Rollup merge of #111573 - compiler-errors:erase-re-error, r=WaffleLapkin

Erase `ReError` properly

Fixes #111341

Since we check whether a type has free regions before erasing (to short circuit unnecesary folding), we need to consider `ReError` as a free region, or else we'll skip it when erasing a type that only mentions `ReError`.

cc `@nnethercote`

compiler/rustc_middle/src/ty/sty.rs

@@ -1708,7 +1708,9 @@ pub fn type_flags(self) -> TypeFlags {
             ty::ReErased => {
                 flags = flags | TypeFlags::HAS_RE_ERASED;
             }
-            ty::ReError(_) => {}
+            ty::ReError(_) => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+            }
         }
 
         debug!("type_flags({:?}) = {:?}", self, flags);

tests/ui/borrowck/erase-error-in-mir-drop-tracking.rs

@@ -0,0 +1,23 @@
+// compile-flags: -Zdrop-tracking-mir
+// edition:2021
+
+use std::future::Future;
+
+trait Client {
+    type Connecting<'a>: Future + Send
+    where
+        Self: 'a;
+
+    fn connect(&'_ self) -> Self::Connecting<'a>;
+    //~^ ERROR use of undeclared lifetime name `'a`
+}
+
+fn call_connect<C>(c: &'_ C) -> impl '_ + Future + Send
+where
+    C: Client + Send + Sync,
+{
+    async move { c.connect().await }
+    //~^ ERROR `C` does not live long enough
+}
+
+fn main() {}

tests/ui/borrowck/erase-error-in-mir-drop-tracking.stderr

@@ -0,0 +1,24 @@
+error[E0261]: use of undeclared lifetime name `'a`
+  --> $DIR/erase-error-in-mir-drop-tracking.rs:11:46
+   |
+LL |     fn connect(&'_ self) -> Self::Connecting<'a>;
+   |                                              ^^ undeclared lifetime
+   |
+help: consider introducing lifetime `'a` here
+   |
+LL |     fn connect<'a>(&'_ self) -> Self::Connecting<'a>;
+   |               ++++
+help: consider introducing lifetime `'a` here
+   |
+LL | trait Client<'a> {
+   |             ++++
+
+error: `C` does not live long enough
+  --> $DIR/erase-error-in-mir-drop-tracking.rs:19:5
+   |
+LL |     async move { c.connect().await }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0261`.