysis
+```
+(Note this example is missing changes explained below)
+
+- Print the path to the closure instead of the `[closure]` notation, similar to how types defined within the closure are printed. If `-Zverbose` is passed to the compiler then we print generics as part of the path.
+
+```rust
+mod mod1 {
+    pub fn f<T: std::fmt::Display>(t: T) 
+    {
+        let x = 20;
+    
+        // pretty print: mod1::f::{{closure}}#0
+        let c = || println!("{} {}", t, x);
+        c(); // *
+    }
+}
+
+fn main() {
+    f(format!("S"));
+}
+```
+
+If `-Zverbose` is set then pretty print for the closure will be `mod1::f<T>::{{closure}}#0 (&T, &i32)`.
+
+If  `-Zverbose` is set and the line marked with `*` is changed to `let c1 :() = c`, that is it prevents us from starting capture analysis, the pretty print will be `mod1::f<T>::{{closure}}#0 (unavailable)`.