Add hooks for Miri panic unwinding

src/libcore/intrinsics.rs

@@ -1348,6 +1348,11 @@ extern "rust-intrinsic" {
     /// See documentation of `<*const T>::offset_from` for details.
     #[cfg(not(bootstrap))]
     pub fn ptr_offset_from<T>(ptr: *const T, base: *const T) -> isize;
+
+    /// Internal hook used by Miri to implement unwinding.
+    /// Perma-unstable: do not use
+    #[cfg(not(bootstrap))]
+    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> !;
 }
 
 // Some functions are defined here because they accidentally got made

src/libpanic_unwind/lib.rs

@@ -36,7 +36,10 @@ use core::raw;
 use core::panic::BoxMeUp;
 
 cfg_if::cfg_if! {
-    if #[cfg(target_os = "emscripten")] {
+    if #[cfg(miri)] {
+        #[path = "miri.rs"]
+        mod imp;
+    } else if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
         mod imp;
     } else if #[cfg(target_arch = "wasm32")] {

src/libpanic_unwind/miri.rs

@@ -0,0 +1,23 @@
+use core::any::Any;
+use alloc::boxed::Box;
+
+pub fn payload() -> *mut u8 {
+    core::ptr::null_mut()
+}
+
+pub unsafe fn panic(data: Box<dyn Any + Send>) -> ! {
+    core::intrinsics::miri_start_panic(Box::into_raw(data))
+}
+
+pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
+    Box::from_raw(ptr)
+}
+
+
+// This is required by the compiler to exist (e.g., it's a lang item),
+// but is never used by Miri. Therefore, we just use a stub here
+#[lang = "eh_personality"]
+#[cfg(not(test))]
+fn rust_eh_personality() {
+    unsafe { core::intrinsics::abort() }
+}

src/librustc_codegen_ssa/mir/block.rs

@@ -528,6 +528,21 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             _ => FnAbi::new(&bx, sig, &extra_args)
         };
 
+        // This should never be reachable at runtime:
+        // We should only emit a call to this intrinsic in #[cfg(miri)] mode,
+        // which means that we will never actually use the generate object files
+        // (we will just be interpreting the MIR)
+        //
+        // Note that we still need to be able to codegen *something* for this intrisnic:
+        // Miri currently uses Xargo to build a special libstd. As a side effect,
+        // we generate normal object files for libstd - while these are never used,
+        // we still need to be able to build them.
+        if intrinsic == Some("miri_start_panic") {
+            bx.abort();
+            bx.unreachable();
+            return;
+        }
+
         // Emit a panic or a no-op for `panic_if_uninhabited`.
         if intrinsic == Some("panic_if_uninhabited") {
             let ty = instance.unwrap().substs.type_at(0);

src/librustc_mir/const_eval.rs

@@ -325,6 +325,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         args: &[OpTy<'tcx>],
         dest: Option<PlaceTy<'tcx>>,
         ret: Option<mir::BasicBlock>,
+        _unwind: Option<mir::BasicBlock> // unwinding is not supported in consts
     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
         debug!("eval_fn_call: {:?}", instance);
         // Only check non-glue functions
@@ -336,7 +337,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 // Some functions we support even if they are non-const -- but avoid testing
                 // that for const fn!  We certainly do *not* want to actually call the fn
                 // though, so be sure we return here.
-                return if ecx.hook_fn(instance, args, dest)? {
+                return if ecx.hook_panic_fn(instance, args, dest)? {
                     ecx.goto_block(ret)?; // fully evaluated and done
                     Ok(None)
                 } else {
@@ -374,7 +375,9 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         span: Span,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
-        dest: PlaceTy<'tcx>,
+        dest: Option<PlaceTy<'tcx>>,
+        _ret: Option<mir::BasicBlock>,
+        _unwind: Option<mir::BasicBlock>
     ) -> InterpResult<'tcx> {
         if ecx.emulate_intrinsic(span, instance, args, dest)? {
             return Ok(());
@@ -469,12 +472,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         Ok(())
     }
-
-    /// Called immediately before a stack frame gets popped.
-    #[inline(always)]
-    fn stack_pop(_ecx: &mut InterpCx<'mir, 'tcx, Self>, _extra: ()) -> InterpResult<'tcx> {
-        Ok(())
-    }
 }
 
 /// Extracts a field of a (variant of a) const.

src/librustc_mir/interpret/eval_context.rs

@@ -21,7 +21,7 @@ use rustc_data_structures::fx::FxHashMap;
 
 use super::{
     Immediate, Operand, MemPlace, MPlaceTy, Place, PlaceTy, ScalarMaybeUndef,
-    Memory, Machine
+    Memory, Machine, StackPopInfo
 };
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
@@ -60,6 +60,9 @@ pub struct Frame<'mir, 'tcx, Tag=(), Extra=()> {
     /// The span of the call site.
     pub span: source_map::Span,
 
+    /// Extra data for the machine.
+    pub extra: Extra,
+
     ////////////////////////////////////////////////////////////////////////////////
     // Return place and locals
     ////////////////////////////////////////////////////////////////////////////////
@@ -82,21 +85,22 @@ pub struct Frame<'mir, 'tcx, Tag=(), Extra=()> {
     ////////////////////////////////////////////////////////////////////////////////
     /// The block that is currently executed (or will be executed after the above call stacks
     /// return).
-    pub block: mir::BasicBlock,
+    /// If this is `None`, we are unwinding and this function doesn't need any clean-up.
+    /// Just continue the same as with `Resume`.
+    pub block: Option<mir::BasicBlock>,
 
     /// The index of the currently evaluated statement.
     pub stmt: usize,
-
-    /// Extra data for the machine.
-    pub extra: Extra,
 }
 
 #[derive(Clone, Eq, PartialEq, Debug)] // Miri debug-prints these
 pub enum StackPopCleanup {
     /// Jump to the next block in the caller, or cause UB if None (that's a function
     /// that may never return). Also store layout of return place so
     /// we can validate it at that layout.
-    Goto(Option<mir::BasicBlock>),
+    /// `ret` stores the block we jump to on a normal return, while 'unwind'
+    /// stores the block used for cleanup during unwinding
+    Goto { ret: Option<mir::BasicBlock>, unwind: Option<mir::BasicBlock> },
     /// Just do nohing: Used by Main and for the box_alloc hook in miri.
     /// `cleanup` says whether locals are deallocated. Static computation
     /// wants them leaked to intern what they need (and just throw away
@@ -489,7 +493,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let extra = M::stack_push(self)?;
         self.stack.push(Frame {
             body,
-            block: mir::START_BLOCK,
+            block: Some(mir::START_BLOCK),
             return_to_block,
             return_place,
             // empty local array, we fill it in below, after we are inside the stack frame and
@@ -547,60 +551,118 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    pub(super) fn pop_stack_frame(&mut self) -> InterpResult<'tcx> {
-        info!("LEAVING({}) {}", self.cur_frame(), self.frame().instance);
+    /// Pops the current frame from the stack, deallocating the
+    /// memory for allocated locals.
+    ///
+    /// If `unwinding` is `false`, then we are performing a normal return
+    /// from a function. In this case, we jump back into the frame of the caller,
+    /// and continue execution as normal.
+    ///
+    /// If `unwinding` is `true`, then we are in the middle of a panic,
+    /// and need to unwind this frame. In this case, we jump to the
+    /// `cleanup` block for the function, which is responsible for running
+    /// `Drop` impls for any locals that have been initialized at this point.
+    /// The cleanup block ends with a special `Resume` terminator, which will
+    /// cause us to continue unwinding.
+    pub(super) fn pop_stack_frame(
+        &mut self,
+        unwinding: bool
+    ) -> InterpResult<'tcx> {
+        info!("LEAVING({}) {} (unwinding = {})",
+            self.cur_frame(), self.frame().instance, unwinding);
+
+        // Sanity check `unwinding`.
+        assert_eq!(
+            unwinding,
+            match self.frame().block {
+                None => true,
+                Some(block) => self.body().basic_blocks()[block].is_cleanup
+            }
+        );
+
         ::log_settings::settings().indentation -= 1;
         let frame = self.stack.pop().expect(
             "tried to pop a stack frame, but there were none",
         );
-        M::stack_pop(self, frame.extra)?;
-        // Abort early if we do not want to clean up: We also avoid validation in that case,
+        let stack_pop_info = M::stack_pop(self, frame.extra, unwinding)?;
+        if let (false, StackPopInfo::StopUnwinding) = (unwinding, stack_pop_info) {
+            bug!("Attempted to stop unwinding while there is no unwinding!");
+        }
+
+        // Now where do we jump next?
+
+        // Determine if we leave this function normally or via unwinding.
+        let cur_unwinding = if let StackPopInfo::StopUnwinding = stack_pop_info {
+            false
+        } else {
+            unwinding
+        };
+
+        // Usually we want to clean up (deallocate locals), but in a few rare cases we don't.
+        // In that case, we return early. We also avoid validation in that case,
         // because this is CTFE and the final value will be thoroughly validated anyway.
-        match frame.return_to_block {
-            StackPopCleanup::Goto(_) => {},
-            StackPopCleanup::None { cleanup } => {
-                if !cleanup {
-                    assert!(self.stack.is_empty(), "only the topmost frame should ever be leaked");
-                    // Leak the locals, skip validation.
-                    return Ok(());
-                }
-            }
+        let (cleanup, next_block) = match frame.return_to_block {
+            StackPopCleanup::Goto { ret, unwind } => {
+                (true, Some(if cur_unwinding { unwind } else { ret }))
+            },
+            StackPopCleanup::None { cleanup, .. } => (cleanup, None)
+        };
+
+        if !cleanup {
+            assert!(self.stack.is_empty(), "only the topmost frame should ever be leaked");
+            assert!(next_block.is_none(), "tried to skip cleanup when we have a next block!");
+            // Leak the locals, skip validation.
+            return Ok(());
         }
-        // Deallocate all locals that are backed by an allocation.
+
+        // Cleanup: deallocate all locals that are backed by an allocation.
         for local in frame.locals {
             self.deallocate_local(local.value)?;
         }
-        // Validate the return value. Do this after deallocating so that we catch dangling
-        // references.
-        if let Some(return_place) = frame.return_place {
-            if M::enforce_validity(self) {
-                // Data got changed, better make sure it matches the type!
-                // It is still possible that the return place held invalid data while
-                // the function is running, but that's okay because nobody could have
-                // accessed that same data from the "outside" to observe any broken
-                // invariant -- that is, unless a function somehow has a ptr to
-                // its return place... but the way MIR is currently generated, the
-                // return place is always a local and then this cannot happen.
-                self.validate_operand(
-                    self.place_to_op(return_place)?,
-                    vec![],
-                    None,
-                )?;
-            }
+
+
+        trace!("StackPopCleanup: {:?} StackPopInfo: {:?} cur_unwinding = {:?}",
+               frame.return_to_block, stack_pop_info, cur_unwinding);
+        if cur_unwinding {
+            // Follow the unwind edge.
+            let unwind = next_block.expect("Encounted StackPopCleanup::None when unwinding!");
+            let next_frame = self.frame_mut();
+            // If `unwind` is `None`, we'll leave that function immediately again.
+            next_frame.block = unwind;
+            next_frame.stmt = 0;
         } else {
-            // Uh, that shouldn't happen... the function did not intend to return
-            throw_ub!(Unreachable)
-        }
-        // Jump to new block -- *after* validation so that the spans make more sense.
-        match frame.return_to_block {
-            StackPopCleanup::Goto(block) => {
-                self.goto_block(block)?;
+            // Follow the normal return edge.
+            // Validate the return value. Do this after deallocating so that we catch dangling
+            // references.
+            if let Some(return_place) = frame.return_place {
+                if M::enforce_validity(self) {
+                    // Data got changed, better make sure it matches the type!
+                    // It is still possible that the return place held invalid data while
+                    // the function is running, but that's okay because nobody could have
+                    // accessed that same data from the "outside" to observe any broken
+                    // invariant -- that is, unless a function somehow has a ptr to
+                    // its return place... but the way MIR is currently generated, the
+                    // return place is always a local and then this cannot happen.
+                    self.validate_operand(
+                        self.place_to_op(return_place)?,
+                        vec![],
+                        None,
+                    )?;
+                }
+            } else {
+                // Uh, that shouldn't happen... the function did not intend to return
+                throw_ub!(Unreachable);
+            }
+
+            // Jump to new block -- *after* validation so that the spans make more sense.
+            if let Some(ret) = next_block {
+                self.goto_block(ret)?;
             }
-            StackPopCleanup::None { .. } => {}
         }
 
         if self.stack.len() > 0 {
-            info!("CONTINUING({}) {}", self.cur_frame(), self.frame().instance);
+            info!("CONTINUING({}) {} (unwinding = {})",
+                self.cur_frame(), self.frame().instance, cur_unwinding);
         }
 
         Ok(())
@@ -745,16 +807,20 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             } else {
                 last_span = Some(span);
             }
-            let block = &body.basic_blocks()[block];
-            let source_info = if stmt < block.statements.len() {
-                block.statements[stmt].source_info
-            } else {
-                block.terminator().source_info
-            };
-            let lint_root = match body.source_scope_local_data {
-                mir::ClearCrossCrate::Set(ref ivs) => Some(ivs[source_info.scope].lint_root),
-                mir::ClearCrossCrate::Clear => None,
-            };
+
+            let lint_root = block.and_then(|block| {
+                let block = &body.basic_blocks()[block];
+                let source_info = if stmt < block.statements.len() {
+                    block.statements[stmt].source_info
+                } else {
+                    block.terminator().source_info
+                };
+                match body.source_scope_local_data {
+                    mir::ClearCrossCrate::Set(ref ivs) => Some(ivs[source_info.scope].lint_root),
+                    mir::ClearCrossCrate::Clear => None,
+                }
+            });
+
             frames.push(FrameInfo { call_site: span, instance, lint_root });
         }
         trace!("generate stacktrace: {:#?}, {:?}", frames, explicit_span);

src/librustc_mir/interpret/intrinsics.rs

@@ -91,11 +91,17 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         span: Span,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, M::PointerTag>],
-        dest: PlaceTy<'tcx, M::PointerTag>,
+        dest: Option<PlaceTy<'tcx, M::PointerTag>>,
     ) -> InterpResult<'tcx, bool> {
         let substs = instance.substs;
 
+        // We currently do not handle any diverging intrinsics.
+        let dest = match dest {
+            Some(dest) => dest,
+            None => return Ok(false)
+        };
         let intrinsic_name = &*self.tcx.item_name(instance.def_id()).as_str();
+
         match intrinsic_name {
             "caller_location" => {
                 let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
@@ -347,9 +353,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         Ok(true)
     }
 
-    /// "Intercept" a function call because we have something special to do for it.
+    /// "Intercept" a function call to a panic-related function
+    /// because we have something special to do for it.
     /// Returns `true` if an intercept happened.
-    pub fn hook_fn(
+    pub fn hook_panic_fn(
         &mut self,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, M::PointerTag>],