path: root/compiler/rustc_middle/src/mir/terminator.rs

use crate::mir;
use crate::mir::interpret::Scalar;
use crate::ty::{self, Ty, TyCtxt};
use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
use smallvec::{smallvec, SmallVec};

use super::{
    AssertMessage, BasicBlock, InlineAsmOperand, Operand, Place, SourceInfo, Successors,
    SuccessorsMut,
};
pub use rustc_ast::Mutability;
use rustc_macros::HashStable;
use rustc_span::Span;
use std::borrow::Cow;
use std::fmt::{self, Debug, Formatter, Write};
use std::iter;
use std::slice;

pub use super::query::*;

#[derive(Debug, Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, PartialOrd)]
pub struct SwitchTargets {
    /// Possible values. The locations to branch to in each case
    /// are found in the corresponding indices from the `targets` vector.
    values: SmallVec<[u128; 1]>,

    /// Possible branch sites. The last element of this vector is used
    /// for the otherwise branch, so targets.len() == values.len() + 1
    /// should hold.
    //
    // This invariant is quite non-obvious and also could be improved.
    // One way to make this invariant is to have something like this instead:
    //
    // branches: Vec<(ConstInt, BasicBlock)>,
    // otherwise: Option<BasicBlock> // exhaustive if None
    //
    // However we’ve decided to keep this as-is until we figure a case
    // where some other approach seems to be strictly better than other.
    targets: SmallVec<[BasicBlock; 2]>,
}

impl SwitchTargets {
    /// Creates switch targets from an iterator of values and target blocks.
    ///
    /// The iterator may be empty, in which case the `SwitchInt` instruction is equivalent to
    /// `goto otherwise;`.
    pub fn new(targets: impl Iterator<Item = (u128, BasicBlock)>, otherwise: BasicBlock) -> Self {
        let (values, mut targets): (SmallVec<_>, SmallVec<_>) = targets.unzip();
        targets.push(otherwise);
        Self { values, targets }
    }

    /// Builds a switch targets definition that jumps to `then` if the tested value equals `value`,
    /// and to `else_` if not.
    pub fn static_if(value: u128, then: BasicBlock, else_: BasicBlock) -> Self {
        Self { values: smallvec![value], targets: smallvec![then, else_] }
    }

    /// Returns the fallback target that is jumped to when none of the values match the operand.
    pub fn otherwise(&self) -> BasicBlock {
        *self.targets.last().unwrap()
    }

    /// Returns an iterator over the switch targets.
    ///
    /// The iterator will yield tuples containing the value and corresponding target to jump to, not
    /// including the `otherwise` fallback target.
    ///
    /// Note that this may yield 0 elements. Only the `otherwise` branch is mandatory.
    pub fn iter(&self) -> SwitchTargetsIter<'_> {
        SwitchTargetsIter { inner: iter::zip(&self.values, &self.targets) }
    }

    /// Returns a slice with all possible jump targets (including the fallback target).
    pub fn all_targets(&self) -> &[BasicBlock] {
        &self.targets
    }

    pub fn all_targets_mut(&mut self) -> &mut [BasicBlock] {
        &mut self.targets
    }

    /// Finds the `BasicBlock` to which this `SwitchInt` will branch given the
    /// specific value.  This cannot fail, as it'll return the `otherwise`
    /// branch if there's not a specific match for the value.
    pub fn target_for_value(&self, value: u128) -> BasicBlock {
        self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
    }
}

pub struct SwitchTargetsIter<'a> {
    inner: iter::Zip<slice::Iter<'a, u128>, slice::Iter<'a, BasicBlock>>,
}

impl<'a> Iterator for SwitchTargetsIter<'a> {
    type Item = (u128, BasicBlock);

    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next().map(|(val, bb)| (*val, *bb))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

impl<'a> ExactSizeIterator for SwitchTargetsIter<'a> {}

/// A note on unwinding: Panics may occur during the execution of some terminators. Depending on the
/// `-C panic` flag, this may either cause the program to abort or the call stack to unwind. Such
/// terminators have a `cleanup: Option<BasicBlock>` field on them. If stack unwinding occurs, then
/// once the current function is reached, execution continues at the given basic block, if any. If
/// `cleanup` is `None` then no cleanup is performed, and the stack continues unwinding. This is
/// equivalent to the execution of a `Resume` terminator.
///
/// The basic block pointed to by a `cleanup` field must have its `cleanup` flag set. `cleanup`
/// basic blocks have a couple restrictions:
///  1. All `cleanup` fields in them must be `None`.
///  2. `Return` terminators are not allowed in them. `Abort` and `Unwind` terminators are.
///  3. All other basic blocks (in the current body) that are reachable from `cleanup` basic blocks
///     must also be `cleanup`. This is a part of the type system and checked statically, so it is
///     still an error to have such an edge in the CFG even if it's known that it won't be taken at
///     runtime.
#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)]
pub enum TerminatorKind<'tcx> {
    /// Block has one successor; we continue execution there.
    Goto { target: BasicBlock },

    /// Switches based on the computed value.
    ///
    /// First, evaluates the `discr` operand. The type of the operand must be a signed or unsigned
    /// integer, char, or bool, and must match the given type. Then, if the list of switch targets
    /// contains the computed value, continues execution at the associated basic block. Otherwise,
    /// continues execution at the "otherwise" basic block.
    ///
    /// Target values may not appear more than once.
    SwitchInt {
        /// The discriminant value being tested.
        discr: Operand<'tcx>,

        /// The type of value being tested.
        /// This is always the same as the type of `discr`.
        /// FIXME: remove this redundant information. Currently, it is relied on by pretty-printing.
        switch_ty: Ty<'tcx>,

        targets: SwitchTargets,
    },

    /// Indicates that the landing pad is finished and that the process should continue unwinding.
    ///
    /// Like a return, this marks the end of this invocation of the function.
    ///
    /// Only permitted in cleanup blocks. `Resume` is not permitted with `-C unwind=abort` after
    /// deaggregation runs.
    Resume,

    /// Indicates that the landing pad is finished and that the process should abort.
    ///
    /// Used to prevent unwinding for foreign items or with `-C unwind=abort`. Only permitted in
    /// cleanup blocks.
    Abort,

    /// Returns from the function.
    ///
    /// Like function calls, the exact semantics of returns in Rust are unclear. Returning very
    /// likely at least assigns the value currently in the return place (`_0`) to the place
    /// specified in the associated `Call` terminator in the calling function, as if assigned via
    /// `dest = move _0`. It might additionally do other things, like have side-effects in the
    /// aliasing model.
    ///
    /// If the body is a generator body, this has slightly different semantics; it instead causes a
    /// `GeneratorState::Returned(_0)` to be created (as if by an `Aggregate` rvalue) and assigned
    /// to the return place.
    Return,

    /// Indicates a terminator that can never be reached.
    ///
    /// Executing this terminator is UB.
    Unreachable,

    /// The behavior of this statement differs significantly before and after drop elaboration.
    /// After drop elaboration, `Drop` executes the drop glue for the specified place, after which
    /// it continues execution/unwinds at the given basic blocks. It is possible that executing drop
    /// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an
    /// issue tracking if drop glue has any interesting semantics in addition to those of a function
    /// call?)
    ///
    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the
    /// `Drop` will be executed if...
    ///
    /// **Needs clarification**: End of that sentence. This in effect should document the exact
    /// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure:
    ///
    /// > The drop glue is executed if, among all statements executed within this `Body`, an assignment to
    /// > the place or one of its "parents" occurred more recently than a move out of it. This does not
    /// > consider indirect assignments.
    Drop { place: Place<'tcx>, target: BasicBlock, unwind: Option<BasicBlock> },

    /// Drops the place and assigns a new value to it.
    ///
    /// This first performs the exact same operation as the pre drop-elaboration `Drop` terminator;
    /// it then additionally assigns the `value` to the `place` as if by an assignment statement.
    /// This assignment occurs both in the unwind and the regular code paths. The semantics are best
    /// explained by the elaboration:
    ///
    /// ```ignore (MIR)
    /// BB0 {
    ///   DropAndReplace(P <- V, goto BB1, unwind BB2)
    /// }
    /// ```
    ///
    /// becomes
    ///
    /// ```ignore (MIR)
    /// BB0 {
    ///   Drop(P, goto BB1, unwind BB2)
    /// }
    /// BB1 {
    ///   // P is now uninitialized
    ///   P <- V
    /// }
    /// BB2 {
    ///   // P is now uninitialized -- its dtor panicked
    ///   P <- V
    /// }
    /// ```
    ///
    /// Disallowed after drop elaboration.
    DropAndReplace {
        place: Place<'tcx>,
        value: Operand<'tcx>,
        target: BasicBlock,
        unwind: Option<BasicBlock>,
    },

    /// Roughly speaking, evaluates the `func` operand and the arguments, and starts execution of
    /// the referred to function. The operand types must match the argument types of the function.
    /// The return place type must match the return type. The type of the `func` operand must be
    /// callable, meaning either a function pointer, a function type, or a closure type.
    ///
    /// **Needs clarification**: The exact semantics of this. Current backends rely on `move`
    /// operands not aliasing the return place. It is unclear how this is justified in MIR, see
    /// [#71117].
    ///
    /// [#71117]: https://github.com/rust-lang/rust/issues/71117
    Call {
        /// The function that’s being called.
        func: Operand<'tcx>,
        /// Arguments the function is called with.
        /// These are owned by the callee, which is free to modify them.
        /// This allows the memory occupied by "by-value" arguments to be
        /// reused across function calls without duplicating the contents.
        args: Vec<Operand<'tcx>>,
        /// Where the returned value will be written
        destination: Place<'tcx>,
        /// Where to go after this call returns. If none, the call necessarily diverges.
        target: Option<BasicBlock>,
        /// Cleanups to be done if the call unwinds.
        cleanup: Option<BasicBlock>,
        /// `true` if this is from a call in HIR rather than from an overloaded
        /// operator. True for overloaded function call.
        from_hir_call: bool,
        /// This `Span` is the span of the function, without the dot and receiver
        /// (e.g. `foo(a, b)` in `x.foo(a, b)`
        fn_span: Span,
    },

    /// Evaluates the operand, which must have type `bool`. If it is not equal to `expected`,
    /// initiates a panic. Initiating a panic corresponds to a `Call` terminator with some
    /// unspecified constant as the function to call, all the operands stored in the `AssertMessage`
    /// as parameters, and `None` for the destination. Keep in mind that the `cleanup` path is not
    /// necessarily executed even in the case of a panic, for example in `-C panic=abort`. If the
    /// assertion does not fail, execution continues at the specified basic block.
    Assert {
        cond: Operand<'tcx>,
        expected: bool,
        msg: AssertMessage<'tcx>,
        target: BasicBlock,
        cleanup: Option<BasicBlock>,
    },

    /// Marks a suspend point.
    ///
    /// Like `Return` terminators in generator bodies, this computes `value` and then a
    /// `GeneratorState::Yielded(value)` as if by `Aggregate` rvalue. That value is then assigned to
    /// the return place of the function calling this one, and execution continues in the calling
    /// function. When next invoked with the same first argument, execution of this function
    /// continues at the `resume` basic block, with the second argument written to the `resume_arg`
    /// place. If the generator is dropped before then, the `drop` basic block is invoked.
    ///
    /// Not permitted in bodies that are not generator bodies, or after generator lowering.
    ///
    /// **Needs clarification**: What about the evaluation order of the `resume_arg` and `value`?
    Yield {
        /// The value to return.
        value: Operand<'tcx>,
        /// Where to resume to.
        resume: BasicBlock,
        /// The place to store the resume argument in.
        resume_arg: Place<'tcx>,
        /// Cleanup to be done if the generator is dropped at this suspend point.
        drop: Option<BasicBlock>,
    },

    /// Indicates the end of dropping a generator.
    ///
    /// Semantically just a `return` (from the generators drop glue). Only permitted in the same situations
    /// as `yield`.
    ///
    /// **Needs clarification**: Is that even correct? The generator drop code is always confusing
    /// to me, because it's not even really in the current body.
    ///
    /// **Needs clarification**: Are there type system constraints on these terminators? Should
    /// there be a "block type" like `cleanup` blocks for them?
    GeneratorDrop,

    /// A block where control flow only ever takes one real path, but borrowck needs to be more
    /// conservative.
    ///
    /// At runtime this is semantically just a goto.
    ///
    /// Disallowed after drop elaboration.
    FalseEdge {
        /// The target normal control flow will take.
        real_target: BasicBlock,
        /// A block control flow could conceptually jump to, but won't in
        /// practice.
        imaginary_target: BasicBlock,
    },

    /// A terminator for blocks that only take one path in reality, but where we reserve the right
    /// to unwind in borrowck, even if it won't happen in practice. This can arise in infinite loops
    /// with no function calls for example.
    ///
    /// At runtime this is semantically just a goto.
    ///
    /// Disallowed after drop elaboration.
    FalseUnwind {
        /// The target normal control flow will take.
        real_target: BasicBlock,
        /// The imaginary cleanup block link. This particular path will never be taken
        /// in practice, but in order to avoid fragility we want to always
        /// consider it in borrowck. We don't want to accept programs which
        /// pass borrowck only when `panic=abort` or some assertions are disabled
        /// due to release vs. debug mode builds. This needs to be an `Option` because
        /// of the `remove_noop_landing_pads` and `abort_unwinding_calls` passes.
        unwind: Option<BasicBlock>,
    },

    /// Block ends with an inline assembly block. This is a terminator since
    /// inline assembly is allowed to diverge.
    InlineAsm {
        /// The template for the inline assembly, with placeholders.
        template: &'tcx [InlineAsmTemplatePiece],

        /// The operands for the inline assembly, as `Operand`s or `Place`s.
        operands: Vec<InlineAsmOperand<'tcx>>,

        /// Miscellaneous options for the inline assembly.
        options: InlineAsmOptions,

        /// Source spans for each line of the inline assembly code. These are
        /// used to map assembler errors back to the line in the source code.
        line_spans: &'tcx [Span],

        /// Destination block after the inline assembly returns, unless it is
        /// diverging (InlineAsmOptions::NORETURN).
        destination: Option<BasicBlock>,

        /// Cleanup to be done if the inline assembly unwinds. This is present
        /// if and only if InlineAsmOptions::MAY_UNWIND is set.
        cleanup: Option<BasicBlock>,
    },
}
#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
pub struct Terminator<'tcx> {
    pub source_info: SourceInfo,
    pub kind: TerminatorKind<'tcx>,
}

impl<'tcx> Terminator<'tcx> {
    pub fn successors(&self) -> Successors<'_> {
        self.kind.successors()
    }

    pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
        self.kind.successors_mut()
    }

    pub fn unwind(&self) -> Option<&Option<BasicBlock>> {
        self.kind.unwind()
    }

    pub fn unwind_mut(&mut self) -> Option<&mut Option<BasicBlock>> {
        self.kind.unwind_mut()
    }
}

impl<'tcx> TerminatorKind<'tcx> {
    pub fn if_(
        tcx: TyCtxt<'tcx>,
        cond: Operand<'tcx>,
        t: BasicBlock,
        f: BasicBlock,
    ) -> TerminatorKind<'tcx> {
        TerminatorKind::SwitchInt {
            discr: cond,
            switch_ty: tcx.types.bool,
            targets: SwitchTargets::static_if(0, f, t),
        }
    }

    pub fn successors(&self) -> Successors<'_> {
        use self::TerminatorKind::*;
        match *self {
            Resume
            | Abort
            | GeneratorDrop
            | Return
            | Unreachable
            | Call { target: None, cleanup: None, .. }
            | InlineAsm { destination: None, cleanup: None, .. } => {
                None.into_iter().chain((&[]).into_iter().copied())
            }
            Goto { target: t }
            | Call { target: None, cleanup: Some(t), .. }
            | Call { target: Some(t), cleanup: None, .. }
            | Yield { resume: t, drop: None, .. }
            | DropAndReplace { target: t, unwind: None, .. }
            | Drop { target: t, unwind: None, .. }
            | Assert { target: t, cleanup: None, .. }
            | FalseUnwind { real_target: t, unwind: None }
            | InlineAsm { destination: Some(t), cleanup: None, .. }
            | InlineAsm { destination: None, cleanup: Some(t), .. } => {
                Some(t).into_iter().chain((&[]).into_iter().copied())
            }
            Call { target: Some(t), cleanup: Some(ref u), .. }
            | Yield { resume: t, drop: Some(ref u), .. }
            | DropAndReplace { target: t, unwind: Some(ref u), .. }
            | Drop { target: t, unwind: Some(ref u), .. }
            | Assert { target: t, cleanup: Some(ref u), .. }
            | FalseUnwind { real_target: t, unwind: Some(ref u) }
            | InlineAsm { destination: Some(t), cleanup: Some(ref u), .. } => {
                Some(t).into_iter().chain(slice::from_ref(u).into_iter().copied())
            }
            SwitchInt { ref targets, .. } => {
                None.into_iter().chain(targets.targets.iter().copied())
            }
            FalseEdge { real_target, ref imaginary_target } => Some(real_target)
                .into_iter()
                .chain(slice::from_ref(imaginary_target).into_iter().copied()),
        }
    }

    pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
        use self::TerminatorKind::*;
        match *self {
            Resume
            | Abort
            | GeneratorDrop
            | Return
            | Unreachable
            | Call { target: None, cleanup: None, .. }
            | InlineAsm { destination: None, cleanup: None, .. } => None.into_iter().chain(&mut []),
            Goto { target: ref mut t }
            | Call { target: None, cleanup: Some(ref mut t), .. }
            | Call { target: Some(ref mut t), cleanup: None, .. }
            | Yield { resume: ref mut t, drop: None, .. }
            | DropAndReplace { target: ref mut t, unwind: None, .. }
            | Drop { target: ref mut t, unwind: None, .. }
            | Assert { target: ref mut t, cleanup: None, .. }
            | FalseUnwind { real_target: ref mut t, unwind: None }
            | InlineAsm { destination: Some(ref mut t), cleanup: None, .. }
            | InlineAsm { destination: None, cleanup: Some(ref mut t), .. } => {
                Some(t).into_iter().chain(&mut [])
            }
            Call { target: Some(ref mut t), cleanup: Some(ref mut u), .. }
            | Yield { resume: ref mut t, drop: Some(ref mut u), .. }
            | DropAndReplace { target: ref mut t, unwind: Some(ref mut u), .. }
            | Drop { target: ref mut t, unwind: Some(ref mut u), .. }
            | Assert { target: ref mut t, cleanup: Some(ref mut u), .. }
            | FalseUnwind { real_target: ref mut t, unwind: Some(ref mut u) }
            | InlineAsm { destination: Some(ref mut t), cleanup: Some(ref mut u), .. } => {
                Some(t).into_iter().chain(slice::from_mut(u))
            }
            SwitchInt { ref mut targets, .. } => None.into_iter().chain(&mut targets.targets),
            FalseEdge { ref mut real_target, ref mut imaginary_target } => {
                Some(real_target).into_iter().chain(slice::from_mut(imaginary_target))
            }
        }
    }

    pub fn unwind(&self) -> Option<&Option<BasicBlock>> {
        match *self {
            TerminatorKind::Goto { .. }
            | TerminatorKind::Resume
            | TerminatorKind::Abort
            | TerminatorKind::Return
            | TerminatorKind::Unreachable
            | TerminatorKind::GeneratorDrop
            | TerminatorKind::Yield { .. }
            | TerminatorKind::SwitchInt { .. }
            | TerminatorKind::FalseEdge { .. } => None,
            TerminatorKind::Call { cleanup: ref unwind, .. }
            | TerminatorKind::Assert { cleanup: ref unwind, .. }
            | TerminatorKind::DropAndReplace { ref unwind, .. }
            | TerminatorKind::Drop { ref unwind, .. }
            | TerminatorKind::FalseUnwind { ref unwind, .. }
            | TerminatorKind::InlineAsm { cleanup: ref unwind, .. } => Some(unwind),
        }
    }

    pub fn unwind_mut(&mut self) -> Option<&mut Option<BasicBlock>> {
        match *self {
            TerminatorKind::Goto { .. }
            | TerminatorKind::Resume
            | TerminatorKind::Abort
            | TerminatorKind::Return
            | TerminatorKind::Unreachable
            | TerminatorKind::GeneratorDrop
            | TerminatorKind::Yield { .. }
            | TerminatorKind::SwitchInt { .. }
            | TerminatorKind::FalseEdge { .. } => None,
            TerminatorKind::Call { cleanup: ref mut unwind, .. }
            | TerminatorKind::Assert { cleanup: ref mut unwind, .. }
            | TerminatorKind::DropAndReplace { ref mut unwind, .. }
            | TerminatorKind::Drop { ref mut unwind, .. }
            | TerminatorKind::FalseUnwind { ref mut unwind, .. }
            | TerminatorKind::InlineAsm { cleanup: ref mut unwind, .. } => Some(unwind),
        }
    }

    pub fn as_switch(&self) -> Option<(&Operand<'tcx>, Ty<'tcx>, &SwitchTargets)> {
        match self {
            TerminatorKind::SwitchInt { discr, switch_ty, targets } => {
                Some((discr, *switch_ty, targets))
            }
            _ => None,
        }
    }

    pub fn as_goto(&self) -> Option<BasicBlock> {
        match self {
            TerminatorKind::Goto { target } => Some(*target),
            _ => None,
        }
    }
}

impl<'tcx> Debug for TerminatorKind<'tcx> {
    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
        self.fmt_head(fmt)?;
        let successor_count = self.successors().count();
        let labels = self.fmt_successor_labels();
        assert_eq!(successor_count, labels.len());

        match successor_count {
            0 => Ok(()),

            1 => write!(fmt, " -> {:?}", self.successors().next().unwrap()),

            _ => {
                write!(fmt, " -> [")?;
                for (i, target) in self.successors().enumerate() {
                    if i > 0 {
                        write!(fmt, ", ")?;
                    }
                    write!(fmt, "{}: {:?}", labels[i], target)?;
                }
                write!(fmt, "]")
            }
        }
    }
}

impl<'tcx> TerminatorKind<'tcx> {
    /// Writes the "head" part of the terminator; that is, its name and the data it uses to pick the
    /// successor basic block, if any. The only information not included is the list of possible
    /// successors, which may be rendered differently between the text and the graphviz format.
    pub fn fmt_head<W: Write>(&self, fmt: &mut W) -> fmt::Result {
        use self::TerminatorKind::*;
        match self {
            Goto { .. } => write!(fmt, "goto"),
            SwitchInt { discr, .. } => write!(fmt, "switchInt({:?})", discr),
            Return => write!(fmt, "return"),
            GeneratorDrop => write!(fmt, "generator_drop"),
            Resume => write!(fmt, "resume"),
            Abort => write!(fmt, "abort"),
            Yield { value, resume_arg, .. } => write!(fmt, "{:?} = yield({:?})", resume_arg, value),
            Unreachable => write!(fmt, "unreachable"),
            Drop { place, .. } => write!(fmt, "drop({:?})", place),
            DropAndReplace { place, value, .. } => {
                write!(fmt, "replace({:?} <- {:?})", place, value)
            }
            Call { func, args, destination, .. } => {
                write!(fmt, "{:?} = ", destination)?;
                write!(fmt, "{:?}(", func)?;
                for (index, arg) in args.iter().enumerate() {
                    if index > 0 {
                        write!(fmt, ", ")?;
                    }
                    write!(fmt, "{:?}", arg)?;
                }
                write!(fmt, ")")
            }
            Assert { cond, expected, msg, .. } => {
                write!(fmt, "assert(")?;
                if !expected {
                    write!(fmt, "!")?;
                }
                write!(fmt, "{:?}, ", cond)?;
                msg.fmt_assert_args(fmt)?;
                write!(fmt, ")")
            }
            FalseEdge { .. } => write!(fmt, "falseEdge"),
            FalseUnwind { .. } => write!(fmt, "falseUnwind"),
            InlineAsm { template, ref operands, options, .. } => {
                write!(fmt, "asm!(\"{}\"", InlineAsmTemplatePiece::to_string(template))?;
                for op in operands {
                    write!(fmt, ", ")?;
                    let print_late = |&late| if late { "late" } else { "" };
                    match op {
                        InlineAsmOperand::In { reg, value } => {
                            write!(fmt, "in({}) {:?}", reg, value)?;
                        }
                        InlineAsmOperand::Out { reg, late, place: Some(place) } => {
                            write!(fmt, "{}out({}) {:?}", print_late(late), reg, place)?;
                        }
                        InlineAsmOperand::Out { reg, late, place: None } => {
                            write!(fmt, "{}out({}) _", print_late(late), reg)?;
                        }
                        InlineAsmOperand::InOut {
                            reg,
                            late,
                            in_value,
                            out_place: Some(out_place),
                        } => {
                            write!(
                                fmt,
                                "in{}out({}) {:?} => {:?}",
                                print_late(late),
                                reg,
                                in_value,
                                out_place
                            )?;
                        }
                        InlineAsmOperand::InOut { reg, late, in_value, out_place: None } => {
                            write!(fmt, "in{}out({}) {:?} => _", print_late(late), reg, in_value)?;
                        }
                        InlineAsmOperand::Const { value } => {
                            write!(fmt, "const {:?}", value)?;
                        }
                        InlineAsmOperand::SymFn { value } => {
                            write!(fmt, "sym_fn {:?}", value)?;
                        }
                        InlineAsmOperand::SymStatic { def_id } => {
                            write!(fmt, "sym_static {:?}", def_id)?;
                        }
                    }
                }
                write!(fmt, ", options({:?}))", options)
            }
        }
    }

    /// Returns the list of labels for the edges to the successor basic blocks.
    pub fn fmt_successor_labels(&self) -> Vec<Cow<'static, str>> {
        use self::TerminatorKind::*;
        match *self {
            Return | Resume | Abort | Unreachable | GeneratorDrop => vec![],
            Goto { .. } => vec!["".into()],
            SwitchInt { ref targets, switch_ty, .. } => ty::tls::with(|tcx| {
                let param_env = ty::ParamEnv::empty();
                let switch_ty = tcx.lift(switch_ty).unwrap();
                let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
                targets
                    .values
                    .iter()
                    .map(|&u| {
                        mir::ConstantKind::from_scalar(tcx, Scalar::from_uint(u, size), switch_ty)
                            .to_string()
                            .into()
                    })
                    .chain(iter::once("otherwise".into()))
                    .collect()
            }),
            Call { target: Some(_), cleanup: Some(_), .. } => {
                vec!["return".into(), "unwind".into()]
            }
            Call { target: Some(_), cleanup: None, .. } => vec!["return".into()],
            Call { target: None, cleanup: Some(_), .. } => vec!["unwind".into()],
            Call { target: None, cleanup: None, .. } => vec![],
            Yield { drop: Some(_), .. } => vec!["resume".into(), "drop".into()],
            Yield { drop: None, .. } => vec!["resume".into()],
            DropAndReplace { unwind: None, .. } | Drop { unwind: None, .. } => {
                vec!["return".into()]
            }
            DropAndReplace { unwind: Some(_), .. } | Drop { unwind: Some(_), .. } => {
                vec!["return".into(), "unwind".into()]
            }
            Assert { cleanup: None, .. } => vec!["".into()],
            Assert { .. } => vec!["success".into(), "unwind".into()],
            FalseEdge { .. } => vec!["real".into(), "imaginary".into()],
            FalseUnwind { unwind: Some(_), .. } => vec!["real".into(), "cleanup".into()],
            FalseUnwind { unwind: None, .. } => vec!["real".into()],
            InlineAsm { destination: Some(_), cleanup: Some(_), .. } => {
                vec!["return".into(), "unwind".into()]
            }
            InlineAsm { destination: Some(_), cleanup: None, .. } => vec!["return".into()],
            InlineAsm { destination: None, cleanup: Some(_), .. } => vec!["unwind".into()],
            InlineAsm { destination: None, cleanup: None, .. } => vec![],
        }
    }
}