Handle projection predicates in the param env for auto-trait docs

Fixes #72213

Any predicates in the param env are guaranteed to hold, so we don't need
to do any additional processing of them if we come across them as
sub-obligations of a different predicate. This allows us to avoid adding
the same predicate to the computed ParamEnv multiple times (but with
different regions each time), which causes an ambiguity error during
fulfillment.

src/librustc_trait_selection/traits/auto_trait.rs

@@ -270,6 +270,13 @@ impl AutoTraitFinder<'tcx> {
     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
         let tcx = infcx.tcx;
 
+        // Don't try to proess any nested obligations involving predicates
+        // that are already in the `ParamEnv` (modulo regions): we already
+        // know that they must hold.
+        for predicate in param_env.caller_bounds() {
+            fresh_preds.insert(self.clean_pred(infcx, predicate));
+        }
+
         let mut select = SelectionContext::with_negative(&infcx, true);
 
         let mut already_visited = FxHashSet::default();

src/test/rustdoc/synthetic_auto/issue-72213-projection-lifetime.rs

@@ -0,0 +1,25 @@
+// Regression test for issue #72213
+// Tests that we don't ICE when we have projection predicates
+// in our initial ParamEnv
+
+pub struct Lines<'a, L>
+where
+    L: Iterator<Item = &'a ()>,
+{
+    words: std::iter::Peekable<Words<'a, L>>,
+}
+
+pub struct Words<'a, L> {
+    _m: std::marker::PhantomData<&'a L>,
+}
+
+impl<'a, L> Iterator for Words<'a, L>
+where
+    L: Iterator<Item = &'a ()>,
+{
+    type Item = ();
+
+    fn next(&mut self) -> Option<Self::Item> {
+        unimplemented!()
+    }
+}