custom mir: make it clear what the return block is

Author: RalfJung

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -61,7 +61,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
             @call(mir_drop, args) => {
                 Ok(TerminatorKind::Drop {
                     place: self.parse_place(args[0])?,
-                    target: self.parse_block(args[1])?,
+                    target: self.parse_return_to(args[1])?,
                     unwind: self.parse_unwind_action(args[2])?,
                     replace: false,
                 })
@@ -104,6 +104,14 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
         )
     }
 
+    fn parse_return_to(&self, expr_id: ExprId) -> PResult<BasicBlock> {
+        parse_by_kind!(self, expr_id, _, "return block",
+            @call(mir_return_to, args) => {
+                self.parse_block(args[0])
+            },
+        )
+    }
+
     fn parse_match(&self, arms: &[ArmId], span: Span) -> PResult<SwitchTargets> {
         let Some((otherwise, rest)) = arms.split_last() else {
             return Err(ParseError {
@@ -146,7 +154,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
             ExprKind::Assign { lhs, rhs } => (*lhs, *rhs),
         );
         let destination = self.parse_place(destination)?;
-        let target = self.parse_block(args[1])?;
+        let target = self.parse_return_to(args[1])?;
         let unwind = self.parse_unwind_action(args[2])?;
 
         parse_by_kind!(self, call, _, "function call",

compiler/rustc_span/src/symbol.rs

@@ -1040,6 +1040,7 @@ symbols! {
         mir_offset,
         mir_retag,
         mir_return,
+        mir_return_to,
         mir_set_discriminant,
         mir_static,
         mir_static_mut,

library/core/src/intrinsics/mir.rs

@@ -104,21 +104,22 @@
 //! }
 //!
 //! #[custom_mir(dialect = "runtime", phase = "optimized")]
+#![cfg_attr(bootstrap, doc = "#[cfg(any())]")] // disable the following function in doctests when `bootstrap` is set
 //! fn push_and_pop<T>(v: &mut Vec<T>, value: T) {
 //!     mir!(
 //!         let _unused;
 //!         let popped;
 //!
 //!         {
-//!             Call(_unused = Vec::push(v, value), pop, UnwindContinue())
+//!             Call(_unused = Vec::push(v, value), ReturnTo(pop), UnwindContinue())
 //!         }
 //!
 //!         pop = {
-//!             Call(popped = Vec::pop(v), drop, UnwindContinue())
+//!             Call(popped = Vec::pop(v), ReturnTo(drop), UnwindContinue())
 //!         }
 //!
 //!         drop = {
-//!             Drop(popped, ret, UnwindContinue())
+//!             Drop(popped, ReturnTo(ret), UnwindContinue())
 //!         }
 //!
 //!         ret = {
@@ -242,9 +243,8 @@
 //!  - `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 expression, for example `my_other_function(a, 5)`.
-//!
+//!  - [`Call`] has an associated function as well, with special syntax:
+//!    `Call(ret_val = function(arg1, arg2, ...), ReturnTo(next_block), UnwindContinue())`.
 
 #![unstable(
     feature = "custom_mir",
@@ -295,7 +295,7 @@ define!(
 define!(
     "mir_unwind_unreachable",
     /// An unwind action that triggers undefined behaviour.
-    fn UnwindUnreachable() -> BasicBlock
+    fn UnwindUnreachable()
 );
 define!(
     "mir_unwind_terminate",
@@ -310,12 +310,43 @@ define!(
     fn UnwindCleanup(goto: BasicBlock)
 );
 
+// Return destination for `Call`
+define!("mir_return_to", fn ReturnTo(goto: BasicBlock));
+
 // Terminators
 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, U>(place: T, goto: BasicBlock, unwind_action: U));
-define!("mir_call", fn Call<U>(call: (), goto: BasicBlock, unwind_action: U));
+define!("mir_drop",
+    /// Drop the contents of a place.
+    ///
+    /// The first argument must be a place.
+    ///
+    /// The second argument must be of the form `ReturnTo(bb)`, where `bb` is the basic block that
+    /// will be jumped to after the destructor returns.
+    ///
+    /// The third argument describes what happens on unwind. It can be one of:
+    /// - [`UnwindContinue`]
+    /// - [`UnwindUnreachable`]
+    /// - [`UnwindTerminate`]
+    /// - [`UnwindCleanup`]
+    fn Drop<T>(place: T, goto: (), unwind_action: ())
+);
+define!("mir_call",
+    /// Call a function.
+    ///
+    /// The first argument must be of the form `ret_val = fun(arg1, arg2, ...)`.
+    ///
+    /// The second argument must be of the form `ReturnTo(bb)`, where `bb` is the basic block that
+    /// will be jumped to after the function returns.
+    ///
+    /// The third argument describes what happens on unwind. It can be one of:
+    /// - [`UnwindContinue`]
+    /// - [`UnwindUnreachable`]
+    /// - [`UnwindTerminate`]
+    /// - [`UnwindCleanup`]
+    fn Call(call: (), goto: (), unwind_action: ())
+);
 define!("mir_unwind_resume",
     /// A terminator that resumes the unwinding.
     fn UnwindResume()

src/tools/miri/tests/fail/validity/cast_fn_ptr_invalid_caller_arg.rs

@@ -20,7 +20,7 @@ fn call(f: fn(NonZeroU32)) {
             let tmp = ptr::addr_of!(c);
             let ptr = tmp as *const NonZeroU32;
             // The call site now is a NonZeroU32-to-u32 transmute.
-            Call(_res = f(*ptr), retblock, UnwindContinue()) //~ERROR: expected something greater or equal to 1
+            Call(_res = f(*ptr), ReturnTo(retblock), UnwindContinue()) //~ERROR: expected something greater or equal to 1
         }
         retblock = {
             Return()

src/tools/miri/tests/fail/validity/cast_fn_ptr_invalid_caller_arg.stderr

@@ -1,7 +1,7 @@
 error: Undefined Behavior: constructing invalid value: encountered 0, but expected something greater or equal to 1
   --> $DIR/cast_fn_ptr_invalid_caller_arg.rs:LL:CC
    |
-LL |             Call(_res = f(*ptr), retblock, UnwindContinue())
+LL |             Call(_res = f(*ptr), ReturnTo(retblock), UnwindContinue())
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0, but expected something greater or equal to 1
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior

tests/mir-opt/building/custom/terminators.rs

@@ -13,7 +13,7 @@ fn ident<T>(t: T) -> T {
 fn direct_call(x: i32) -> i32 {
     mir!(
         {
-            Call(RET = ident(x), retblock, UnwindContinue())
+            Call(RET = ident(x), ReturnTo(retblock), UnwindContinue())
         }
 
         retblock = {
@@ -27,7 +27,7 @@ fn direct_call(x: i32) -> i32 {
 fn indirect_call(x: i32, f: fn(i32) -> i32) -> i32 {
     mir!(
         {
-            Call(RET = f(x), retblock, UnwindContinue())
+            Call(RET = f(x), ReturnTo(retblock), UnwindContinue())
         }
 
         retblock = {
@@ -49,7 +49,7 @@ impl<'a> Drop for WriteOnDrop<'a> {
 fn drop_first<'a>(a: WriteOnDrop<'a>, b: WriteOnDrop<'a>) {
     mir!(
         {
-            Drop(a, retblock, UnwindContinue())
+            Drop(a, ReturnTo(retblock), UnwindContinue())
         }
 
         retblock = {
@@ -64,7 +64,7 @@ fn drop_first<'a>(a: WriteOnDrop<'a>, b: WriteOnDrop<'a>) {
 fn drop_second<'a>(a: WriteOnDrop<'a>, b: WriteOnDrop<'a>) {
     mir!(
         {
-            Drop(b, retblock, UnwindContinue())
+            Drop(b, ReturnTo(retblock), UnwindContinue())
         }
 
         retblock = {

tests/mir-opt/building/custom/unwind_action.rs

@@ -11,7 +11,7 @@ use core::intrinsics::mir::*;
 pub fn a() {
     mir!(
         {
-            Call(RET = a(), bb1, UnwindUnreachable())
+            Call(RET = a(), ReturnTo(bb1), UnwindUnreachable())
         }
         bb1 = {
             Return()
@@ -26,7 +26,7 @@ pub fn a() {
 pub fn b() {
     mir!(
         {
-            Call(RET = b(), bb1, UnwindContinue())
+            Call(RET = b(), ReturnTo(bb1), UnwindContinue())
         }
         bb1 = {
             Return()
@@ -41,7 +41,7 @@ pub fn b() {
 pub fn c() {
     mir!(
         {
-            Call(RET = c(), bb1, UnwindTerminate(ReasonAbi))
+            Call(RET = c(), ReturnTo(bb1), UnwindTerminate(ReasonAbi))
         }
         bb1 = {
             Return()
@@ -56,7 +56,7 @@ pub fn c() {
 pub fn d() {
     mir!(
         {
-            Call(RET = d(), bb1, UnwindCleanup(bb2))
+            Call(RET = d(), ReturnTo(bb1), UnwindCleanup(bb2))
         }
         bb1 = {
             Return()

tests/mir-opt/copy-prop/borrowed_local.rs

@@ -22,11 +22,11 @@ fn f() -> bool {
             let b = a;
             // We cannot propagate the place `a`.
             let r2 = &b;
-            Call(RET = cmp_ref(r1, r2), next, UnwindContinue())
+            Call(RET = cmp_ref(r1, r2), ReturnTo(next), UnwindContinue())
         }
         next = {
             // But we can propagate the value `a`.
-            Call(RET = opaque(b), ret, UnwindContinue())
+            Call(RET = opaque(b), ReturnTo(ret), UnwindContinue())
         }
         ret = {
             Return()

tests/mir-opt/copy-prop/calls.rs

@@ -26,7 +26,7 @@ fn multiple_edges(t: bool) -> u8 {
             match t { true => bbt, _ => ret }
         }
         bbt = {
-            Call(x = dummy(13), ret, UnwindContinue())
+            Call(x = dummy(13), ReturnTo(ret), UnwindContinue())
         }
         ret = {
             // `x` is not assigned on the `bb0 -> ret` edge,

tests/mir-opt/copy-prop/custom_move_arg.rs

@@ -14,11 +14,11 @@ struct NotCopy(bool);
 fn f(_1: NotCopy) {
     mir!({
         let _2 = _1;
-        Call(RET = opaque(Move(_1)), bb1, UnwindContinue())
+        Call(RET = opaque(Move(_1)), ReturnTo(bb1), UnwindContinue())
     }
     bb1 = {
         let _3 = Move(_2);
-        Call(RET = opaque(_3), bb2, UnwindContinue())
+        Call(RET = opaque(_3), ReturnTo(bb2), UnwindContinue())
     }
     bb2 = {
         Return()