doc fixes

Author: WaffleLapkin

compiler/rustc_hir_typeck/src/fallback.rs

@@ -102,21 +102,21 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         fallback_occurred
     }
 
-    // Tries to apply a fallback to `ty` if it is an unsolved variable.
-    //
-    // - Unconstrained ints are replaced with `i32`.
-    //
-    // - Unconstrained floats are replaced with `f64`.
-    //
-    // - Non-numerics may get replaced with `()` or `!`, depending on
-    //   how they were categorized by `calculate_diverging_fallback`
-    //   (and the setting of `#![feature(never_type_fallback)]`).
-    //
-    // Fallback becomes very dubious if we have encountered
-    // type-checking errors. In that case, fallback to Error.
-    //
-    // Sets `FnCtxt::fallback_has_occurred` if fallback is performed
-    // during this call.
+    /// Tries to apply a fallback to `ty` if it is an unsolved variable.
+    ///
+    /// - Unconstrained ints are replaced with `i32`.
+    ///
+    /// - Unconstrained floats are replaced with `f64`.
+    ///
+    /// - Non-numerics may get replaced with `()` or `!`, depending on how they
+    ///   were categorized by [`Self::calculate_diverging_fallback`], crate's
+    ///   edition, and the setting of `#![rustc_never_type_options(fallback = ...)]`.
+    ///
+    /// Fallback becomes very dubious if we have encountered
+    /// type-checking errors. In that case, fallback to Error.
+    ///
+    /// Sets [`FnCtxt::fallback_has_occurred`] if fallback is performed
+    /// during this call.
     fn fallback_if_possible(
         &self,
         ty: Ty<'tcx>,

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -548,6 +548,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             });
                             let unit_obligation = obligation.with(tcx, predicate);
                             if self.predicate_may_hold(&unit_obligation) {
+                                // FIXME: make a new issue for this
                                 err.note(
                                     "this error might have been caused by changes to \
                                     Rust's type-inference algorithm (see issue #48950 \