Move rustdoc::clean::utils::find_nearest_parent_module to TyCtxt

`find_nearest_parent_module` was extracted from
`rustdoc::passes::collect_intra_doc_links` in rust-lang#80368.

We (me and Joshua Nelson) thought at the time that it should be in rustc
instead, and Joshua suggested it be a method on `TyCtxt`.
However, since rust-lang#80368 was fixing a significant regression that was about
to land on beta, we agreed that to be able to fix this quickly I should
leave the code in rustdoc.

This is a followup PR to move it to `TyCtxt`.

compiler/rustc_middle/src/ty/context.rs

@@ -1285,6 +1285,33 @@ impl<'tcx> TyCtxt<'tcx> {
         )
     }
 
+    /// Find the nearest parent module of a [`DefId`].
+    /// This is different from [`TyCtxt::parent()`] because this returns
+    /// the nearest parent `mod`, whereas [`TyCtxt::parent()`] returns
+    /// the nearest module-like item (e.g., the enum that a variant belongs to).
+    ///
+    /// Returns `def_id` if it's the crate root.
+    ///
+    /// Note for users in rustdoc: **panics if the item it belongs to is fake**
+    /// (see `rustdoc::clean::types::Item::is_fake()`).
+    pub fn find_nearest_parent_module(self, def_id: DefId) -> Option<DefId> {
+        if def_id.is_top_level_module() {
+            // The crate root has no parent. Use it as the root instead.
+            Some(def_id)
+        } else {
+            let mut current = def_id;
+            // The immediate parent might not always be a module.
+            // Find the first parent which is.
+            while let Some(parent) = self.parent(current) {
+                if self.def_kind(parent) == DefKind::Mod {
+                    return Some(parent);
+                }
+                current = parent;
+            }
+            None
+        }
+    }
+
     pub fn metadata_encoding_version(self) -> Vec<u8> {
         self.cstore.metadata_encoding_version().to_vec()
     }

src/librustdoc/clean/utils.rs

@@ -14,7 +14,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_middle::mir::interpret::ConstValue;
 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
-use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt};
+use rustc_middle::ty::{self, DefIdTree, Ty};
 use rustc_span::symbol::{kw, sym, Symbol};
 use std::mem;
 
@@ -623,24 +623,3 @@ where
     *cx.impl_trait_bounds.borrow_mut() = old_bounds;
     r
 }
-
-/// Find the nearest parent module of a [`DefId`].
-///
-/// **Panics if the item it belongs to [is fake][Item::is_fake].**
-crate fn find_nearest_parent_module(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
-    if def_id.is_top_level_module() {
-        // The crate root has no parent. Use it as the root instead.
-        Some(def_id)
-    } else {
-        let mut current = def_id;
-        // The immediate parent might not always be a module.
-        // Find the first parent which is.
-        while let Some(parent) = tcx.parent(current) {
-            if tcx.def_kind(parent) == DefKind::Mod {
-                return Some(parent);
-            }
-            current = parent;
-        }
-        None
-    }
-}

src/librustdoc/html/format.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::{DefId, CRATE_DEF_INDEX};
 use rustc_target::spec::abi::Abi;
 
-use crate::clean::{self, utils::find_nearest_parent_module, PrimitiveType};
+use crate::clean::{self, PrimitiveType};
 use crate::formats::cache::cache;
 use crate::formats::item_type::ItemType;
 use crate::html::escape::Escape;
@@ -1100,7 +1100,7 @@ impl clean::Visibility {
                 // FIXME(camelid): This may not work correctly if `item_did` is a module.
                 //                 However, rustdoc currently never displays a module's
                 //                 visibility, so it shouldn't matter.
-                let parent_module = find_nearest_parent_module(tcx, item_did);
+                let parent_module = tcx.find_nearest_parent_module(item_did);
 
                 if vis_did.index == CRATE_DEF_INDEX {
                     write!(f, "pub(crate) ")
@@ -1109,7 +1109,7 @@ impl clean::Visibility {
                     // is the same as no visibility modifier
                     Ok(())
                 } else if parent_module
-                    .map(|parent| find_nearest_parent_module(tcx, parent))
+                    .map(|parent| tcx.find_nearest_parent_module(parent))
                     .flatten()
                     == Some(vis_did)
                 {

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -33,7 +33,7 @@ use std::convert::{TryFrom, TryInto};
 use std::mem;
 use std::ops::Range;
 
-use crate::clean::{self, utils::find_nearest_parent_module, Crate, Item, ItemLink, PrimitiveType};
+use crate::clean::{self, Crate, Item, ItemLink, PrimitiveType};
 use crate::core::DocContext;
 use crate::fold::DocFolder;
 use crate::html::markdown::{markdown_links, MarkdownLink};
@@ -832,11 +832,8 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
     fn fold_item(&mut self, mut item: Item) -> Option<Item> {
         use rustc_middle::ty::DefIdTree;
 
-        let parent_node = if item.is_fake() {
-            None
-        } else {
-            find_nearest_parent_module(self.cx.tcx, item.def_id)
-        };
+        let parent_node =
+            if item.is_fake() { None } else { self.cx.tcx.find_nearest_parent_module(item.def_id) };
 
         if parent_node.is_some() {
             trace!("got parent node for {:?} {:?}, id {:?}", item.type_(), item.name, item.def_id);