improve case with both anonymous lifetime parameters in Structs

[gaurikholkar-zz]

Generate error message for the case where , for structs, we have missing lifetime parameters.
For e.g. a sample error message

struct Ref<'a> { x: &'a u32 }

fn foo(mut x: Vec<Ref>, y: Ref) {
                  ---      --- must have the same lifetime
  x.push(y);
|            ^ data from `y` flows into `x` here

}

There are cases where more than one lifetime parameters come into play.

struct Ref<'a, 'b> { a: &'a u32, b: &'b u32 }

fn foo(mut x: Ref, y: Ref) {
              ---     --- 2nd lifetime parameter on `Ref` must match
              2nd lifetime parameter on `Ref` must match
  x.b = y.b;
}

struct Ref<'a, 'b> { a: &'a u32, b: &'b u32 }

fn foo<'x1, 'x2, 'y1>(mut x: Ref<'x1, 'x2>, y: Ref<'y1, 'y2>) {
  x.b = y.b;
}

cc @nikomatsakis

[arielb1]

Also for when you are dealing with the same struct

struct Ref<'a, 'b> { a: &'a u32, b: &'b u32 }

fn foo(mut x: Ref) {
              ---
              1st lifetime parameter on `Ref` must match
              2nd lifetime parameter on `Ref` must match
  x.a = x.b;
}

[gaurikholkar-zz]

working on this.

[gaurikholkar-zz]

Making a generalized message as follows

  | fn foo(mut x: Vec<Ref<T>>,
  |                  --- 
  |       y: Ref<T>) 
  |          --- these two structs are not declared with the same lifetime ...
  |{
  | x.push(y);
  |        ^ ... but data flows from `y` into `x` here
  | }

and

struct Ref<'a> { x: &'a u32 }

fn foo(mut x: Vec<Ref>, y: Ref) {
                  ---      --- the reference and struct are not declared with the same lifetime ...
  x.push(y);
|            ^ data from `y` flows into `x` here

}

[gaurikholkar-zz]

Further modifying it as follows

  | fn foo(mut x: Vec<Ref<T>>,
  |                  --- 
  |       y: Ref<T>) 
  |          --- these two structs are not declared with the same lifetime ...
  |{
  | x.push(y);
  |        ^ ... but data flows from `y` into `x` here
  | }
help: consider changing the signature to:
  | fn foo<'a>(mut x: Vec<'a, Ref<T>>, y: Ref<'a, T>)

[gaurikholkar-zz]

The error message based on the current PR is

+error[E0623]: lifetime mismatch
+  --> $DIR/ex3-both-anon-regions-both-are-structs.rs:15:12
+   |
+14 | fn foo(mut x: Vec<Ref>, y: Ref) {
+   |                   ---      --- these structs are declared with different lifetimes...
+15 |     x.push(y);
+   |            ^ ...but data from `y` flows into `x` here
+
+error: aborting due to previous error

[gaurikholkar-zz]

+error[E0623]: lifetime mismatch
+  --> $DIR/ex3-both-anon-regions-both-are-structs.rs:15:12
+   |
+14 | fn foo(mut x: Vec<Ref>, y: Ref) {
+   |                   ---      --- these two types are declared with different lifetimes...
+15 |     x.push(y);
+   |            ^ ...but data from `y` flows into `x` here
+
+error: aborting due to previous error

[nikomatsakis]

Seems like this is basically done. Closing.