Add the actual chain of projections to UserTypeProjection.

Update the existing NLL `patterns.rs` test accordingly.

includes changes addressing review feedback:

 * Added example to docs for `UserTypeProjections` illustrating how we
   build up multiple projections when descending into a pattern with
   type ascriptions.

 * Adapted niko's suggested docs for `UserTypeProjection`.

 * Factored out `projection_ty` from more general `projection_ty_core`
   (as a drive-by, made its callback an `FnMut`, as I discovered later
   that I need that).

 * Add note to docs that `PlaceTy.field_ty(..)` does not normalize its result.

 * Normalize as we project out `field_ty`.

src/test/ui/nll/user-annotations/patterns.rs

@@ -9,11 +9,11 @@ fn variable_no_initializer() {
 }
 
 fn tuple_no_initializer() {
-    // FIXME(#47187): We are not propagating ascribed type through tuples.
+
 
     let x = 22;
     let (y, z): (&'static u32, &'static u32);
-    y = &x;
+    y = &x; //~ ERROR
 }
 
 fn ref_with_ascribed_static_type() -> u32 {
@@ -34,11 +34,11 @@ fn ref_with_ascribed_any_type() -> u32 {
 struct Single<T> { value: T }
 
 fn struct_no_initializer() {
-    // FIXME(#47187): We are not propagating ascribed type through patterns.
+
 
     let x = 22;
     let Single { value: y }: Single<&'static u32>;
-    y = &x;
+    y = &x; //~ ERROR
 }
 
 fn variable_with_initializer() {
@@ -91,26 +91,26 @@ fn struct_double_field_underscore_with_initializer() {
 }
 
 fn static_to_a_to_static_through_variable<'a>(x: &'a u32) -> &'static u32 {
-    // The error in this test is inconsistency with
-    // `static_to_a_to_static_through_tuple`, but "feels right" to
-    // me. It occurs because we special case the single binding case
-    // and force the type of `y` to be `&'a u32`, even though the
-    // right-hand side has type `&'static u32`.
+
+
+
+
+
 
     let y: &'a u32 = &22;
     y //~ ERROR
 }
 
 fn static_to_a_to_static_through_tuple<'a>(x: &'a u32) -> &'static u32 {
-    // FIXME(#47187): The fact that this type-checks is perhaps surprising.
-    // What happens is that the right-hand side is constrained to have
-    // type `&'a u32`, which is possible, because it has type
-    // `&'static u32`. The variable `y` is then forced to have type
-    // `&'static u32`, but it is constrained only by the right-hand
-    // side, not the ascribed type, and hence it passes.
+
+
+
+
+
+
 
     let (y, _z): (&'a u32, u32) = (&22, 44);
-    y
+    y //~ ERROR
 }
 
 fn a_to_static_then_static<'a>(x: &'a u32) -> &'static u32 {

src/test/ui/nll/user-annotations/patterns.stderr

@@ -8,6 +8,16 @@ LL |     y = &x; //~ ERROR
 LL | }
    | - `x` dropped here while still borrowed
 
+error[E0597]: `x` does not live long enough
+  --> $DIR/patterns.rs:16:9
+   |
+LL |     let (y, z): (&'static u32, &'static u32);
+   |                 ---------------------------- type annotation requires that `x` is borrowed for `'static`
+LL |     y = &x; //~ ERROR
+   |         ^^ borrowed value does not live long enough
+LL | }
+   | - `x` dropped here while still borrowed
+
 error[E0597]: `x` does not live long enough
   --> $DIR/patterns.rs:22:13
    |
@@ -19,6 +29,16 @@ LL |     **z
 LL | }
    | - `x` dropped here while still borrowed
 
+error[E0597]: `x` does not live long enough
+  --> $DIR/patterns.rs:41:9
+   |
+LL |     let Single { value: y }: Single<&'static u32>;
+   |                              -------------------- type annotation requires that `x` is borrowed for `'static`
+LL |     y = &x; //~ ERROR
+   |         ^^ borrowed value does not live long enough
+LL | }
+   | - `x` dropped here while still borrowed
+
 error[E0597]: `x` does not live long enough
   --> $DIR/patterns.rs:46:27
    |
@@ -127,6 +147,15 @@ LL | fn static_to_a_to_static_through_variable<'a>(x: &'a u32) -> &'static u32 {
 LL |     y //~ ERROR
    |     ^ returning this value requires that `'a` must outlive `'static`
 
+error: unsatisfied lifetime constraints
+  --> $DIR/patterns.rs:113:5
+   |
+LL | fn static_to_a_to_static_through_tuple<'a>(x: &'a u32) -> &'static u32 {
+   |                                        -- lifetime `'a` defined here
+...
+LL |     y //~ ERROR
+   |     ^ returning this value requires that `'a` must outlive `'static`
+
 error: unsatisfied lifetime constraints
   --> $DIR/patterns.rs:117:18
    |
@@ -135,7 +164,7 @@ LL | fn a_to_static_then_static<'a>(x: &'a u32) -> &'static u32 {
 LL |     let (y, _z): (&'static u32, u32) = (x, 44); //~ ERROR
    |                  ^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
 
-error: aborting due to 14 previous errors
+error: aborting due to 17 previous errors
 
 Some errors occurred: E0597, E0716.
 For more information about an error, try `rustc --explain E0597`.