1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
//! 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 temp2: i32;
//!
//!         {
//!             let temp1 = x;
//!             Goto(my_second_block)
//!         }
//!
//!         my_second_block = {
//!             temp2 = Move(temp1);
//!             RET = temp2;
//!             Return()
//!         }
//!     )
//! }
//! ```
//!
//! 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][dialect
//! docs] you are inserting here. Generally you'll want to use `#![custom_mir(dialect = "built")]`
//! if you want your MIR to be modified by the full MIR pipeline, or `#![custom_mir(dialect =
//! "runtime", phase = "optimized")] if you don't.
//!
//! [dialect docs]:
//!     https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.MirPhase.html
//!
//! The input to the [`mir!`] macro is:
//!
//!  - A possibly empty list of local declarations. Locals can also be declared inline on
//!    assignments via `let`. Type inference generally works. Shadowing does not.
//!  - A list of basic blocks. The first of these is the start block and is where execution begins.
//!    All blocks other than the start block need to be given a name, so that they can be referred
//!    to later.
//!     - Each block is a list of semicolon terminated statements, followed by a terminator. The
//!       syntax for the various statements and terminators is designed to be as similar as possible
//!       to the syntax for analogous concepts in native Rust. See below for a list.
//!
//! # Examples
//!
#![cfg_attr(bootstrap, doc = "```rust,compile_fail")]
#![cfg_attr(not(bootstrap), doc = "```rust")]
//! #![feature(core_intrinsics, custom_mir)]
//!
//! extern crate core;
//! use core::intrinsics::mir::*;
//!
//! #[custom_mir(dialect = "built")]
//! pub fn choose_load(a: &i32, b: &i32, c: bool) -> i32 {
//!     mir!(
//!         {
//!             match c {
//!                 true => t,
//!                 _ => f,
//!             }
//!         }
//!
//!         t = {
//!             let temp = a;
//!             Goto(load_and_exit)
//!         }
//!
//!         f = {
//!             temp = b;
//!             Goto(load_and_exit)
//!         }
//!
//!         load_and_exit = {
//!             RET = *temp;
//!             Return()
//!         }
//!     )
//! }
//!
//! #[custom_mir(dialect = "built")]
//! fn unwrap_unchecked<T>(opt: Option<T>) -> T {
//!     mir!({
//!         RET = Move(Field(Variant(opt, 1), 0));
//!         Return()
//!     })
//! }
//!
//! #[custom_mir(dialect = "runtime", phase = "optimized")]
//! fn push_and_pop<T>(v: &mut Vec<T>, value: T) {
//!     mir!(
//!         let unused;
//!         let popped;
//!
//!         {
//!             Call(unused, pop, Vec::push(v, value))
//!         }
//!
//!         pop = {
//!             Call(popped, drop, Vec::pop(v))
//!         }
//!
//!         drop = {
//!             Drop(popped, ret)
//!         }
//!
//!         ret = {
//!             Return()
//!         }
//!     )
//! }
//! ```
//!
//! We can also set off compilation failures that happen in sufficiently late stages of the
//! compiler:
//!
//! ```rust,compile_fail
//! #![feature(core_intrinsics, custom_mir)]
//!
//! extern crate core;
//! use core::intrinsics::mir::*;
//!
//! #[custom_mir(dialect = "built")]
//! fn borrow_error(should_init: bool) -> i32 {
//!     mir!(
//!         let temp: i32;
//!
//!         {
//!             match should_init {
//!                 true => init,
//!                 _ => use_temp,
//!             }
//!         }
//!
//!         init = {
//!             temp = 0;
//!             Goto(use_temp)
//!         }
//!
//!         use_temp = {
//!             RET = temp;
//!             Return()
//!         }
//!     )
//! }
//! ```
//!
//! ```text
//! error[E0381]: used binding is possibly-uninitialized
//!   --> test.rs:24:13
//!    |
//! 8  | /     mir!(
//! 9  | |         let temp: i32;
//! 10 | |
//! 11 | |         {
//! ...  |
//! 19 | |             temp = 0;
//!    | |             -------- binding initialized here in some conditions
//! ...  |
//! 24 | |             RET = temp;
//!    | |             ^^^^^^^^^^ value used here but it is possibly-uninitialized
//! 25 | |             Return()
//! 26 | |         }
//! 27 | |     )
//!    | |_____- binding declared here but left uninitialized
//!
//! error: aborting due to previous error
//!
//! For more information about this error, try `rustc --explain E0381`.
//! ```
//!
//! # Syntax
//!
//! The lists below are an exhaustive description of how various MIR constructs can be created.
//! Anything missing from the list should be assumed to not be supported, PRs welcome.
//!
//! #### Locals
//!
//!  - The `_0` return local can always be accessed via `RET`.
//!  - Arguments can be accessed via their regular name.
//!  - All other locals need to be declared with `let` somewhere and then can be accessed by name.
//!
//! #### Places
//!  - Locals implicit convert to places.
//!  - Field accesses, derefs, and indexing work normally.
//!  - Fields in variants can be accessed via the [`Variant`] and [`Field`] associated functions,
//!    see their documentation for details.
//!
//! #### Operands
//!  - Places implicitly convert to `Copy` operands.
//!  - `Move` operands can be created via [`Move`].
//!  - Const blocks, literals, named constants, and const params all just work.
//!  - [`Static`] and [`StaticMut`] can be used to create `&T` and `*mut T`s to statics. These are
//!    constants in MIR and the only way to access statics.
//!
//! #### Statements
//!  - Assign statements work via normal Rust assignment.
//!  - [`Retag`] statements have an associated function.
//!
//! #### Rvalues
//!
//!  - Operands implicitly convert to `Use` rvalues.
//!  - `&`, `&mut`, `addr_of!`, and `addr_of_mut!` all work to create their associated rvalue.
//!  - [`Discriminant`] has an associated function.
//!
//! #### Terminators
//!
//! Custom MIR does not currently support cleanup blocks or non-trivial unwind paths. As such, there
//! are no resume and abort terminators, and terminators that might unwind do not have any way to
//! indicate the unwind block.
//!
//!  - [`Goto`], [`Return`], [`Unreachable`], [`Drop`](Drop()), and [`DropAndReplace`] have associated functions.
//!  - `match some_int_operand` becomes a `SwitchInt`. Each arm should be `literal => basic_block`
//!     - The exception is the last arm, which must be `_ => basic_block` and corresponds to the
//!       otherwise branch.
//!  - [`Call`] has an associated function as well. The third argument of this function is a normal
//!    function call expresion, for example `my_other_function(a, 5)`.
//!

#![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, $( #[ $meta:meta ] )* fn $($sig:tt)*) => {
        #[rustc_diagnostic_item = $name]
        $( #[ $meta ] )*
        pub fn $($sig)* { panic!() }
    }
}

define!("mir_return", fn Return() -> BasicBlock);
define!("mir_goto", fn Goto(destination: BasicBlock) -> BasicBlock);
define!("mir_unreachable", fn Unreachable() -> BasicBlock);
define!("mir_drop", fn Drop<T>(place: T, goto: BasicBlock));
define!("mir_drop_and_replace", fn DropAndReplace<T>(place: T, value: T, goto: BasicBlock));
define!("mir_call", fn Call<T>(place: T, goto: BasicBlock, call: T));
define!("mir_retag", fn Retag<T>(place: T));
define!("mir_move", fn Move<T>(place: T) -> T);
define!("mir_static", fn Static<T>(s: T) -> &'static T);
define!("mir_static_mut", fn StaticMut<T>(s: T) -> *mut T);
define!(
    "mir_discriminant",
    /// Gets the discriminant of a place.
    fn Discriminant<T>(place: T) -> <T as ::core::marker::DiscriminantKind>::Discriminant
);
define!("mir_set_discriminant", fn SetDiscriminant<T>(place: T, index: u32));
define!(
    "mir_field",
    /// Access the field with the given index of some place.
    ///
    /// This only makes sense to use in conjunction with [`Variant`]. If the type you are looking to
    /// access the field of does not have variants, you can use normal field projection syntax.
    ///
    /// There is no proper way to do a place projection to a variant in Rust, and so these two
    /// functions are a workaround. You can access a field of a variant via `Field(Variant(place,
    /// var_idx), field_idx)`, where `var_idx` and `field_idx` are appropriate literals. Some
    /// caveats:
    ///
    ///  - The return type of `Variant` is always `()`. Don't worry about that, the correct MIR will
    ///    still be generated.
    ///  - In some situations, the return type of `Field` cannot be inferred. You may need to
    ///    annotate it on the function in these cases.
    ///  - Since `Field` is a function call which is not a place expression, using this on the left
    ///    hand side of an expression is rejected by the compiler. [`place!`] is a macro provided to
    ///    work around that issue. Wrap the left hand side of an assignment in the macro to convince
    ///    the compiler that it's ok.
    ///
    /// # Examples
    ///
    #[cfg_attr(bootstrap, doc = "```rust,compile_fail")]
    #[cfg_attr(not(bootstrap), doc = "```rust")]
    /// #![feature(custom_mir, core_intrinsics)]
    ///
    /// extern crate core;
    /// use core::intrinsics::mir::*;
    ///
    /// #[custom_mir(dialect = "built")]
    /// fn unwrap_deref(opt: Option<&i32>) -> i32 {
    ///     mir!({
    ///         RET = *Field::<&i32>(Variant(opt, 1), 0);
    ///         Return()
    ///     })
    /// }
    ///
    /// #[custom_mir(dialect = "built")]
    /// fn set(opt: &mut Option<i32>) {
    ///     mir!({
    ///         place!(Field(Variant(*opt, 1), 0)) = 5;
    ///         Return()
    ///     })
    /// }
    /// ```
    fn Field<F>(place: (), field: u32) -> F
);
define!(
    "mir_variant",
    /// Adds a variant projection with the given index to the place.
    ///
    /// See [`Field`] for documentation.
    fn Variant<T>(place: T, index: u32) -> ()
);
define!(
    "mir_make_place",
    #[doc(hidden)]
    fn __internal_make_place<T>(place: T) -> *mut T
);

/// 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:tt)*
        }

        $(
            $block_name:ident = {
                $($block:tt)*
            }
        )*
    ) => {{
        // 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)? ;
            )*

            ::core::intrinsics::mir::__internal_extract_let!($($entry)*);
            $(
                ::core::intrinsics::mir::__internal_extract_let!($($block)*);
            )*

            {
                // Finally, the contents of the basic blocks
                ::core::intrinsics::mir::__internal_remove_let!({
                    {}
                    { $($entry)* }
                });
                $(
                    ::core::intrinsics::mir::__internal_remove_let!({
                        {}
                        { $($block)* }
                    });
                )*

                RET
            }
        }
    }}
}

/// Helper macro that allows you to treat a value expression like a place expression.
///
/// See the documentation on [`Variant`] for why this is necessary and how to use it.
pub macro place($e:expr) {
    (*::core::intrinsics::mir::__internal_make_place($e))
}

/// Helper macro that extracts the `let` declarations out of a bunch of statements.
///
/// This macro is written using the "statement muncher" strategy. Each invocation parses the first
/// statement out of the input, does the appropriate thing with it, and then recursively calls the
/// same macro on the remainder of the input.
#[doc(hidden)]
pub macro __internal_extract_let {
    // If it's a `let` like statement, keep the `let`
    (
        let $var:ident $(: $ty:ty)? = $expr:expr; $($rest:tt)*
    ) => {
        let $var $(: $ty)?;
        ::core::intrinsics::mir::__internal_extract_let!($($rest)*);
    },
    // Due to #86730, we have to handle const blocks separately
    (
        let $var:ident $(: $ty:ty)? = const $block:block; $($rest:tt)*
    ) => {
        let $var $(: $ty)?;
        ::core::intrinsics::mir::__internal_extract_let!($($rest)*);
    },
    // Otherwise, output nothing
    (
        $stmt:stmt; $($rest:tt)*
    ) => {
        ::core::intrinsics::mir::__internal_extract_let!($($rest)*);
    },
    (
        $expr:expr
    ) => {}
}

/// Helper macro that removes the `let` declarations from a bunch of statements.
///
/// Because expression position macros cannot expand to statements + expressions, we need to be
/// slightly creative here. The general strategy is also statement munching as above, but the output
/// of the macro is "stored" in the subsequent macro invocation. Easiest understood via example:
/// ```text
/// invoke!(
///     {
///         {
///             x = 5;
///         }
///         {
///             let d = e;
///             Call()
///         }
///     }
/// )
/// ```
/// becomes
/// ```text
/// invoke!(
///     {
///         {
///             x = 5;
///             d = e;
///         }
///         {
///             Call()
///         }
///     }
/// )
/// ```
#[doc(hidden)]
pub macro __internal_remove_let {
    // If it's a `let` like statement, remove the `let`
    (
        {
            {
                $($already_parsed:tt)*
            }
            {
                let $var:ident $(: $ty:ty)? = $expr:expr;
                $($rest:tt)*
            }
        }
    ) => { ::core::intrinsics::mir::__internal_remove_let!(
        {
            {
                $($already_parsed)*
                $var = $expr;
            }
            {
                $($rest)*
            }
        }
    )},
    // Due to #86730 , we have to handle const blocks separately
    (
        {
            {
                $($already_parsed:tt)*
            }
            {
                let $var:ident $(: $ty:ty)? = const $block:block;
                $($rest:tt)*
            }
        }
    ) => { ::core::intrinsics::mir::__internal_remove_let!(
        {
            {
                $($already_parsed)*
                $var = const $block;
            }
            {
                $($rest)*
            }
        }
    )},
    // Otherwise, keep going
    (
        {
            {
                $($already_parsed:tt)*
            }
            {
                $stmt:stmt;
                $($rest:tt)*
            }
        }
    ) => { ::core::intrinsics::mir::__internal_remove_let!(
        {
            {
                $($already_parsed)*
                $stmt;
            }
            {
                $($rest)*
            }
        }
    )},
    (
        {
            {
                $($already_parsed:tt)*
            }
            {
                $expr:expr
            }
        }
    ) => {
        {
            $($already_parsed)*
            $expr
        }
    },
}