Rollup merge of #58347 - matthewjasper:closure-bounds-fixes, r=pnkfelix

Closure bounds fixes

* Ensures that "nice region errors" are buffered so that they are sorted and migrated correctly.
* Propagates fewer constraints for closures (cc #58178)
* Propagate constraints from closures more precisely (#58127)

Closes #58127

r? @nikomatsakis

src/test/ui/nll/closure-requirements/issue-58127-mutliple-requirements.rs

@@ -0,0 +1,40 @@
+// revisions: migrate nll
+//[migrate]compile-flags: -Z borrowck=migrate
+#![cfg_attr(nll, feature(nll))]
+
+// compile-pass
+
+// Test that we propagate region relations from closures precisely when there is
+// more than one non-local lower bound.
+
+// In this case the closure has signature
+// |x: &'4 mut (&'5 (&'1 str, &'2 str), &'3 str)| -> ..
+// We end up with a `'3: '5` constraint that we can propagate as
+// `'3: '1`, `'3: '2`, but previously we approximated it as `'3: 'static`.
+
+// As an optimization, we primarily propagate bounds for the "representative"
+// of each SCC. As such we have these two similar cases where hopefully one
+// of them will test the case we want (case2, when this test was added).
+mod case1 {
+    fn f(s: &str) {
+        g(s, |x| h(x));
+    }
+
+    fn g<T, F>(_: T, _: F)
+    where F: Fn(&mut (&(T, T), T)) {}
+
+    fn h<T>(_: &mut (&(T, T), T)) {}
+}
+
+mod case2 {
+    fn f(s: &str) {
+        g(s, |x| h(x));
+    }
+
+    fn g<T, F>(_: T, _: F)
+    where F: Fn(&mut (T, &(T, T))) {}
+
+    fn h<T>(_: &mut (T, &(T, T))) {}
+}
+
+fn main() {}