Handle lifetime annotations in unreachable code

We  equate the type in the annotation with the inferred type first so
that we have a fully inferred type to perform the well-formedness check
on.

src/test/ui/issue-54943.rs

@@ -1,7 +1,3 @@
-// compile-pass
-// FIXME(#54943) This test targets the scenario where proving the WF requirements of a user
-// type annotation requires checking dead code. This test should actually fail to compile.
-
 #![feature(nll)]
 #![allow(warnings)]
 
@@ -11,7 +7,7 @@ fn boo<'a>() {
     return;
 
     let x = foo::<&'a u32>();
-    //~^ ERROR the type `&'a u32` does not fulfill the required lifetime [E0477]
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/issue-54943.stderr

@@ -0,0 +1,11 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-54943.rs:9:13
+   |
+LL | fn boo<'a>() {
+   |        -- lifetime `'a` defined here
+...
+LL |     let x = foo::<&'a u32>();
+   |             ^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
+
+error: aborting due to previous error
+

src/test/ui/nll/ty-outlives/projection-where-clause-env-wrong-bound.stderr

@@ -2,7 +2,7 @@ error[E0309]: the associated type `<T as MyTrait<'_>>::Output` may not live long
   --> $DIR/projection-where-clause-env-wrong-bound.rs:17:5
    |
 LL |     bar::<T::Output>() //~ ERROR may not live long enough
-   |     ^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^
    |
    = help: consider adding an explicit lifetime bound `<T as MyTrait<'_>>::Output: 'a`...
 

src/test/ui/nll/ty-outlives/projection-where-clause-none.stderr

@@ -2,7 +2,7 @@ error[E0309]: the parameter type `T` may not live long enough
   --> $DIR/projection-where-clause-none.rs:16:5
    |
 LL |     bar::<T::Output>() //~ ERROR the parameter type `T` may not live long enough
-   |     ^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^
    |
    = help: consider adding an explicit lifetime bound `T: 'a`...
 

src/test/ui/nll/ty-outlives/wf-unreachable.rs

@@ -0,0 +1,54 @@
+// Test that we check that user type annotations are well-formed, even in dead
+// code.
+
+#![feature(nll)]
+
+fn uninit<'a>() {
+    return;
+    let x: &'static &'a ();                         //~ ERROR lifetime may not live long enough
+}
+
+fn var_type<'a>() {
+    return;
+    let x: &'static &'a () = &&();                  //~ ERROR lifetime may not live long enough
+}
+
+fn uninit_infer<'a>() {
+    let x: &'static &'a _;                          //~ ERROR lifetime may not live long enough
+    x = && ();
+}
+
+fn infer<'a>() {
+    return;
+    let x: &'static &'a _ = &&();                   //~ ERROR lifetime may not live long enough
+}
+
+fn uninit_no_var<'a>() {
+    return;
+    let _: &'static &'a ();                         //~ ERROR lifetime may not live long enough
+}
+
+fn no_var<'a>() {
+    return;
+    let _: &'static &'a () = &&();                  //~ ERROR lifetime may not live long enough
+}
+
+fn infer_no_var<'a>() {
+    return;
+    let _: &'static &'a _ = &&();                   //~ ERROR lifetime may not live long enough
+}
+
+trait X<'a, 'b> {}
+
+struct C<'a, 'b, T: X<'a, 'b>>(T, &'a (), &'b ());
+
+impl X<'_, '_> for i32 {}
+impl<'a> X<'a, 'a> for () {}
+
+// This type annotation is not well-formed because we substitute `()` for `_`.
+fn required_substs<'a>() {
+    return;
+    let _: C<'static, 'a, _> = C((), &(), &());     //~ ERROR lifetime may not live long enough
+}
+
+fn main() {}

src/test/ui/nll/ty-outlives/wf-unreachable.stderr

@@ -0,0 +1,73 @@
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:8:12
+   |
+LL | fn uninit<'a>() {
+   |           -- lifetime `'a` defined here
+LL |     return;
+LL |     let x: &'static &'a ();                         //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:13:12
+   |
+LL | fn var_type<'a>() {
+   |             -- lifetime `'a` defined here
+LL |     return;
+LL |     let x: &'static &'a () = &&();                  //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:17:12
+   |
+LL | fn uninit_infer<'a>() {
+   |                 -- lifetime `'a` defined here
+LL |     let x: &'static &'a _;                          //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:23:12
+   |
+LL | fn infer<'a>() {
+   |          -- lifetime `'a` defined here
+LL |     return;
+LL |     let x: &'static &'a _ = &&();                   //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:28:12
+   |
+LL | fn uninit_no_var<'a>() {
+   |                  -- lifetime `'a` defined here
+LL |     return;
+LL |     let _: &'static &'a ();                         //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:33:12
+   |
+LL | fn no_var<'a>() {
+   |           -- lifetime `'a` defined here
+LL |     return;
+LL |     let _: &'static &'a () = &&();                  //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:38:12
+   |
+LL | fn infer_no_var<'a>() {
+   |                 -- lifetime `'a` defined here
+LL |     return;
+LL |     let _: &'static &'a _ = &&();                   //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/wf-unreachable.rs:51:12
+   |
+LL | fn required_substs<'a>() {
+   |                    -- lifetime `'a` defined here
+LL |     return;
+LL |     let _: C<'static, 'a, _> = C((), &(), &());     //~ ERROR lifetime may not live long enough
+   |            ^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
+
+error: aborting due to 8 previous errors
+

src/test/ui/nll/user-annotations/downcast-infer.rs

@@ -0,0 +1,11 @@
+// compile-pass
+
+// Check that we don't try to downcast `_` when type-checking the annotation.
+fn main() {
+    let x = Some(Some(Some(1)));
+
+    match x {
+        Some::<Option<_>>(Some(Some(v))) => (),
+        _ => (),
+    }
+}

src/test/ui/regions/regions-assoc-type-in-supertrait-outlives-container.ast.stderr

@@ -0,0 +1,20 @@
+error[E0491]: in type `&'a WithAssoc<TheType<'b>>`, reference has a longer lifetime than the data it references
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:45:13
+   |
+LL |     let _x: &'a WithAssoc<TheType<'b>> = loop { };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime 'a as defined on the function body at 37:15
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:37:15
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               ^^
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 37:18
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:37:18
+   |
+LL | fn with_assoc<'a,'b>() {
+   |                  ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0491`.

src/test/ui/regions/regions-assoc-type-in-supertrait-outlives-container.mir.stderr

@@ -0,0 +1,13 @@
+error: lifetime may not live long enough
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:45:13
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               -- -- lifetime `'b` defined here
+   |               |
+   |               lifetime `'a` defined here
+...
+LL |     let _x: &'a WithAssoc<TheType<'b>> = loop { };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: aborting due to previous error
+

src/test/ui/regions/regions-assoc-type-in-supertrait-outlives-container.rs

@@ -3,6 +3,9 @@
 // outlive the location in which the type appears, even when the
 // associted type is in a supertype. Issue #22246.
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
+
 #![allow(dead_code)]
 
 ///////////////////////////////////////////////////////////////////////////
@@ -40,7 +43,8 @@ fn with_assoc<'a,'b>() {
     // FIXME (#54943) NLL doesn't enforce WF condition in unreachable code if
     // `_x` is changed to `_`
     let _x: &'a WithAssoc<TheType<'b>> = loop { };
-    //~^ ERROR reference has a longer lifetime
+    //[ast]~^ ERROR reference has a longer lifetime
+    //[mir]~^^ ERROR lifetime may not live long enough
 }
 
 fn main() {

src/test/ui/regions/regions-free-region-ordering-caller.ast.stderr

@@ -0,0 +1,32 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/regions-free-region-ordering-caller.rs:11:12
+   |
+LL | fn call2<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |                     ---------     ---------
+   |                     |
+   |                     these two types are declared with different lifetimes...
+LL |     let z: Option<&'b &'a usize> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^ ...but data from `a` flows into `b` here
+
+error[E0623]: lifetime mismatch
+  --> $DIR/regions-free-region-ordering-caller.rs:17:12
+   |
+LL | fn call3<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |                     ---------     ---------
+   |                     |
+   |                     these two types are declared with different lifetimes...
+LL |     let y: Paramd<'a> = Paramd { x: a };
+LL |     let z: Option<&'b Paramd<'a>> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^^ ...but data from `a` flows into `b` here
+
+error[E0623]: lifetime mismatch
+  --> $DIR/regions-free-region-ordering-caller.rs:22:12
+   |
+LL | fn call4<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |                     ---------     --------- these two types are declared with different lifetimes...
+LL |     let z: Option<&'a &'b usize> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^ ...but data from `b` flows into `a` here
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0623`.

src/test/ui/regions/regions-free-region-ordering-caller.mir.stderr

@@ -0,0 +1,33 @@
+error: lifetime may not live long enough
+  --> $DIR/regions-free-region-ordering-caller.rs:11:12
+   |
+LL | fn call2<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |          --  -- lifetime `'b` defined here
+   |          |
+   |          lifetime `'a` defined here
+LL |     let z: Option<&'b &'a usize> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'b`
+
+error: lifetime may not live long enough
+  --> $DIR/regions-free-region-ordering-caller.rs:17:12
+   |
+LL | fn call3<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |          --  -- lifetime `'b` defined here
+   |          |
+   |          lifetime `'a` defined here
+LL |     let y: Paramd<'a> = Paramd { x: a };
+LL |     let z: Option<&'b Paramd<'a>> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'b`
+
+error: lifetime may not live long enough
+  --> $DIR/regions-free-region-ordering-caller.rs:22:12
+   |
+LL | fn call4<'a, 'b>(a: &'a usize, b: &'b usize) {
+   |          --  -- lifetime `'b` defined here
+   |          |
+   |          lifetime `'a` defined here
+LL |     let z: Option<&'a &'b usize> = None;//[ast]~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: aborting due to 3 previous errors
+

src/test/ui/regions/regions-free-region-ordering-caller.rs

@@ -2,19 +2,25 @@
 // than the thing it points at and ensure that they result in
 // errors. See also regions-free-region-ordering-callee.rs
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
+
 struct Paramd<'a> { x: &'a usize }
 
 fn call2<'a, 'b>(a: &'a usize, b: &'b usize) {
-    let z: Option<&'b &'a usize> = None;//~ ERROR E0623
+    let z: Option<&'b &'a usize> = None;//[ast]~ ERROR E0623
+    //[mir]~^ ERROR lifetime may not live long enough
 }
 
 fn call3<'a, 'b>(a: &'a usize, b: &'b usize) {
     let y: Paramd<'a> = Paramd { x: a };
-    let z: Option<&'b Paramd<'a>> = None;//~ ERROR E0623
+    let z: Option<&'b Paramd<'a>> = None;//[ast]~ ERROR E0623
+    //[mir]~^ ERROR lifetime may not live long enough
 }
 
 fn call4<'a, 'b>(a: &'a usize, b: &'b usize) {
-    let z: Option<&'a &'b usize> = None;//~ ERROR E0623
+    let z: Option<&'a &'b usize> = None;//[ast]~ ERROR E0623
+    //[mir]~^ ERROR lifetime may not live long enough
 }
 
 fn main() {}

src/test/ui/regions/regions-free-region-ordering-caller1.nll.stderr

@@ -12,6 +12,21 @@ LL |     let z: &'a & usize = &(&y);
 LL | }
    | - temporary value is freed at the end of this statement
 
-error: aborting due to previous error
+error[E0597]: `y` does not live long enough
+  --> $DIR/regions-free-region-ordering-caller1.rs:9:27
+   |
+LL | fn call1<'a>(x: &'a usize) {
+   |          -- lifetime `'a` defined here
+...
+LL |     let z: &'a & usize = &(&y);
+   |            -----------    ^^^^ borrowed value does not live long enough
+   |            |
+   |            type annotation requires that `y` is borrowed for `'a`
+...
+LL | }
+   | - `y` dropped here while still borrowed
 
-For more information about this error, try `rustc --explain E0716`.
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0597, E0716.
+For more information about an error, try `rustc --explain E0597`.

src/test/ui/regions/regions-outlives-projection-container-hrtb.ast.stderr

@@ -0,0 +1,37 @@
+error[E0491]: in type `&'a WithHrAssoc<TheType<'b>>`, reference has a longer lifetime than the data it references
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:35:12
+   |
+LL |     let _: &'a WithHrAssoc<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime 'a as defined on the function body at 32:15
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:32:15
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               ^^
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 32:18
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:32:18
+   |
+LL | fn with_assoc<'a,'b>() {
+   |                  ^^
+
+error[E0491]: in type `&'a WithHrAssocSub<TheType<'b>>`, reference has a longer lifetime than the data it references
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:57:12
+   |
+LL |     let _: &'a WithHrAssocSub<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime 'a as defined on the function body at 53:19
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:53:19
+   |
+LL | fn with_assoc_sub<'a,'b>() {
+   |                   ^^
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 53:22
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:53:22
+   |
+LL | fn with_assoc_sub<'a,'b>() {
+   |                      ^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0491`.

src/test/ui/regions/regions-outlives-projection-container-hrtb.mir.stderr

@@ -0,0 +1,24 @@
+error: lifetime may not live long enough
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:35:12
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               -- -- lifetime `'b` defined here
+   |               |
+   |               lifetime `'a` defined here
+...
+LL |     let _: &'a WithHrAssoc<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: lifetime may not live long enough
+  --> $DIR/regions-outlives-projection-container-hrtb.rs:57:12
+   |
+LL | fn with_assoc_sub<'a,'b>() {
+   |                   -- -- lifetime `'b` defined here
+   |                   |
+   |                   lifetime `'a` defined here
+...
+LL |     let _: &'a WithHrAssocSub<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: aborting due to 2 previous errors
+

src/test/ui/regions/regions-outlives-projection-container-hrtb.rs

@@ -1,6 +1,9 @@
 // Test that structs with higher-ranked where clauses don't generate
 // "outlives" requirements. Issue #22246.
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
+
 #![allow(dead_code)]
 
 
@@ -30,7 +33,8 @@ fn with_assoc<'a,'b>() {
     // We get an error because 'b:'a does not hold:
 
     let _: &'a WithHrAssoc<TheType<'b>> = loop { };
-    //~^ ERROR reference has a longer lifetime
+    //[ast]~^ ERROR reference has a longer lifetime
+    //[mir]~^^ ERROR lifetime may not live long enough
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -51,7 +55,8 @@ fn with_assoc_sub<'a,'b>() {
     // below to be well-formed, it is not related to the HR relation.
 
     let _: &'a WithHrAssocSub<TheType<'b>> = loop { };
-    //~^ ERROR reference has a longer lifetime
+    //[ast]~^ ERROR reference has a longer lifetime
+    //[mir]~^^ ERROR lifetime may not live long enough
 }
 
 

src/test/ui/regions/regions-outlives-projection-container-wc.ast.stderr

@@ -0,0 +1,20 @@
+error[E0491]: in type `&'a WithAssoc<TheType<'b>>`, reference has a longer lifetime than the data it references
+  --> $DIR/regions-outlives-projection-container-wc.rs:37:12
+   |
+LL |     let _: &'a WithAssoc<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime 'a as defined on the function body at 31:15
+  --> $DIR/regions-outlives-projection-container-wc.rs:31:15
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               ^^
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 31:18
+  --> $DIR/regions-outlives-projection-container-wc.rs:31:18
+   |
+LL | fn with_assoc<'a,'b>() {
+   |                  ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0491`.

src/test/ui/regions/regions-outlives-projection-container-wc.mir.stderr

@@ -0,0 +1,13 @@
+error: lifetime may not live long enough
+  --> $DIR/regions-outlives-projection-container-wc.rs:37:12
+   |
+LL | fn with_assoc<'a,'b>() {
+   |               -- -- lifetime `'b` defined here
+   |               |
+   |               lifetime `'a` defined here
+...
+LL |     let _: &'a WithAssoc<TheType<'b>> = loop { };
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: aborting due to previous error
+

src/test/ui/regions/regions-outlives-projection-container-wc.rs

@@ -3,6 +3,9 @@
 // outlive the location in which the type appears, even when the
 // constraint is in a where clause not a bound. Issue #22246.
 
+// revisions: ast mir
+//[mir]compile-flags: -Z borrowck=mir
+
 #![allow(dead_code)]
 
 ///////////////////////////////////////////////////////////////////////////
@@ -32,7 +35,8 @@ fn with_assoc<'a,'b>() {
     // which is &'b (), must outlive 'a.
 
     let _: &'a WithAssoc<TheType<'b>> = loop { };
-    //~^ ERROR reference has a longer lifetime
+    //[ast]~^ ERROR reference has a longer lifetime
+    //[mir]~^^ ERROR lifetime may not live long enough
 }
 
 fn main() {