Auto merge of rust-lang#18349 - dqkqd:issue-18344, r=Veykril

feat: render docs from aliased type when type has no docs

Trying to close rust-lang#18344

- [x] ~Find the docs by traversing upwards if the type itself has none but aliasing for another type that might have.~
- [x] Show docs from aliased type.
- [x] Showing description that we are displaying documentation for different definition in hover box.

![image](https://github.com/user-attachments/assets/820d6f97-aa2c-4dc4-8a25-75746e32d950)

Author: bors

src/tools/rust-analyzer/crates/ide-db/src/defs.rs

@@ -178,7 +178,19 @@ impl Definition {
             Definition::Static(it) => it.docs(db),
             Definition::Trait(it) => it.docs(db),
             Definition::TraitAlias(it) => it.docs(db),
-            Definition::TypeAlias(it) => it.docs(db),
+            Definition::TypeAlias(it) => {
+                it.docs(db).or_else(|| {
+                    // docs are missing, try to fall back to the docs of the aliased item.
+                    let adt = it.ty(db).as_adt()?;
+                    let docs = adt.docs(db)?;
+                    let docs = format!(
+                        "*This is the documentation for* `{}`\n\n{}",
+                        adt.display(db, edition),
+                        docs.as_str()
+                    );
+                    Some(Documentation::new(docs))
+                })
+            }
             Definition::BuiltinType(it) => {
                 famous_defs.and_then(|fd| {
                     // std exposes prim_{} modules with docstrings on the root to document the builtins

src/tools/rust-analyzer/crates/ide/src/hover/tests.rs

@@ -9018,3 +9018,156 @@ foo!(BAR_$0);
         "#]],
     );
 }
+
+#[test]
+fn type_alias_without_docs() {
+    // Simple.
+    check(
+        r#"
+/// Docs for B
+struct B;
+
+type A$0 = B;
+"#,
+        expect![[r#"
+            *A*
+
+            ```rust
+            test
+            ```
+
+            ```rust
+            // size = 0, align = 1
+            type A = B
+            ```
+
+            ---
+
+            *This is the documentation for* `struct B`
+
+            Docs for B
+        "#]],
+    );
+
+    // Nested.
+    check(
+        r#"
+/// Docs for C
+struct C;
+
+type B = C;
+
+type A$0 = B;
+"#,
+        expect![[r#"
+            *A*
+
+            ```rust
+            test
+            ```
+
+            ```rust
+            // size = 0, align = 1
+            type A = B
+            ```
+
+            ---
+
+            *This is the documentation for* `struct C`
+
+            Docs for C
+        "#]],
+    );
+
+    // Showing the docs for aliased struct instead of intermediate type.
+    check(
+        r#"
+/// Docs for C
+struct C;
+
+/// Docs for B
+type B = C;
+
+type A$0 = B;
+"#,
+        expect![[r#"
+            *A*
+
+            ```rust
+            test
+            ```
+
+            ```rust
+            // size = 0, align = 1
+            type A = B
+            ```
+
+            ---
+
+            *This is the documentation for* `struct C`
+
+            Docs for C
+        "#]],
+    );
+
+    // No docs found.
+    check(
+        r#"
+struct C;
+
+type B = C;
+
+type A$0 = B;
+"#,
+        expect![[r#"
+            *A*
+
+            ```rust
+            test
+            ```
+
+            ```rust
+            // size = 0, align = 1
+            type A = B
+            ```
+        "#]],
+    );
+
+    // Multiple nested crate.
+    check(
+        r#"
+//- /lib.rs crate:c
+/// Docs for C
+pub struct C;
+
+//- /lib.rs crate:b deps:c
+pub use c::C;
+pub type B = C;
+
+//- /lib.rs crate:a deps:b
+pub use b::B;
+pub type A = B;
+
+//- /main.rs crate:main deps:a
+use a::A$0;
+"#,
+        expect![[r#"
+            *A*
+
+            ```rust
+            a
+            ```
+
+            ```rust
+            // size = 0, align = 1
+            pub type A = B
+            ```
+
+            ---
+
+            *This is the documentation for* `pub struct C`
+
+            Docs for C
+        "#]],
+    );
+}