Fix rust-lang#104639, find the right lower bound region in the scenario of partial order relations

Author: chenyukang

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -758,27 +758,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // Otherwise, we need to find the minimum remaining choice, if
         // any, and take that.
         debug!("choice_regions remaining are {:#?}", choice_regions);
-        let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
-            let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
-            let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
-            match (r1_outlives_r2, r2_outlives_r1) {
-                (true, true) => Some(r1.min(r2)),
-                (true, false) => Some(r2),
-                (false, true) => Some(r1),
-                (false, false) => None,
-            }
+        let Some(&min_choice) = choice_regions.iter().find(|&r1| {
+            choice_regions.iter().all(|&r2| {
+                self.universal_region_relations.outlives(r2, *r1)
+            })
+        }) else {
+            debug!("no choice region outlived by all others");
+            return false;
         };
-        let mut min_choice = choice_regions[0];
-        for &other_option in &choice_regions[1..] {
-            debug!(?min_choice, ?other_option,);
-            match min(min_choice, other_option) {
-                Some(m) => min_choice = m,
-                None => {
-                    debug!(?min_choice, ?other_option, "incomparable; no min choice",);
-                    return false;
-                }
-            }
-        }
 
         let min_choice_scc = self.constraint_sccs.scc(min_choice);
         debug!(?min_choice, ?min_choice_scc);

src/test/ui/borrowck/issue-104639-lifetime-order.rs

@@ -0,0 +1,10 @@
+// edition:2018
+// check-pass
+
+#![allow(dead_code)]
+async fn fail<'a, 'b, 'c>(_: &'static str) where 'a: 'c, 'b: 'c, {}
+async fn pass<'a, 'c, 'b>(_: &'static str) where 'a: 'c, 'b: 'c, {}
+async fn pass2<'a, 'b, 'c>(_: &'static str) where 'a: 'c, 'b: 'c, 'c: 'a, {}
+async fn pass3<'a, 'b, 'c>(_: &'static str) where 'a: 'b, 'b: 'c, 'c: 'a, {}
+
+fn main() { }