Fix comment about placeholders

compiler/rustc_next_trait_solver/src/placeholder.rs

@@ -32,8 +32,10 @@ where
     Infcx: InferCtxtLike<Interner = I>,
     I: Interner,
 {
-    /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
-    /// use a binding level above `universe_indices.len()`, we fail.
+    /// Returns a type with all bound vars replaced by placeholders,
+    /// together with mappings from the new placeholders back to the original variable.
+    ///
+    /// Panics if there are any bound vars that use a binding level above `universe_indices.len()`.
     pub fn replace_bound_vars<T: TypeFoldable<I>>(
         infcx: &'a Infcx,
         universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -39,9 +39,9 @@ use crate::solve::assembly::Candidate;
 ///
 /// We previously used  `cx.recursion_limit().0.checked_ilog2().unwrap_or(0)` for this.
 /// However, it feels unlikely that uncreasing the recursion limit by a power of two
-/// to get one more iteration is every useful or desirable. We now instead used a constant
+/// to get one more iteration is ever useful or desirable. We now instead used a constant
 /// here. If there ever ends up some use-cases where a bigger number of fixpoint iterations
-/// is required, we can add a new attribute for that or revert this to be dependant on the
+/// is required, we can add a new attribute for that or revert this to be dependent on the
 /// recursion limit again. However, this feels very unlikely.
 const FIXPOINT_STEP_LIMIT: usize = 8;