Handle pub use Trait as _ cases correctly: the item is unnameable. (#…

…269)

src/indexed_crate.rs

@@ -601,7 +601,10 @@ mod tests {
                     "duplicates found: {actual_items:?}"
                 );
 
-                assert_eq!(expected_importable_paths, &deduplicated_actual_items);
+                assert_eq!(
+                    expected_importable_paths, &deduplicated_actual_items,
+                    "mismatch for item name {expected_item_name}",
+                );
             }
         }
 
@@ -1789,5 +1792,77 @@ expected exactly one importable path for `Foo` items in this crate but got: {act
 
             assert_exported_items_match(test_crate, &expected_items);
         }
+
+        #[test]
+        fn reexport_as_underscore() {
+            let test_crate = "reexport_as_underscore";
+            let expected_items = btreemap! {
+                "Struct" => btreeset![
+                    "reexport_as_underscore::Struct",
+                ],
+                "Trait" => btreeset![],
+                "hidden" => btreeset![],
+                "UnderscoreImported" => btreeset![],
+            };
+
+            assert_exported_items_match(test_crate, &expected_items);
+        }
+
+        #[test]
+        fn nested_reexport_as_underscore() {
+            let test_crate = "nested_reexport_as_underscore";
+            let expected_items = btreemap! {
+                "Trait" => btreeset![],  // no importable paths!
+            };
+
+            assert_exported_items_match(test_crate, &expected_items);
+        }
+
+        #[test]
+        fn overlapping_reexport_as_underscore() {
+            let test_crate = "overlapping_reexport_as_underscore";
+
+            let rustdoc = load_pregenerated_rustdoc(test_crate);
+            let indexed_crate = IndexedCrate::new(&rustdoc);
+
+            let item_id_candidates = rustdoc
+                .index
+                .iter()
+                .filter_map(|(id, item)| (item.name.as_deref() == Some("Example")).then_some(id))
+                .collect_vec();
+            if item_id_candidates.len() != 2 {
+                panic!(
+                    "Expected to find exactly 2 items with name \
+                    Example, but found these matching IDs: {item_id_candidates:?}"
+                );
+            }
+
+            for item_id in item_id_candidates {
+                let importable_paths: Vec<_> = indexed_crate
+                    .publicly_importable_names(item_id)
+                    .into_iter()
+                    .map(|components| components.into_iter().join("::"))
+                    .collect();
+
+                match &rustdoc.index[item_id].inner {
+                    ItemEnum::Struct(..) => {
+                        assert_eq!(
+                            vec!["overlapping_reexport_as_underscore::Example"],
+                            importable_paths,
+                        );
+                    }
+                    ItemEnum::Trait(..) => {
+                        assert!(
+                            importable_paths.is_empty(),
+                            "expected no importable item names but found {importable_paths:?}"
+                        );
+                    }
+                    _ => unreachable!(
+                        "unexpected item for ID {item_id:?}: {:?}",
+                        rustdoc.index[item_id]
+                    ),
+                }
+            }
+        }
     }
 }

src/visibility_tracker.rs

@@ -61,7 +61,11 @@ impl<'a> VisibilityTracker<'a> {
 
         let (push_name, popped_name) = match &item.inner {
             rustdoc_types::ItemEnum::Import(import_item) => {
-                if import_item.glob {
+                if import_item.name == "_" {
+                    // Items re-exported as `_` are not nameable. They cannot be directly imported.
+                    // They can be used via a glob import, but we are not interested in that here.
+                    return;
+                } else if import_item.glob {
                     // Glob imports refer to the *contents* of the named item, not the item itself.
                     // Rust doesn't allow glob imports to rename items, so there's no name to add.
                     (None, None)
@@ -328,6 +332,17 @@ fn resolve_crate_names(crate_: &Crate) -> NameResolution<'_> {
                         .or_default()
                         .insert(inner_id);
                 } else if let Some(target) = imp.id.as_ref().and_then(|id| crate_.index.get(id)) {
+                    if imp.name == "_" {
+                        // `_` is a special name which causes the imported item to be available
+                        // but unnameable. `pub use Trait as _` makes sense when constructing
+                        // modules intended to be used as a prelude, since glob imports will
+                        // include the (unnameable) trait and make its methods available for use.
+                        //
+                        // For importable path purposes, items re-exported as `_` do not exist
+                        // since they cannot be directly imported due to lack of a usable name.
+                        continue;
+                    }
+
                     for name in get_names_for_item(crate_, target) {
                         // Handle renaming imports like `use some::foo as bar;`
                         let name = name.rename(&imp.name);

test_crates/nested_reexport_as_underscore/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+publish = false
+name = "nested_reexport_as_underscore"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

test_crates/nested_reexport_as_underscore/src/lib.rs

@@ -0,0 +1,26 @@
+//! This crate does not re-export any *nameable* items.
+//!
+//! However, glob imports of this file (in the style of a prelude)
+//! make `Trait::method()` visible, making `().method()` valid Rust.
+//!
+//! Docs: <https://doc.rust-lang.org/reference/items/use-declarations.html#underscore-imports>
+
+mod inner {
+    pub trait Trait {
+        fn method(&self) {}
+    }
+
+    impl Trait for () {}
+}
+
+mod second {
+    pub use super::inner::Trait as _;
+}
+
+pub use second::*;
+
+/// Verify that the trait is indeed visible.
+#[allow(dead_code)]
+fn proof() {
+    ().method();
+}

test_crates/overlapping_reexport_as_underscore/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+publish = false
+name = "overlapping_reexport_as_underscore"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

test_crates/overlapping_reexport_as_underscore/src/lib.rs

@@ -0,0 +1,23 @@
+//! This crate re-exports only the struct `Example`.
+//!
+//! Even though `second` imports two items named `Example`, one of them is renamed to `_`
+//! making it unnameable.
+//!
+//! Docs: <https://doc.rust-lang.org/reference/items/use-declarations.html#underscore-imports>
+
+mod inner {
+    pub trait Example {
+        fn method(&self) {}
+    }
+}
+
+mod inner2 {
+    pub struct Example {}
+}
+
+mod second {
+    pub use super::inner::Example as _;
+    pub use super::inner2::Example;
+}
+
+pub use second::*;

test_crates/reexport_as_underscore/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+publish = false
+name = "reexport_as_underscore"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

test_crates/reexport_as_underscore/src/lib.rs

@@ -0,0 +1,32 @@
+//! This crate re-exports only the name `Struct`.
+//!
+//! However, glob imports of this file (in the style of a prelude)
+//! also make `Trait::method()` visible as well, making `().method()` valid Rust.
+//!
+//! The `_` re-export of the module `hidden` is not nameable at all, and here has no effect.
+//! `hidden::UnderscoreImported` is not nameable outside this crate.
+//!
+//! Docs: <https://doc.rust-lang.org/reference/items/use-declarations.html#underscore-imports>
+
+mod inner {
+    pub trait Trait {
+        fn method(&self) {}
+    }
+
+    impl Trait for () {}
+
+    pub struct Struct {}
+}
+
+mod nested {
+    pub mod hidden {
+        pub struct UnderscoreImported;
+    }
+}
+
+pub use inner::{
+    Struct,
+    Trait as _,
+};
+
+pub use nested::hidden as _;