Rollup merge of #146373 - calvinhirsch:trait-solver-cycle-head-docs, r=lcnr

fix comments about trait solver cycle heads

update some comments that use "cycle root" rather than "cycle head"
also fixed a random other nearby typo in `StackEntry` docs

zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/quick.20question.20about.20SearchGraph.3A.3Aevaluate_goal_in_task/with/538004295

?r lcnr

Author: matthiaskrgr

compiler/rustc_type_ir/src/search_graph/mod.rs

@@ -1262,7 +1262,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
             encountered_overflow |= stack_entry.encountered_overflow;
             debug_assert_eq!(stack_entry.input, input);
 
-            // If the current goal is not the root of a cycle, we are done.
+            // If the current goal is not a cycle head, we are done.
             //
             // There are no provisional cache entries which depend on this goal.
             let Some(usages) = stack_entry.usages else {
@@ -1278,7 +1278,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
             //
             // Check whether we reached a fixpoint, either because the final result
             // is equal to the provisional result of the previous iteration, or because
-            // this was only the root of either coinductive or inductive cycles, and the
+            // this was only the head of either coinductive or inductive cycles, and the
             // final result is equal to the initial response for that case.
             if self.reached_fixpoint(cx, &stack_entry, usages, result) {
                 self.rebase_provisional_cache_entries(&stack_entry, |_, result| result);

compiler/rustc_type_ir/src/search_graph/stack.rs

@@ -13,7 +13,7 @@ rustc_index::newtype_index! {
     pub(super) struct StackDepth {}
 }
 
-/// Stack entries of the evaluation stack. Its fields tend to be lazily
+/// Stack entries of the evaluation stack. Its fields tend to be lazily updated
 /// when popping a child goal or completely immutable.
 #[derive_where(Debug; X: Cx)]
 pub(super) struct StackEntry<X: Cx> {
@@ -42,7 +42,7 @@ pub(super) struct StackEntry<X: Cx> {
     /// Whether evaluating this goal encountered overflow. Lazily updated.
     pub encountered_overflow: bool,
 
-    /// Whether and how this goal has been used as the root of a cycle. Lazily updated.
+    /// Whether and how this goal has been used as a cycle head. Lazily updated.
     pub usages: Option<HeadUsages>,
 
     /// We want to be able to ignore head usages if they happen inside of candidates