Treat vars created for opaque subtyping as live everywhere

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -144,7 +144,31 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
                 variance,
                 ty,
             )?;
-            Ok(infcx.resolve_vars_if_possible(Ty::new_infer(infcx.tcx, ty::TyVar(ty_vid))))
+            let new_var =
+                infcx.resolve_vars_if_possible(Ty::new_infer(infcx.tcx, ty::TyVar(ty_vid)));
+
+            // Any regions in this new type must be live everywhere, so we mark them as such.
+            // (It may be that it only needs to be live where the opaque type itself is - which
+            // includes defining use sites, but we'll be conservative and mark all points as live).
+            // This is needed for Polonius, which doesn't propagate constraints
+            // through dead regions. See issue #160669.
+            //
+            // We only do this in the root universe. Inside a binder these regions live in a higher
+            // universe and their values contain placeholders; marking them live at every point
+            // leaks those placeholders, turning the higher-ranked check into an error which then
+            // suppresses the deferred opaque type diagnostics.
+            if infcx.universe() == ty::UniverseIndex::ROOT {
+                let tcx = infcx.tcx;
+                let liveness = &mut self.type_checker.constraints.liveness_constraints;
+                ty::fold_regions(tcx, new_var, |r, _| {
+                    if let ty::ReVar(vid) = r.kind() {
+                        liveness.add_all_points(vid);
+                    }
+                    r
+                });
+            }
+
+            Ok(new_var)
         };
 
         let (a, b) = match (a.kind(), b.kind()) {

tests/ui/nll/polonius/opaque-multiple-defining-uses-160669.next.stderr

@@ -0,0 +1,69 @@
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:31:16
+   |
+LL |         let local = String::from("dangling");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+LL |     s
+LL | }
+   |  - borrow later used here
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:41:16
+   |
+LL |         let local = String::from("dangling0");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+...
+LL | }
+   |  - borrow later used here
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:45:16
+   |
+LL |         let local = String::from("dangling1");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+LL |     s
+LL | }
+   |  - borrow later used here
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:57:5
+   |
+LL |     let local = String::from("dangling");
+   |         ----- binding `local` declared here
+LL |     &local
+   |     ^^^^^^ borrowed value does not live long enough
+LL | }
+   | -- borrow later used here
+   | |
+   | `local` dropped here while still borrowed
+
+error[E0597]: `short` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:64:16
+   |
+LL | fn minimal(short: (), out: &'static ()) -> impl Sized {
+   |            ----- binding `short` declared here
+LL |     if true {
+LL |         return &short;
+   |                ^^^^^^ borrowed value does not live long enough
+...
+LL | }
+   |  -
+   |  |
+   |  `short` dropped here while still borrowed
+   |  borrow later used here
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

tests/ui/nll/polonius/opaque-multiple-defining-uses-160669.nll.stderr

@@ -0,0 +1,80 @@
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:31:16
+   |
+LL | fn two_uses<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+   |             -- lifetime `'a` defined here
+LL |     if flag {
+LL |         let local = String::from("dangling");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+LL |     s
+   |     - opaque type requires that `local` is borrowed for `'a`
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:41:16
+   |
+LL | fn three_uses<'a>(s: &'a String, flag: u8) -> impl Display + use<'a> {
+   |               -- lifetime `'a` defined here
+LL |     if flag == 0 {
+LL |         let local = String::from("dangling0");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+...
+LL |         return &local;
+   |                ------ opaque type requires that `local` is borrowed for `'a`
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:45:16
+   |
+LL | fn three_uses<'a>(s: &'a String, flag: u8) -> impl Display + use<'a> {
+   |               -- lifetime `'a` defined here
+...
+LL |         let local = String::from("dangling1");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^
+   |                |
+   |                borrowed value does not live long enough
+   |                opaque type requires that `local` is borrowed for `'a`
+LL |     }
+   |     - `local` dropped here while still borrowed
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:57:5
+   |
+LL | fn reversed_order<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+   |                   -- lifetime `'a` defined here
+...
+LL |     let local = String::from("dangling");
+   |         ----- binding `local` declared here
+LL |     &local
+   |     ^^^^^^
+   |     |
+   |     borrowed value does not live long enough
+   |     opaque type requires that `local` is borrowed for `'a`
+LL | }
+   | - `local` dropped here while still borrowed
+
+error[E0597]: `short` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:64:16
+   |
+LL | fn minimal(short: (), out: &'static ()) -> impl Sized {
+   |            ----- binding `short` declared here
+LL |     if true {
+LL |         return &short;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+LL |     out
+   |     --- opaque type requires that `short` is borrowed for `'static`
+LL | }
+   |  - `short` dropped here while still borrowed
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

tests/ui/nll/polonius/opaque-multiple-defining-uses-160669.polonius.stderr

@@ -0,0 +1,80 @@
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:31:16
+   |
+LL | fn two_uses<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+   |             -- lifetime `'a` defined here
+LL |     if flag {
+LL |         let local = String::from("dangling");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+LL |     s
+   |     - opaque type requires that `local` is borrowed for `'a`
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:41:16
+   |
+LL | fn three_uses<'a>(s: &'a String, flag: u8) -> impl Display + use<'a> {
+   |               -- lifetime `'a` defined here
+LL |     if flag == 0 {
+LL |         let local = String::from("dangling0");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+   |     - `local` dropped here while still borrowed
+...
+LL |         return &local;
+   |                ------ opaque type requires that `local` is borrowed for `'a`
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:45:16
+   |
+LL | fn three_uses<'a>(s: &'a String, flag: u8) -> impl Display + use<'a> {
+   |               -- lifetime `'a` defined here
+...
+LL |         let local = String::from("dangling1");
+   |             ----- binding `local` declared here
+LL |         return &local;
+   |                ^^^^^^
+   |                |
+   |                borrowed value does not live long enough
+   |                opaque type requires that `local` is borrowed for `'a`
+LL |     }
+   |     - `local` dropped here while still borrowed
+
+error[E0597]: `local` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:57:5
+   |
+LL | fn reversed_order<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+   |                   -- lifetime `'a` defined here
+...
+LL |     let local = String::from("dangling");
+   |         ----- binding `local` declared here
+LL |     &local
+   |     ^^^^^^
+   |     |
+   |     borrowed value does not live long enough
+   |     opaque type requires that `local` is borrowed for `'a`
+LL | }
+   | - `local` dropped here while still borrowed
+
+error[E0597]: `short` does not live long enough
+  --> $DIR/opaque-multiple-defining-uses-160669.rs:64:16
+   |
+LL | fn minimal(short: (), out: &'static ()) -> impl Sized {
+   |            ----- binding `short` declared here
+LL |     if true {
+LL |         return &short;
+   |                ^^^^^^ borrowed value does not live long enough
+LL |     }
+LL |     out
+   |     --- opaque type requires that `short` is borrowed for `'static`
+LL | }
+   |  - `short` dropped here while still borrowed
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

tests/ui/nll/polonius/opaque-multiple-defining-uses-160669.rs

@@ -0,0 +1,69 @@
+// An opaque type has a single hidden type, and the opaque type storage only keeps a
+// single hidden type per key. So, a second defining use replaces the first.
+//
+// In the old solver, MIR is built with unnormalized opaques; and, a defining use
+// is generalized prior to being registered as the hidden type, to allow for
+// subtyping.
+//
+// During this generalization, any regions created were previously not
+// considered live, which means that under Polonius Alpha, outlives constraints
+// were not propogated between a previous hidden type and a new one.
+//
+// We now consider all these lifetimes live at all points.
+//
+// In the new solver, this is all moot, because MIR has *normalized* opaques,
+// and so there is no special subtyping code.
+//
+// Regression test for #160669.
+
+//@ ignore-compare-mode-polonius (explicit revisions)
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@ revisions: nll polonius next
+//@ [nll] compile-flags: -Z polonius=off
+//@ [polonius] compile-flags: -Z polonius=next
+//@ [next] compile-flags: -Z next-solver -Z polonius=next
+
+use std::fmt::Display;
+
+fn two_uses<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+    if flag {
+        let local = String::from("dangling");
+        return &local; //~ ERROR `local` does not live long enough
+    }
+    s
+}
+
+// The same, with a borrow of a local in multiple branchs, to check that we
+// don't only report the defining use which happens to be last in MIR order.
+fn three_uses<'a>(s: &'a String, flag: u8) -> impl Display + use<'a> {
+    if flag == 0 {
+        let local = String::from("dangling0");
+        return &local; //~ ERROR `local` does not live long enough
+    }
+    if flag == 1 {
+        let local = String::from("dangling1");
+        return &local; //~ ERROR `local` does not live long enough
+    }
+    s
+}
+
+// Control for the order dependence: here the bad defining use is the last one in MIR order,
+// so it stayed anchored and was caught even before the fix.
+fn reversed_order<'a>(s: &'a String, flag: bool) -> impl Display + use<'a> {
+    if flag {
+        return s;
+    }
+    let local = String::from("dangling");
+    &local //~ ERROR `local` does not live long enough
+}
+
+// This is a much more minimal MIR representation of the same bug. Useful for
+// debugging.
+fn minimal(short: (), out: &'static ()) -> impl Sized {
+    if true {
+        return &short; //~ ERROR `short` does not live long enough
+    }
+    out
+}
+
+fn main() {}

tests/ui/suggestions/lifetimes/explicit-lifetime-suggestion-in-proper-span-issue-121267.rs

@@ -8,6 +8,7 @@ fn bar(src: &crate::Foo) -> impl Iterator<Item = i32> {
     [0].into_iter()
  //~^ ERROR hidden type for `impl Iterator<Item = i32>` captures lifetime that does not appear in bounds
         .filter_map(|_| foo(src))
+        //~^ ERROR `src` does not live long enough
 }
 
 struct Foo<'a>(&'a str);

tests/ui/suggestions/lifetimes/explicit-lifetime-suggestion-in-proper-span-issue-121267.stderr

@@ -10,6 +10,21 @@ LL | |         .filter_map(|_| foo(src))
    |
    = note: hidden type `FilterMap<Iter<'_, i32>, {closure@...}>` captures lifetime `'_`
 
-error: aborting due to 1 previous error
+error[E0597]: `src` does not live long enough
+  --> $DIR/explicit-lifetime-suggestion-in-proper-span-issue-121267.rs:10:29
+   |
+LL | fn bar(src: &crate::Foo) -> impl Iterator<Item = i32> {
+   |        --- binding `src` declared here
+...
+LL |         .filter_map(|_| foo(src))
+   |                     ---     ^^^ borrowed value does not live long enough
+   |                     |
+   |                     value captured here
+LL |
+LL | }
+   |  - `src` dropped here while still borrowed
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0700`.
+Some errors have detailed explanations: E0597, E0700.
+For more information about an error, try `rustc --explain E0597`.