Fix doctest errors related to rustc_middle

compiler/rustc_middle/Cargo.toml

@@ -4,7 +4,6 @@ version = "0.0.0"
 edition = "2021"
 
 [lib]
-doctest = false
 
 [dependencies]
 bitflags = "1.2.1"

compiler/rustc_middle/src/query/mod.rs

@@ -1648,6 +1648,8 @@ rustc_queries! {
     /// a generic type parameter will panic if you call this method on it:
     ///
     /// ```
+    /// use std::fmt::Debug;
+    ///
     /// pub trait Foo<T: Debug> {}
     /// ```
     ///

compiler/rustc_middle/src/ty/inhabitedness/mod.rs

@@ -5,7 +5,7 @@
 //!
 //! # Example
 //! ```rust
-//! enum Void {}
+//! #![feature(never_type)]
 //! mod a {
 //!     pub mod b {
 //!         pub struct SecretlyUninhabited {
@@ -15,6 +15,7 @@
 //! }
 //!
 //! mod c {
+//!     enum Void {}
 //!     pub struct AlsoSecretlyUninhabited {
 //!         _priv: Void,
 //!     }
@@ -35,7 +36,7 @@
 //! `Foo`.
 //!
 //! # Example
-//! ```rust
+//! ```ignore(illustrative)
 //! let foo_result: Result<T, Foo> = ... ;
 //! let Ok(t) = foo_result;
 //! ```