Check user types are well-formed in MIR borrow check

Also update some tests so that they don't have user types on `_` in
unreachable code.

Author: matthewjasper

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -970,15 +970,21 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
         locations: Locations,
         category: ConstraintCategory,
     ) -> Fallible<()> {
-        relate_tys::relate_type_and_user_type(
+        let ty = relate_tys::relate_type_and_user_type(
             self.infcx,
             a,
             v,
             b,
             locations,
             category,
             self.borrowck_context.as_mut().map(|x| &mut **x),
-        )
+        )?;
+        self.prove_predicate(
+            ty::Predicate::WellFormed(ty),
+            locations,
+            category,
+        );
+        Ok(())
     }
 
     fn eq_opaque_type_and_type(

src/librustc_mir/borrow_check/nll/type_check/relate_tys.rs

@@ -10,7 +10,7 @@
 
 use borrow_check::nll::constraints::OutlivesConstraint;
 use borrow_check::nll::type_check::{BorrowCheckContext, Locations};
-use rustc::infer::canonical::{Canonical, CanonicalVarInfos};
+use rustc::infer::canonical::{Canonical, CanonicalVarInfos, CanonicalVarValues};
 use rustc::infer::{InferCtxt, NLLRegionVariableOrigin};
 use rustc::mir::ConstraintCategory;
 use rustc::traits::query::Fallible;
@@ -70,7 +70,7 @@ pub(super) fn relate_type_and_user_type<'tcx>(
     locations: Locations,
     category: ConstraintCategory,
     borrowck_context: Option<&mut BorrowCheckContext<'_, 'tcx>>,
-) -> Fallible<()> {
+) -> Fallible<Ty<'tcx>> {
     debug!(
         "sub_type_and_user_type(a={:?}, b={:?}, locations={:?})",
         a, b, locations
@@ -85,13 +85,24 @@ pub(super) fn relate_type_and_user_type<'tcx>(
     // variance to get the right relationship.
     let v1 = ty::Contravariant.xform(v);
 
-    TypeRelating::new(
+    let mut type_relating = TypeRelating::new(
         infcx.tcx,
         NllTypeRelatingDelegate::new(infcx, borrowck_context, locations, category),
         v1,
         b_variables,
-    ).relate(&b_value, &a)?;
-    Ok(())
+    );
+    type_relating.relate(&b_value, &a)?;
+
+    Ok(b.substitute(
+        infcx.tcx,
+        &CanonicalVarValues {
+            var_values: type_relating
+                .canonical_var_values
+                .into_iter()
+                .map(|x| x.expect("unsubstituted canonical variable"))
+                .collect(),
+        },
+    ))
 }
 
 struct TypeRelating<'me, 'gcx: 'tcx, 'tcx: 'me, D>

src/test/ui/associated-types/associated-types-subtyping-1.nll.stderr

@@ -0,0 +1,24 @@
+error: unsatisfied lifetime constraints
+  --> $DIR/associated-types-subtyping-1.rs:36:13
+   |
+LL | fn method2<'a,'b,T>(x: &'a T, y: &'b T)
+   |            -- -- lifetime `'b` defined here
+   |            |
+   |            lifetime `'a` defined here
+...
+LL |     let _c: <T as Trait<'b>>::Type = a; //~ ERROR E0623
+   |             ^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: unsatisfied lifetime constraints
+  --> $DIR/associated-types-subtyping-1.rs:44:12
+   |
+LL | fn method3<'a,'b,T>(x: &'a T, y: &'b T)
+   |            -- -- lifetime `'b` defined here
+   |            |
+   |            lifetime `'a` defined here
+...
+LL |     let b: <T as Trait<'b>>::Type = make_any();
+   |            ^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
+
+error: aborting due to 2 previous errors
+

src/test/ui/associated-types/associated-types-subtyping-1.rs

@@ -8,10 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 #![allow(unused_variables)]
 
+fn make_any<T>() -> T {  loop {} }
+
 trait Trait<'a> {
     type Type;
 
@@ -22,35 +22,35 @@ fn method1<'a,'b,T>(x: &'a T, y: &'b T)
     where T : for<'z> Trait<'z>, 'a : 'b
 {
     // Note that &'static T <: &'a T.
-    let a: <T as Trait<'a>>::Type = loop { };
-    let b: <T as Trait<'b>>::Type = loop { };
+    let a: <T as Trait<'a>>::Type = make_any();
+    let b: <T as Trait<'b>>::Type = make_any();
     let _c: <T as Trait<'a>>::Type = a;
 }
 
 fn method2<'a,'b,T>(x: &'a T, y: &'b T)
     where T : for<'z> Trait<'z>, 'a : 'b
 {
     // Note that &'static T <: &'a T.
-    let a: <T as Trait<'a>>::Type = loop { };
-    let b: <T as Trait<'b>>::Type = loop { };
+    let a: <T as Trait<'a>>::Type = make_any();
+    let b: <T as Trait<'b>>::Type = make_any();
     let _c: <T as Trait<'b>>::Type = a; //~ ERROR E0623
 }
 
 fn method3<'a,'b,T>(x: &'a T, y: &'b T)
     where T : for<'z> Trait<'z>, 'a : 'b
 {
     // Note that &'static T <: &'a T.
-    let a: <T as Trait<'a>>::Type = loop { };
-    let b: <T as Trait<'b>>::Type = loop { };
+    let a: <T as Trait<'a>>::Type = make_any();
+    let b: <T as Trait<'b>>::Type = make_any();
     let _c: <T as Trait<'a>>::Type = b; //~ ERROR E0623
 }
 
 fn method4<'a,'b,T>(x: &'a T, y: &'b T)
     where T : for<'z> Trait<'z>, 'a : 'b
 {
     // Note that &'static T <: &'a T.
-    let a: <T as Trait<'a>>::Type = loop { };
-    let b: <T as Trait<'b>>::Type = loop { };
+    let a: <T as Trait<'a>>::Type = make_any();
+    let b: <T as Trait<'b>>::Type = make_any();
     let _c: <T as Trait<'b>>::Type = b;
 }
 

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

@@ -0,0 +1,13 @@
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:52:13
+   |
+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

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 // Test that we are imposing the requirement that every associated
 // type of a bound that appears in the where clause on a struct must
 // outlive the location in which the type appears, even when the
@@ -49,7 +47,10 @@ fn with_assoc<'a,'b>() {
     // outlive 'a. In this case, that means TheType<'b>::TheAssocType,
     // which is &'b (), must outlive 'a.
 
-    let _: &'a WithAssoc<TheType<'b>> = loop { }; //~ ERROR reference has a longer lifetime
+    // 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
 }
 
 fn main() {

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

@@ -1,16 +1,16 @@
 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:52:12
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:52:13
    |
-LL |     let _: &'a WithAssoc<TheType<'b>> = loop { }; //~ ERROR reference has a longer lifetime
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _x: &'a WithAssoc<TheType<'b>> = loop { };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: the pointer is valid for the lifetime 'a as defined on the function body at 46:15
-  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:46:15
+note: the pointer is valid for the lifetime 'a as defined on the function body at 44:15
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:44:15
    |
 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 46:18
-  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:46:18
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 44:18
+  --> $DIR/regions-assoc-type-in-supertrait-outlives-container.rs:44:18
    |
 LL | fn with_assoc<'a,'b>() {
    |                  ^^

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

@@ -0,0 +1,33 @@
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-free-region-ordering-caller.rs:18:12
+   |
+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;//~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'b`
+
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-free-region-ordering-caller.rs:23:12
+   |
+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;//~ ERROR E0623
+   |            ^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'b`
+
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-free-region-ordering-caller.rs:27:12
+   |
+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;//~ 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

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 // Test various ways to construct a pointer with a longer lifetime
 // than the thing it points at and ensure that they result in
 // errors. See also regions-free-region-ordering-callee.rs

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

@@ -1,5 +1,5 @@
 error[E0623]: lifetime mismatch
-  --> $DIR/regions-free-region-ordering-caller.rs:20:12
+  --> $DIR/regions-free-region-ordering-caller.rs:18:12
    |
 LL | fn call2<'a, 'b>(a: &'a usize, b: &'b usize) {
    |                     ---------     ---------
@@ -9,7 +9,7 @@ LL |     let z: Option<&'b &'a usize> = None;//~ ERROR E0623
    |            ^^^^^^^^^^^^^^^^^^^^^ ...but data from `a` flows into `b` here
 
 error[E0623]: lifetime mismatch
-  --> $DIR/regions-free-region-ordering-caller.rs:25:12
+  --> $DIR/regions-free-region-ordering-caller.rs:23:12
    |
 LL | fn call3<'a, 'b>(a: &'a usize, b: &'b usize) {
    |                     ---------     ---------
@@ -20,7 +20,7 @@ LL |     let z: Option<&'b Paramd<'a>> = None;//~ ERROR E0623
    |            ^^^^^^^^^^^^^^^^^^^^^^ ...but data from `a` flows into `b` here
 
 error[E0623]: lifetime mismatch
-  --> $DIR/regions-free-region-ordering-caller.rs:29:12
+  --> $DIR/regions-free-region-ordering-caller.rs:27:12
    |
 LL | fn call4<'a, 'b>(a: &'a usize, b: &'b usize) {
    |                     ---------     --------- these two types are declared with different lifetimes...

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

@@ -13,6 +13,22 @@ note: borrowed value must be valid for the lifetime 'a as defined on the functio
 LL | fn call1<'a>(x: &'a usize) {
    |          ^^
 
-error: aborting due to previous error
+error[E0597]: `y` does not live long enough
+  --> $DIR/regions-free-region-ordering-caller1.rs:19:27
+   |
+LL |     let z: &'a & usize = &(&y);
+   |                           ^^^^ borrowed value does not live long enough
+...
+LL | }
+   | - `y` dropped here while still borrowed
+   |
+note: borrowed value must be valid for the lifetime 'a as defined on the function body at 15:10...
+  --> $DIR/regions-free-region-ordering-caller1.rs:15:10
+   |
+LL | fn call1<'a>(x: &'a usize) {
+   |          ^^
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0716`.
+Some errors occurred: E0597, E0716.
+For more information about an error, try `rustc --explain E0597`.

src/test/ui/regions/regions-implied-bounds-projection-gap-1.nll.stderr

@@ -0,0 +1,11 @@
+error[E0309]: the parameter type `T` may not live long enough
+  --> $DIR/regions-implied-bounds-projection-gap-1.rs:26:5
+   |
+LL |     wf::<&'x T>();
+   |     ^^^^^^^^^^^^^
+   |
+   = help: consider adding an explicit lifetime bound `T: 'x`...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0309`.

src/test/ui/regions/regions-implied-bounds-projection-gap-1.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 // Illustrates the "projection gap": in this test, even though we know
 // that `T::Foo: 'x`, that does not tell us that `T: 'x`, because
 // there might be other ways for the caller of `func` to show that

src/test/ui/regions/regions-implied-bounds-projection-gap-1.stderr

@@ -1,5 +1,5 @@
 error[E0309]: the parameter type `T` may not live long enough
-  --> $DIR/regions-implied-bounds-projection-gap-1.rs:28:10
+  --> $DIR/regions-implied-bounds-projection-gap-1.rs:26:10
    |
 LL | fn func<'x, T:Trait1<'x>>(t: &'x T::Foo)
    |             -- help: consider adding an explicit lifetime bound `T: 'x`...
@@ -8,7 +8,7 @@ LL |     wf::<&'x T>();
    |          ^^^^^
    |
 note: ...so that the reference type `&'x T` does not outlive the data it points at
-  --> $DIR/regions-implied-bounds-projection-gap-1.rs:28:10
+  --> $DIR/regions-implied-bounds-projection-gap-1.rs:26:10
    |
 LL |     wf::<&'x T>();
    |          ^^^^^

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

@@ -0,0 +1,13 @@
+error: unsatisfied lifetime constraints
+  --> $DIR/regions-outlives-projection-container-wc.rs:46:13
+   |
+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-outlives-projection-container-wc.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 // Test that we are imposing the requirement that every associated
 // type of a bound that appears in the where clause on a struct must
 // outlive the location in which the type appears, even when the
@@ -43,7 +41,9 @@ fn with_assoc<'a,'b>() {
     // outlive 'a. In this case, that means TheType<'b>::TheAssocType,
     // which is &'b (), must outlive 'a.
 
-    let _: &'a WithAssoc<TheType<'b>> = loop { };
+    // 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
 }
 

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

@@ -1,16 +1,16 @@
 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:46:12
+  --> $DIR/regions-outlives-projection-container-wc.rs:46:13
    |
-LL |     let _: &'a WithAssoc<TheType<'b>> = loop { };
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _x: &'a WithAssoc<TheType<'b>> = loop { };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: the pointer is valid for the lifetime 'a as defined on the function body at 40:15
-  --> $DIR/regions-outlives-projection-container-wc.rs:40:15
+note: the pointer is valid for the lifetime 'a as defined on the function body at 38:15
+  --> $DIR/regions-outlives-projection-container-wc.rs:38:15
    |
 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 40:18
-  --> $DIR/regions-outlives-projection-container-wc.rs:40:18
+note: but the referenced data is only valid for the lifetime 'b as defined on the function body at 38:18
+  --> $DIR/regions-outlives-projection-container-wc.rs:38:18
    |
 LL | fn with_assoc<'a,'b>() {
    |                  ^^