Permalink
Find file Copy path
f886376 Mar 23, 2017
@patiences @nikomatsakis @mirandadam @DerSaidin
825 lines (671 sloc) 35 KB

Summary

Introduce a "mid-level IR" (MIR) into the compiler. The MIR desugars most of Rust's surface representation, leaving a simpler form that is well-suited to type-checking and translation.

Motivation

The current compiler uses a single AST from the initial parse all the way to the final generation of LLVM. While this has some advantages, there are also a number of distinct downsides.

  1. The complexity of the compiler is increased because all passes must be written against the full Rust language, rather than being able to consider a reduced subset. The MIR proposed here is radically simpler than the surface Rust syntax -- for example, it contains no "match" statements, and converts both ref bindings and & expresions into a single form.

    a. There are numerous examples of "desugaring" in Rust. In principle, desugaring one language feature into another should make the compiler simpler, but in our current implementation, it tends to make things more complex, because every phase must simulate the desugaring anew. The most prominent example are closure expressions (|| ...), which desugar to a fresh struct instance, but other examples abound: for loops, if let and while let, box expressions, overloaded operators (which desugar to method calls), method calls (which desugar to UFCS notation).

    b. There are a number of features which are almost infeasible to implement today but which should be much easier given a MIR representation. Examples include box patterns and non-lexical lifetimes.

  2. Reasoning about fine-grained control-flow in an AST is rather difficult. The right tool for this job is a control-flow graph (CFG). We currently construct a CFG that lives "on top" of the AST, which allows the borrow checking code to be flow sensitive, but it is awkward to work with. Worse, because this CFG is not used by trans, it is not necessarily the case that the control-flow as seen by the analyses corresponds to the code that will be generated. The MIR is based on a CFG, resolving this situation.

  3. The reliability of safety analyses is reduced because the gap between what is being analyzed (the AST) and what is being executed (LLVM bitcode) is very wide. The MIR is very low-level and hence the translation to LLVM should be straightforward.

  4. The reliability of safety proofs, when we have some, would be reduced because the formal language we are modeling is so far from the full compiler AST. The MIR is simple enough that it should be possible to (eventually) make safety proofs based on the MIR itself.

  5. Rust-specific optimizations, and optimizing trans output, are very challenging. There are numerous cases where it would be nice to be able to do optimizations before translating to LLVM bitcode, or to take advantage of Rust-specific knowledge of which LLVM is unaware. Currently, we are forced to do these optimizations as part of lowering to bitcode, which can get quite complex. Having an intermediate form improves the situation because:

    a. In some cases, we can do the optimizations in the MIR itself before translation.

    b. In other cases, we can do analyses on the MIR to easily determine when the optimization would be safe.

    c. In all cases, whatever we can do on the MIR will be helpful for other targets beyond LLVM (see next bullet).

  6. Migrating away from LLVM is nearly impossible. In the future, it may be advantageous to provide a choice of backends beyond LLVM. Currently though this is infeasible, since so much of the semantics of Rust itself are embedded in the trans step which converts to LLVM IR. Under the MIR design, those semantics are instead described in the translation from AST to MIR, and the LLVM step itself simply applies optimizations.

Given the numerous benefits of a MIR, you may wonder why we have not taken steps in this direction earlier. In fact, we have a number of structures in the compiler that simulate the effect of a MIR:

  1. Adjustments. Every expression can have various adjustments, like autoderefs and so forth. These are computed by the type-checker and then read by later analyses. This is a form of MIR, but not a particularly convenient one.
  2. The CFG. The CFG tries to model the flow of execution as a graph rather than a tree, to help analyses in dealing with complex control-flow formed by things like loops, break, continue, etc. This CFG is however inferior to the MIR in that it is only an approximation of control-flow and does not include all the information one would need to actually execute the program (for example, for an if expression, the CFG would indicate that two branches are possible, but would not contain enough information to decide which branch to take).
  3. ExprUseVisitor. The ExprUseVisitor is designed to work in conjunction with the CFG. It walks the AST and highlights actions of interest to later analyses, such as borrows or moves. For each such action, the analysis gets a callback indicating the point in the CFG where the action occurred along with what happened. Overloaded operators, method calls, and so forth are "desugared" into their more primitive operations. This is effectively a kind of MIR, but it is not complete enough to do translation, since it focuses purely on borrows, moves, and other things of interest to the safety checker.

Each of these things were added in order to try and cope with the complexity of working directly on the AST. The CFG for example consolidates knowledge about control-flow into one piece of code, producing a data structure that can be easily interpreted. Similarly, the ExprUseVisitor consolidates knowledge of how to walk and interpret the current compiler representation.

Goals

It is useful to think about what "knowledge" the MIR should encapsulate. Here is a listing of the kinds of things that should be explicit in the MIR and thus that downstream code won't have to re-encode in the form of repeated logic:

  • Precise ordering of control-flow. The CFG makes this very explicit, and the individual statements and nodes in the MIR are very small and detailed and hence nothing "interesting" happens in the middle of an individual node with respect to control-flow.
  • What needs to be dropped and when. The set of data that needs to be dropped and when is a fairly complex thing to calculate: you have to know what's in scope, including temporary values and so forth. In the MIR, all drops are explicit, including those that result from panics and unwinding.
  • How matches are desugared. Reasoning about matches has been a traditional source of complexity. Matches combine traversing types with borrows, moves, and all sorts of other things, depending on the precise patterns in use. This is all vastly simplified and explicit in MIR.

One thing the current MIR does not make explicit as explicit as it could is when something is moved. For by-value uses of a value, the code must still consult the type of the value to decide if that is a move or not. This could be made more explicit in the IR.

Which analyses are well-suited to the MIR?

Some analyses are better suited to the AST than to a MIR. The following is a list of work the compiler does that would benefit from using a MIR:

  • liveness checking: this is used to issue warnings about unused assignments and the like. The MIR is perfect for this sort of data-flow analysis.
  • borrow and move checking: the borrow checker already uses a combination of the CFG and ExprUseVisitor to try and achieve a similarly low-level of detail.
  • translation to LLVM IR: the MIR is much closer than the AST to the desired end-product.

Some other passes would probably work equally well on the MIR or an AST, but they will likely find the MIR somewhat easier to work with than the current AST simply because it is, well, simpler:

  • rvalue checking, which checks that things are Sized which need to be.
  • reachability and death checking.

These items are likely ill-suited to the MIR as designed:

  • privacy checking, since it relies on explicit knowledge of paths that is not necessarily present in the MIR.
  • lint checking, since it is often dependent on the sort of surface details we are seeking to obscure.

For some passes, the impact is not entirely clear. In particular, match exhaustiveness checking could easily be subsumed by the MIR construction process, which must do a similar analysis during the lowering process. However, once the MIR is built, the match is completely desugared into more primitive switches and so forth, so we will need to leave some markers in order to know where to check for exhaustiveness and to reconstruct counter examples.

Detailed design

What is really being proposed here?

The rest of this section goes into detail on a particular MIR design. However, the true purpose of this RFC is not to nail down every detail of the MIR -- which are expected to evolve and change over time anyway -- but rather to establish some high-level principles which drive the rest of the design:

  1. We should indeed lower the representation from an AST to something else that will drive later analyses, and this representation should be based on a CFG, not a tree.
  2. This representation should be explicitly minimal and not attempt to retain the original syntactic structure, though it should be possible to recover enough of it to make quality error messages.
  3. This representation should encode drops, panics, and other scope-dependent items explicitly.
  4. This representation does not have to be well-typed Rust, though it should be possible to type-check it using a tweaked variant on the Rust type system.

Prototype

The MIR design being described can be found here. In particular, this module defines the MIR representation, and this build module contains the code to create a MIR representation from an AST-like form.

For increased flexibility, as well as to make the code simpler, the prototype is not coded directly against the compiler's AST, but rather against an idealized representation defined by the HAIR trait. Note that this HAIR trait is entirely independent from the HIR discussed by nrc in RFC 1191 -- you can think of it as an abstract trait that any high-level Rust IR could implement, including our current AST. Moreover, it's just an implementation detail and not part of the MIR being proposed here per se. Still, if you want to read the code, you have to understand its design.

The HAIR trait contains a number of opaque associated types for the various aspects of the compiler. For example, the type H::Expr represents an expression. In order to find out what kind of expression it is, the mirror method is called, which converts an H::Expr into an Expr<H> mirror. This mirror then contains embedded ExprRef<H> nodes to refer to further subexpressions; these may either be mirrors themselves, or else they may be additional H::Expr nodes. This allows the tree that is exported to differ in small ways from the actual tree within the compiler; the primary intention is to use this to model "adjustments" like autoderef. The code to convert from our current AST to the HAIR is not yet complete, but it can be found here.

Note that the HAIR mirroring system is an experiment and not really part of the MIR itself. It does however present an interesting option for (eventually) stabilizing access to the compiler's internals.

Overview of the MIR

The proposed MIR always describes the execution of a single fn. At the highest level it consists of a series of declarations regarding the stack storage that will be required and then a set of basic blocks:

MIR = fn({TYPE}) -> TYPE {
    {let [mut] B: TYPE;}  // user-declared bindings and their types
    {let TEMP: TYPE;}     // compiler-introduced temporary
    {BASIC_BLOCK}         // control-flow graph
};

The storage declarations are broken into two categories. User-declared bindings have a 1-to-1 relationship with the variables specified in the program. Temporaries are introduced by the compiler in various cases. For example, borrowing an lvalue (e.g., &foo()) will introduce a temporary to store the result of foo(). Similarly, discarding a value foo(); is translated to something like let tmp = foo(); drop(tmp);). Temporaries are single-assignment, but because they can be borrowed they may be mutated after this assignment and hence they differ somewhat from variables in a pure SSA representation.

The proposed MIR takes the form of a graph where each node is a basic block. A basic block is a standard compiler term for a continuous sequence of instructions with a single entry point. All interesting control-flow happens between basic blocks. Each basic block has an id BB and consists of a sequence of statements and a terminator:

BASIC_BLOCK = BB: {STATEMENT} TERMINATOR

A STATEMENT can have one of three forms:

STATEMENT = LVALUE "=" RVALUE        // assign rvalue into lvalue
          | Drop(DROP_KIND, LVALUE)  // drop value if needed
DROP_KIND = SHALLOW                  // (see discussion below)
          | DEEP

The following sections dives into these various kinds of statements in more detail.

The TERMINATOR for a basic block describes how it connects to subsequent blocks:

TERMINATOR = GOTO(BB)              // normal control-flow
           | PANIC(BB)             // initiate unwinding, branching to BB for cleanup
           | IF(LVALUE, BB0, BB1)  // test LVALUE and branch to BB0 if true, else BB1
           | SWITCH(LVALUE, BB...) // load discriminant from LVALUE (which must be an enum),
                                   // and branch to BB... depending on which variant it is
           | CALL(LVALUE0 = LVALUE1(LVALUE2...), BB0, BB1)
                                   // call LVALUE1 with LVALUE2... as arguments. Write
                                   // result into LVALUE0. Branch to BB0 if it returns
                                   // normally, BB1 if it is unwinding.
           | DIVERGE               // return to caller, unwinding
           | RETURN                // return to caller normally

Most of the terminators should be fairly obvious. The most interesting part is the handling of unwinding. This aligns fairly close with how LLVM works: there is one terminator, PANIC, that initiates unwinding. It immediately branches to a handler (BB) which will perform cleanup and (eventually) reach a block that has a DIVERGE terminator. DIVERGE causes unwinding to continue up the stack.

Because calls to other functions can always (or almost always) panic, calls are themselves a kind of terminator. If we can determine that some function we are calling cannot unwind, we can always modify the IR to make the second basic block optional. (We could also add an RVALUE to represent calls, but it's probably easiest to keep the call as a terminator unless the memory savings of consolidating basic blocks are found to be worthwhile.)

It's worth pointing out that basic blocks are just a kind of compile-time and memory-use optimization; there is no semantic difference between a single block and two blocks joined by a GOTO terminator.

Assignments, values, and rvalues

The primary kind of statement is an assignent:

LVALUE "=" RVALUE

The semantics of this operation are to first evaluate the RVALUE and then store it into the LVALUE (which must represent a memory location of suitable type).

An LVALUE represents a path to a memory location. This is the basic "unit" analyzed by the borrow checker. It is always possible to evaluate an LVALUE without triggering any side-effects (modulo derefences of unsafe pointers, which naturally can trigger arbitrary behavior if the pointer is not valid).

LVALUE = B                   // reference to a user-declared binding
       | TEMP                // a temporary introduced by the compiler
       | ARG                 // a formal argument of the fn
       | STATIC              // a reference to a static or static mut
       | RETURN              // the return pointer of the fn
       | LVALUE.f            // project a field or tuple field, like x.f or x.0
       | *LVALUE             // dereference a pointer
       | LVALUE[LVALUE]      // index into an array (see disc. below about bounds checks)
       | (LVALUE as VARIANT) // downcast to a specific variant of an enum,
                             // see the section on desugaring matches below

An RVALUE represents a computation that yields a result. This result must be stored in memory somewhere to be accessible. The MIR does not contain any kind of nested expressions: everything is flattened out, going through lvalues as intermediaries.

RVALUE = Use(LVALUE)                // just read an lvalue
       | [LVALUE; LVALUE]
       | &'REGION LVALUE
       | &'REGION mut LVALUE
       | LVALUE as TYPE
       | LVALUE <BINOP> LVALUE
       | <UNOP> LVALUE
       | Struct { f: LVALUE0, ... } // aggregates, see section below
       | (LVALUE...LVALUE)
       | [LVALUE...LVALUE]
       | CONSTANT
       | LEN(LVALUE)                // load length from a slice, see section below
       | BOX                        // malloc for builtin box, see section below
BINOP = + | - | * | / | ...         // excluding && and ||
UNOP = ! | -                        // note: no `*`, as that is part of LVALUE

One thing worth pointing out is that the binary and unary operators are only the builtin form, operating on scalar values. Overloaded operators will be desugared to trait calls. Moreover, all method calls are desugared into normal calls via UFCS form.

Constants

Constants are a subset of rvalues that can be evaluated at compilation time:

CONSTANT = INT
         | UINT
         | FLOAT
         | BOOL
         | BYTES
         | STATIC_STRING
         | ITEM<SUBSTS>                 // reference to an item or constant etc
         | <P0 as TRAIT<P1...Pn>>       // projection
         | CONSTANT(CONSTANT...)        // 
         | CAST(CONSTANT, TY)           // foo as bar
         | Struct { (f: CONSTANT)... }  // aggregates...
         | (CONSTANT...)                //
         | [CONSTANT...]                //

Aggregates and further lowering

The set of rvalues includes "aggregate" expressions like (x, y) or Foo { f: x, g: y }. This is a place where the MIR (somewhat) departs from what will be generated compilation time, since (often) an expression like f = (x, y, z) will wind up desugared into a series of piecewise assignments like:

f.0 = x;
f.1 = y;
f.2 = z;

However, there are good reasons to include aggregates as first-class rvalues. For one thing, if we break down each aggregate into the specific assignments that would be used to construct the value, then zero-sized types are never assigned, since there is no data to actually move around at runtime. This means that the compiler couldn't distinguish uninitialized variables from initialized ones. That is, code like this:

let x: (); // note: never initialized
use(x)

and this:

let x: () = ();
use(x);

would desugar to the same MIR. That is a problem, particularly with respect to destructors: imagine that instead of the type (), we used a type like struct Foo; where Foo implements Drop.

Another advantage is that building aggregates in a two-step way assures the proper execution order when unwinding occurs before the complete value is constructed. In particular, we want to drop the intermediate results in the order that they appear in the source, not in the order in which the fields are specified in the struct definition.

A final reason to include aggregates is that, at runtime, the representation of an aggregate may indeed fit within a single word, in which case making a temporary and writing the fields piecemeal may in fact not be the correct representation.

In any case, after the move and correctness checking is done, it is easy enough to remove these aggregate rvalues and replace them with assignments. This could potentially be done during LLVM lowering, or as a pre-pass that transforms MIR statements like:

x = ...x;
y = ...y;
z = ...z;
f = (x, y, z)

to:

x = ...x;
y = ...y;
z = ...z;
f.0 = x;
f.1 = y;
f.2 = z;

combined with another pass that removes temporaries that are only used within a single assignment (and nowhere else):

f.0 = ...x;
f.1 = ...y;
f.2 = ...z;

Going further, once type-checking is done, it is plausible to do further lowering within the MIR purely for optimization purposes. For example, we could introduce intermediate references to cache the results of common lvalue computations and so forth. This may well be better left to LLVM (or at least to the lowering pass).

Bounds checking

Because bounds checks are fallible, it's important to encode them in the MIR whenever we do indexing. Otherwise the trans code would have to figure out on its own how to do unwinding at that point. Because the MIR doesn't "desugar" fat pointers, we include a special rvalue LEN that extracts the length from an array value whose type matches [T] or [T;n] (in the latter case, it yields a constant). Using this, we desugar an array reference like y = arr[x] as follows:

let len: usize;
let idx: usize;
let lt: bool;

B0: {
  len = len(arr);
  idx = x;
  lt = idx < len;
  if lt { B1 } else { B2 }
}

B1: {
  y = arr[idx]
  ...
}

B2: {
  <panic>
}

The key point here is that we create a temporary (idx) capturing the value that we bounds checked and we ensure that there is a comparison against the length.

Overflow checking

Similarly, since overflow checks can trigger a panic, they ought to be exposed in the MIR as well. This is handled by having distinct binary operators for "add with overflow" and so forth, analogous to the LLVM intrinsics. These operators yield a tuple of (result, overflow), so result = left + right might be translated like:

let tmp: (u32, bool);

B0: {
  tmp = left + right;
  if(tmp.1, B2, B1)
}

B1: {
  result = tmp.0
  ...
}

B2: {
  <panic>
}

Matches

One of the goals of the MIR is to desugar matches into something much more primitive, so that we are freed from reasoning about their complexity. This is primarily achieved through a combination of SWITCH terminators and downcasts. To get the idea, consider this simple match statement:

match foo() {
    Some(ref v) => ...0,
    None => ...1
}

This would be converted into MIR as follows (leaving out the unwinding support):

BB0 {
    call(tmp = foo(), BB1, ...);
}

BB1 {
    switch(tmp, BB2, BB3) // two branches, corresponding to the Some and None variants resp.
}

BB2 {
    v = &(tmp as Option::Some).0;
    ...0
}

BB3 {
    ...1
}

There are some interesting cases that arise from matches that are worth examining.

Vector patterns. Currently, (unstable) Rust supports vector patterns which permit borrows that would not otherwise be legal:

let mut vec = [1, 2];
match vec {
    [ref mut p, ref mut q] => { ... }
}

If this code were written using p = &mut vec[0], q = &mut vec[1], the borrow checker would complain. This is because it does not attempt to reason about indices being disjoint, even if they are constant (this is a limitation we may wish to consider lifting at some point in the future, however).

To accommodate these, we plan to desugar such matches into lvalues using the special "constant index" form. The borrow checker would be able to reason that two constant indices are disjoint but it could consider "variable indices" to be (potentially) overlapping with all constant indices. This is a fairly straightforward thing to do (and in fact the borrow checker already includes similar logic, since the ExprUseVisitor encounters a similar dilemna trying to resolve borrows).

Drops

The Drop(DROP_KIND, LVALUE) instruction is intended to represent "automatic" compiler-inserted drops. The semantics of a Drop is that it drops "if needed". This means that the compiler can insert it everywhere that a Drop would make sense (due to scoping), and assume that instrumentation will be done as needed to prevent double drops. Currently, this signaling is done by zeroing out memory at runtime, but we are in the process of introducing stack flags for this purpose: the MIR offers the opportunity to reify those flags if we wanted, and rewrite drops to be more narrow (versus leaving that work for LLVM).

To illustrate how drop works, let's work through a simple example. Imagine that we have a snippet of code like:

{
  let x = Box::new(22);
  send(x);
}

The compiler would generate a drop for x at the end of the block, but the value x would also be moved as part of the call to send. A later analysis could easily strip out this Drop since it is evident that the value is always used on all paths that lead to Drop.

Shallow drops and Box

The MIR includes the distinction between "shallow" and "deep" drop. Deep drop is the normal thing, but shallow drop is used when partially initializing boxes. This is tied to the box keyword. For example, an assignment like the following:

let x = box Foo::new();

would be translated to something like the following:

let tmp: Box<Foo>;

B0: {
  tmp = BOX;
  f = Foo::new; // constant reference
  call(*tmp, f, B1, B2);
} 

B1: { // successful return of the call
  x = use(tmp); // move of tmp
  ...
}

B2: { // calling Foo::new() panic'd
  drop(Shallow, tmp);
  diverge;
}

The interesting part here is the block B2, which indicates the case that Foo::new() invoked unwinding. In that case, we have to free the box that we allocated, but we only want to free the box itself, not its contents (it is not yet initialized).

Note that having this kind of builtin box code is a legacy thing. The more generalized protocol that RFC 809 specifies works in more-or-less exactly the same way: when that is adopted uniformly, the need for shallow drop and the Box rvalue will go away.

Phasing

Ideally, the translation to MIR would be done during type checking, but before "region checking". This is because we would like to implement non-lexical lifetimes eventually, and doing that well would requires access to a control-flow graph. Given that we do very limited reasoning about regions at present, this should not be a problem.

Representing scopes

Lexical scopes in Rust play a large role in terms of when destructors run and how the reasoning about lifetimes works. However, they are completely erased by the graph format. For the most part, this is not an issue, since drops are encoded explicitly into the control-flow where needed. However, one place that we still need to reason about scopes (at least in the short term) is in region checking, because currently regions are encoded in terms of scopes, and we have to be able to map that to a region in the graph. The MIR therefore includes extra information mapping every scope to a SEME region (single-entry, multiple-exit). If/when we move to non-lexical lifetimes, regions would be defined in terms of the graph itself, and the need to retain scoping information should go away.

Monomorphization

Currently, we do monomorphization at LLVM translation time. If we ever chose to do it at a MIR level, that would be fine, but one thing to be careful of is that we may be able to elide Drop nodes based on the specific types.

Unchecked assertions

There are various bits of the MIR that are not trivially type-checked. In general, these are properties which are assured in Rust by construction in the high-level syntax, and thus we must be careful not to do any transformation that would endanger them after the fact.

  • Bounds-checking. We introduce explicit bounds checks into the IR that guard all indexing lvalues, but there is no explicit connection between this check and the later accesses.
  • Downcasts to a specific variant. We test variants with a SWITCH opcode but there is no explicit connection between this test and later downcasts.

This need for unchecked operations results form trying to lower and simplify the representation as much as possible, as well as trying to represent all panics explicitly. We believe the tradeoff to be worthwhile, particularly since:

  1. the existing analyses can continue to generally assume that these properties hold (e.g., that all indices are in bounds and all downcasts are safe); and,
  2. it would be trivial to implement a static dataflow analysis checking that bounds and downcasts only occur downstream of a relevant check.

Drawbacks

Converting from AST to a MIR will take some compilation time. Expectations are that constructing the MIR will be quite fast, and that follow-on code (such as trans and borowck) will execute faster, because they will operate over a simpler and more compact representation. However, this needs to be measured.

More effort is required to make quality error messages. Because the representation the compiler is working with is now quite different from what the user typed, we have to put in extra effort to make sure that we bridge this gap when reporting errors. We have some precedent for dealing with this, however. For example, the ExprUseVisitor (and mem_categorization) includes extra annotations and hints to tell the borrow checker when a reference was introduced as part of a closure versus being explicit in the source code. The current prototype doesn't have much in this direction, but it should be relatively straightforward to add. Hints like those, in addition to spans, should be enough to bridge the error message gap.

Alternatives

Use SSA. In the proposed MIR, temporaries are single-assignment but can be borrowed, making them more analogous to allocas than SSA values. This is helpful to analyses like the borrow checker, because it means that the program operates directly on paths through memory, versus having the stack modeled as allocas. The current model is also helpful for generating debuginfo.

SSA representation can be helpful for more sophisticated backend optimizations. However, we tend to leave those optimizations to LLVM, and hence it makes more sense to have the MIR be based on lvalues instead. There are some cases where it might make sense to do analyses on the MIR that would benefit from SSA, such as bounds check elision. In those cases, we could either quickly identify those temporaries that are not mutably borrowed (and which therefore act like SSA variables); or, further lower into a LIR, (which would be an SSA form); or else simply perform the analyses on the MIR using standard techniques like def-use chains. (CSE and so forth are straightforward both with and without SSA, honestly.)

Exclude unwinding. Excluding unwinding from the MIR would allow us to elide annoying details like bounds and overflow checking. These are not particularly interesting to borrowck, so that is somewhat appealing. But that would mean that consumers of MIR would have to reconstruct the order of drops and so forth on unwinding paths, which would require them reasoning about scopes and other rather complex bits of information. Moreover, having all drops fully exposed in the MIR is likely helpful for better handling of dynamic drop and also for the rules collectively known as dropck, though all details there have not been worked out.

Expand the set of operands. The proposed MIR forces all rvalue operands to be lvalues. This means that integer constants and other "simple" things will wind up introducing a temporary. For example, translating x = 2+2 will generate code like:

tmp0 = 2
tmp1 = 2
x = tmp0 + tmp1

A more common case will be calls to statically known functions like x = foo(3), which desugars to a temporary and a constant reference:

tmp0 = foo;
tmp1 = 3
x = tmp0(tmp1)

There is no particular harm in such constants: it would be very easy to optimize them away when reducing to LLVM bitcode, and if we do not do so, LLVM will do it. However, we could also expand the scope of operands to include both lvalues and some simple rvalues like constants. The main advantage of this is that it would reduce the total number of statements and hence might help with memory consumption.

Totally safe MIR. This MIR includes operations whose safety is not trivially type-checked (see the section on unchecked assertions above). We might design a higher-level MIR where those properties held by construction, or modify the MIR to thread "evidence" of some form that makes it easier to check that the properties hold. The former would make downstream code accommodate more complexity. The latter remains an option in the future but doesn't seem to offer much practical advantage.

Unresolved questions

What additional info is needed to provide for good error messages? Currently the implementation only has spans on statements, not lvalues or rvalues. We'll have to experiment here. I expect we will probably wind up placing "debug info" on all lvalues, which includes not only a span but also a "translation" into terms the user understands. For example, in a closure, a reference to an by-reference upvar foo will be translated to something like *self.foo, and we would like that to be displayed to the user as just foo.

What additional info is needed for debuginfo? It may be that to generate good debuginfo we want to include additional information about control-flow or scoping.

Unsafe blocks. Should we layer unsafe in the MIR so that effect checking can be done on the CFG? It's not the most natural way to do it, but it would make it fairly easy to support (e.g.) autoderef on unsafe pointers, since all the implicit operations are made explicit in the MIR. My hunch is that we can improve our HIR instead.