diff --git a/core/src/intrinsics.rs b/core/src/intrinsics.rs
index bfbd43012..cec603dcb 100644
--- a/core/src/intrinsics.rs
+++ b/core/src/intrinsics.rs
@@ -59,6 +59,9 @@ use crate::marker::DiscriminantKind;
 use crate::marker::Tuple;
 use crate::mem;
 
+#[cfg(not(bootstrap))]
+pub mod mir;
+
 // These imports are used for simplifying intra-doc links
 #[allow(unused_imports)]
 #[cfg(all(target_has_atomic = "8", target_has_atomic = "32", target_has_atomic = "ptr"))]
diff --git a/core/src/intrinsics/mir.rs b/core/src/intrinsics/mir.rs
new file mode 100644
index 000000000..1bacdc391
--- /dev/null
+++ b/core/src/intrinsics/mir.rs
@@ -0,0 +1,123 @@
+//! Rustc internal tooling for hand-writing MIR.
+//!
+//! If for some reasons you are not writing rustc tests and have found yourself considering using
+//! this feature, turn back. This is *exceptionally* unstable. There is no attempt at all to make
+//! anything work besides those things which the rustc test suite happened to need. If you make a
+//! typo you'll probably ICE. Really, this is not the solution to your problems. Consider instead
+//! supporting the [stable MIR project group](https://github.com/rust-lang/project-stable-mir).
+//!
+//! The documentation for this module describes how to use this feature. If you are interested in
+//! hacking on the implementation, most of that documentation lives at
+//! `rustc_mir_building/src/build/custom/mod.rs`.
+//!
+//! Typical usage will look like this:
+//!
+//! ```rust
+//! #![feature(core_intrinsics, custom_mir)]
+//!
+//! extern crate core;
+//! use core::intrinsics::mir::*;
+//!
+//! #[custom_mir(dialect = "built")]
+//! pub fn simple(x: i32) -> i32 {
+//!     mir!(
+//!         let temp1: i32;
+//!         let temp2: _;
+//!
+//!         {
+//!             temp1 = x;
+//!             Goto(exit)
+//!         }
+//!
+//!         exit = {
+//!             temp2 = Move(temp1);
+//!             RET = temp2;
+//!             Return()
+//!         }
+//!     )
+//! }
+//! ```
+//!
+//! Hopefully most of this is fairly self-explanatory. Expanding on some notable details:
+//!
+//!  - The `custom_mir` attribute tells the compiler to treat the function as being custom MIR. This
+//!    attribute only works on functions - there is no way to insert custom MIR into the middle of
+//!    another function.
+//!  - The `dialect` and `phase` parameters indicate which version of MIR you are inserting here.
+//!    This will normally be the phase that corresponds to the thing you are trying to test. The
+//!    phase can be omitted for dialects that have just one.
+//!  - You should define your function signature like you normally would. Externally, this function
+//!    can be called like any other function.
+//!  - Type inference works - you don't have to spell out the type of all of your locals.
+//!
+//! For now, all statements and terminators are parsed from nested invocations of the special
+//! functions provided in this module. We additionally want to (but do not yet) support more
+//! "normal" Rust syntax in places where it makes sense. Also, most kinds of instructions are not
+//! supported yet.
+//!
+
+#![unstable(
+    feature = "custom_mir",
+    reason = "MIR is an implementation detail and extremely unstable",
+    issue = "none"
+)]
+#![allow(unused_variables, non_snake_case, missing_debug_implementations)]
+
+/// Type representing basic blocks.
+///
+/// All terminators will have this type as a return type. It helps achieve some type safety.
+pub struct BasicBlock;
+
+macro_rules! define {
+    ($name:literal, $($sig:tt)*) => {
+        #[rustc_diagnostic_item = $name]
+        pub $($sig)* { panic!() }
+    }
+}
+
+define!("mir_return", fn Return() -> BasicBlock);
+define!("mir_goto", fn Goto(destination: BasicBlock) -> BasicBlock);
+define!("mir_retag", fn Retag<T>(place: T));
+define!("mir_retag_raw", fn RetagRaw<T>(place: T));
+define!("mir_move", fn Move<T>(place: T) -> T);
+
+/// Convenience macro for generating custom MIR.
+///
+/// See the module documentation for syntax details. This macro is not magic - it only transforms
+/// your MIR into something that is easier to parse in the compiler.
+#[rustc_macro_transparency = "transparent"]
+pub macro mir {
+    (
+        $(let $local_decl:ident $(: $local_decl_ty:ty)? ;)*
+
+        $entry_block:block
+
+        $(
+            $block_name:ident = $block:block
+        )*
+    ) => {{
+        // First, we declare all basic blocks.
+        $(
+            let $block_name: ::core::intrinsics::mir::BasicBlock;
+        )*
+
+        {
+            // Now all locals
+            #[allow(non_snake_case)]
+            let RET;
+            $(
+                let $local_decl $(: $local_decl_ty)? ;
+            )*
+
+            {
+                // Finally, the contents of the basic blocks
+                $entry_block;
+                $(
+                    $block;
+                )*
+
+                RET
+            }
+        }
+    }}
+}