If a `BindSymbolicName` is converted to `type` and then to its exact
`FacetType`, we get a `FacetValue` wrapping the `BindSymbolicName` but
providing no different information: it has the same witnesses and
`FacetType` as the original `BindSymbolicName`. Yet it is a different
constant value, creating multiple canonical forms with the same meaning.
Now we make that `FacetValue` with the same `FacetType` as the
`BindSymbolicName` it wraps evaluate back to the `BindSymbolicName`,
making it the unique canonical form.
This makes the "shortcut" in convert for avoiding impl lookup when
converting from `FacetAccessType` to `FacetType` in this exact scenario
work the same as doing the full impl lookup.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
The main direction of this change is the edits to `destroy.carbon`
(matching in both prelude and min_prelude).
Previously there was a no-op blanket impl for `Destroy`, which hid all
missing implementations of `Destroy`. This does a few things:
- Sets up builtin aggregate destruction for struct and tuple types as
before, but also adds C++ class types and array types to the same
handling. (all as a TODO for actual implementation)
- Also maybe-unformed destruction, for now at least. (there's a chance I
may try a different approach on this, but the impl lookup wasn't working
as I'd hope in order to write it in code)
- Adds handlers for simple things that are easy to do in code: `type`,
`bool`, pointers. (because these are no-op destruction)
- Redirect `const T` destruction to `T` destruction.
This leaves as future issues:
- `partial T` destruction. (this can't be done similar to `const`
because it only works for non-`final` class types; I think `class`
definitions should just generate what's needed)
- Destruction of other prelude-provided types. (will probably come up as
we implement class destruction, that the adapted builtin type doesn't
implement `Destroy` -- but may end up special-casing that in a way that
moots it)
This moves the `&` operator from `facet_types.carbon` to
`convert.carbon` because more things need to handle type and now that
we're getting separate copy and destroy interfaces. It should be
low-cost (an interface and builtin) so hopefully this is the right
balance for complexity and re-use.
A few tests are also edited in order to focus them more on what they
intend to test, and avoid a `Destroy` dependency.
Add `Dependent` value and initializing representations for types whose
representations are unknown because they are dependent. When generating
SemIR in such cases, use a worst-case initializing representation that
both provides a destination address and also propagates a potential
result value.
Use this to fix incorrect lowering and lowering crashes for specific
functions involving generic types that don't use a copy value
representation.
In lowering, be careful to distinguish between whether the initializing
representation for the generic return type uses a return slot (which
affects whether the SemIR declaration and call have one) and whether the
initializing representation for the specific return type uses a return
slot (which affects whether the LLVM IR declaration and call have one).
This is in support of a goal of changing the blanket `destroy` impl to
use (roughly):
```
private fn CanAggregateDestroy() -> type = "type.can_aggregate_destroy";
// Handles aggregate type destruction.
impl forall [AggregateDestroyT:! CanAggregateDestroy()] AggregateDestroyT as Destroy {
fn Op[addr self: Self*]() = "type.aggregate_destroy";
}
```
That isn't done here because there's still other issues that migrating
raises. What this *does* do is add the builtin functions, and in
particular, support to `FacetTypeInfo` to make `CanAggregateDestroy`
work.
The "special requirement" approach in `FacetTypeInfo` allows us to
support restricting a blanket impl under the current approach of impls.
Maybe we'll find a cleaner approach that can work in the future, but
this fits into the current model by propagating similar to other
requirements. I'm using an enum mask because we have a number of similar
things to add (e.g. copy, move) but I'm not sure we need a full vector.
A few alternatives considered were:
- Supporting syntax more like `where .Self impls
TypeCanAggregateDestroy(.Self, SupportedInterface,
UnsupportedInterface)`. I think it'd be a little cleaner, but requires
better compile-time evaluation in order to assess the type of the call.
Right now it's expected to be a `FacetType` too early to make this work,
and I was concerned about pouring too much more time down this route.
- Providing an actual interface, in particular doing name lookup back
into `Core.` for an interface. This would've added name lookup overhead,
and the question of whether an `impl` exists.
- Generating an interface. This avoids the name lookup, but would still
raise the question of whether an `impl` should also be generated. Work
I've previously done generating interfaces for class destruction also
feels complex to both write and understand (an unfortunate issue).
- Still modeling as an `ImplsConstraint`, for example by defining a
special `InterfaceId::CanAggregateDestroy = -2` similar to what we do on
other ids. I was hesitant because of how this expands the number of
modes of `InterfaceId`, and things for consuming code to watch out for,
for what feels like a relatively niche set of use-cases that are only
interface-like.
---------
Co-authored-by: Dana Jansens <danakj@orodu.net>
When returning a value from a function whose return type has a by-copy
initializing representation, perform initialization like we do when the
return type has an in-place initializing representation. This makes our
SemIR representation more uniform, as the return expression will now
always be an initializing expression rather than a value expression, but
more importantly it means that attempts to return a non-copyable type by
value now fail, even if the type has a by-copy initializing
representation.
This catches a bunch of places where we were returning a value of an
unconstrained template parameter `T:! type`, which we were incorrectly
allowing because we didn't notice it was not copyable. Unfortunately
this then requires quite a few test updates.
Like #6034, this exposes a lowering issue where lowering crashes when
attempting to lower a specific copy operation for certain types; a
couple more tests are temporarily disabled here. An upcoming PR
dependent on this one will fix the issue and re-enable those tests.
Instead of hardcoding which types are copyable, add a `Core.Copy`
interface to perform copying. Move almost all the current copy support
to that interface. Some remaining pieces are still using builtin logic
after this PR:
* For tuples and structs, builtin logic is used to perform elementwise
copies. This also supports copying *adapters of* tuples and structs,
which seems like it may not be desirable, especially for non-extending
adapters. A `Copy` impl is provided for tuples of at most 2 elements, so
that `Core.Copy` constraints are satisfied, but we can't implement this
generally until we have variadics support, and don't yet have a
mechanism to generalize this to structs.
* For `enum` types imported from C++, builtin logic is used to perform a
copy. This is temporary until we have a mechanism to identify these
types from an impl in the prelude.
One lowering test in `toolchain/lower/testdata/class/generic.carbon` is
disabled for now, as it causes a crash in the lowering code due to an
ABI mismatch between the call signature in the lowered declaration of a
specific function and the call that is generated in the specific callee.
Fixing this is a little involved, and will be done in a separate PR.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
If it's just `TypeType` then the `BindSymbolicName` appears directly in
type positions, but if it is replaced with another facet value, then we
would need to insert a `FacetAccessType` around it. By giving it a
`FacetType` type, like other `BindSymbolicName`s we make it consistent
and avoid having to introduce extra instructions.
This uses each vector's size as a barrier between lists, to eliminate
the possibility of incidental collisions between entries of different
lists. This is the same as is done inside `AddBlock`.
Attach the cleanup to the `Temporary` instruction instead of to the
`TemporaryStorage` instruction. We create `TemporaryStorage`
instructions speculatively when creating an initializing expression, and
may overwrite those instructions with other instructions if it turns out
that a temporary is not required. Instead, wait until we finalize the
temporary and create a `Temporary` instruction to register the cleanup.
This is necessary if the source type is an adapter, as we would not
otherwise be able to determine what type it adapts and hence could be
converted to.
We add a virtual node (`CompileTimeBindingPatternStart`) as the first
child of `CompileTimeBindingPattern` which holds the identifier
underneath it, so that it is checked just before the type expression of
the `CompileTimeBindingPattern`. When we reach this virtual node during
check, we add `.Self` as a name in the current scope, and when we reach
`CompileTimeBindingPattern` we remove it from scope, which ensures it's
present during only the checking of the type expression for the compile
time pattern.
At the moment the `.Self` has a different type (it's a `TypeType`) than
other `.Self` in the facet type (which are a single `FacetType`), but
the intention is to immediately substitute it out of the facet type
entirely, replacing it with a reference to the compile time binding (a
`BindSymbolicName`) itself. A TODO has been added for this.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
The self access is important; for the test `generic_class.carbon` being
added to `toolchain/check/testdata/class/destroy_calls.carbon`, it was
using `%T.as.Destroy` instead of `%D.as.Destroy`, indicating the default
blank impl was being used instead of the type-specific version. That
test is trying to focus on the issue, but the delta is visible in a
couple other files in this PR, for example
`toolchain/check/testdata/class/generic/init.carbon`.
I'm separately working on getting rid of the default impl, which is how
I noticed this.
This makes all `.Self` references in a facet type canonically the same
(which will remain true iff they refer to the same `Self` type in the
future), removing the need to do more complex comparisons between them
using the EntityName, interface, and index. This allows the comparison
of types containing `.Self` references to be done correctly regardless
of where the `.Self` appears, as such type expressions will all be
canonically equal if they otherwise equal now, regardless of whether
they are written in the context where `.Self` could have seen different
`Self` facet types.
In order to retain access to constraints on a base `.Self` facet type,
in the case of applying `where` to an existing facet type, we:
- Give the base facet type as a `RequirementBaseFacetType` constraint so
that eval of `WhereExpr` can find and copy all the constraints off of
it.
- Introduce eager/early rewrite constraint resolution, which allows a
constraint to eagerly resolve access to earlier rewrite constraints
(`where .A = () and .B = .A` is eagerly transformed into `where .A = ()
and .B = ()`) before the full constraint resolution step. This allows
use of rewrite constraints in larger type expressions, such as `where .A
= () and .B = C(.A)` and `C` will know that the argument is `()`.
I was thinking about this for destruction, which I may not be able to
use it for, but still think this may be a good change to keep features
consistent.
Although this focused on `Destroy` support, some choices here around
`implicit_type_impls` are because copy/move will likely follow a similar
approach. I'm trying not to predict too much about how we'll structure
those, but I'm putting `Destroy` impl logic in a file that could perhaps
be shared with those. They'd likely be interested in similar things,
e.g. traversing members of types (particularly class, struct literal,
tuple literal).
At present this sets the destroy function as `no_op` which is consistent
with current logic, but has a TODO to correctly define.
Constant importing for functions changes slightly due to some issues I
was having with `GetFunctionType`. zygoloid suggested this approach to
avoid `EvalInst` logic.
Adds a flag for controlling whether to generating these impls. While
this does generation for `class`, as noted above this'll also need to be
done for tuples and struct literals, which would leave the `none.carbon`
min_prelude unable to use any types. Note if destruction *would* occur,
it'll still look up `Core.Destroy` for that and fail, but that's already
true of any test using `none.carbon`. I'm trying to use the flag to see
if we can keep `none.carbon` working mostly-consistently.
I'd tried separating out the flag to #5852, but that got a lot of
pushback over whether the behavior was appropriate. I'm hoping that the
interactions here make it clearer why the particular approach -- the
goal is not to enable advanced testing, or create some new end-user
behavior that we really support, it's just to keep no-prelude tests
functional. The main question raised there was why not just keep
generating `impl T as Core.Destroy` if `fn destroy` is present -- but I
think here it should be apparent that would require additional
complexity, as the generation of `impl T as Core.Destroy` is not
currently conditioned based on the implementation of `fn destroy`. I'd
rather add complexity to this flag only if it's enabling interesting
test functionality.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
In order to verify rewrite constraints at the end of
`LookupImplWitness()` we need to replace references to associated
constants in the query facet type with values that come from the query's
self. To do this, we find any `ImplWitnessAccess` that is a reference to
`.Self` and replace its witness with the witness found through the impl
lookup process, if the interfaces match. This allows the
`ImplWitnessAccess` to resolve to a concrete value if that witness was
concrete. Then we just need to compare that for each rewrite constraint
the lhs and rhs are the same constant value. If they differ, the self
provided a different value for one side (either through its own facet
value constraints or through an associated impl), or the self did not
provide a value at all.
For now, only .Self references in the top-level facet type are
rewritten. Nested facet types are not, even if they contain a .Self
reference up to the top level facet value. This will be addressed by
adding numbering to the EntityName of of .Self in a BindSymbolicName.
See the third model in
https://docs.google.com/document/d/1Yt-i5AmF76LSvD4TrWRIAE_92kii6j5yFiW-S7ahzlg/edit?tab=t.0
for the plan. For now, there is a TODO addressing this.
A rewrite constraint like `.X = .Y.Z and .Y = .Self and .Z = ()` has a
nested `ImplWitnessAccess` `.Y.Z` (technically `(.Self.Y).Z`). The inner
access `.Self.Y` needs to be resolved (in this case to `.Self`) before
the outer `???.Z` can be resolved as `.Self.Z` which is `()`.
Make Subst perform "recursion" on the RHS instructions as they are
replaced, effectively doing a depth-first traversal through the rewrite
constraints doing replacements. This allows us to fully compute
individual associated constants in the minimal amount of work, and cache
the results so they can be reused cheaply in cases where the rewrite
constraints generate an exponential number of references to associated
constants.
Fixes https://github.com/carbon-language/carbon-lang/issues/5672
Change impls from `<interface>.impl` to `<self>.as.<interface>.impl`,
and *member* functions to `<parent scope>.<fn>` (non-member functions
exclude their parent scope). Stop special-casing builtin functions,
given the new naming scheme.
The purpose of this is to make it clearer when a member function is
being accessed and, if so, which member function. In particular, we
often access interface `Op` functions. The builtin function
special-casing was intended to help with that, but we still have lots of
`Op` functions. This particular approach should make the interactions
clearer.
This changes up queueing of block IDs a little because, in particular,
we need to process bodies of entities only after constants finish
processing. But, it should also result in less memory usage during
processing because it means we have less on the insts stack at any given
time, since we track a block rather than all instructions contained by
the block.
This switches from the `CollectNamesInBlock` approach for entities, to
instead traversing entities as they're encountered. For example, when
traversing constants, when a type is found, the entity will have its
block queued for processing.
This leads to a change in the traversal order, which affects
disambiguation done by numeric sequencing (since that's just showing the
traversal order).
This will allow for simpler "name based on name" logic. This is
something I plan to use for:
- impls: `<type>.as.<interface>.impl`
- functions: `<entity>.<member function>`
- Note an impl may be used as the entity for a bound function.
By naming the entities as they're encountered, I'll be able to rely on
the generated names rather than recalculating them.
To assist this, I'm also differentiating between the ambiguous and
disambiguated name. Otherwise, we could end up with things like
`<function>.<disambiguator>.<call>.<other disambiguator>`, where the
repeated disambiguator may not be necessary in order to get full
disambiguation. It's also a smaller delta from the current output.
Note, changing `Name` to a class felt appropriate given its shape. I was
also noticing that parts of its API were unused, and the class helps
detect unused private members.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
Use `full.carbon` min-prelude for any tests using the full prelude. A
few tests were using it unnecessarily and are changed to a more minimal
one in the process.
Any test which does not include some min-prelude will now fail with an
error.
In `BuildUnaryOperator`, `GetOperatorOpFunction` is treated as
desugaring, but `PerformCompoundMemberAccess` and `PerformCall` are not.
This treats all of destruction as desugaring.
This leads to some instructions being elided, because of `GetOrAddInst`
behaviors:
> // If the instruction has a desugared location and a constant value,
returns
> // the constant value's instruction ID. Otherwise, same as AddInst.
This changes instructions that previously had a non-desugared location
to instead have a desugared location, so if they also have a constant
value then the constant value can be used directly.
Currently if the first operand contains an error, we will return error,
even though the second operands contains a runtime, and it has a
stronger priority (the phase always goes up if possible).
Import is only allowed on instructions with compile-time values, so we
crash if we ever try to import a runtime value. Importable instructions
must diagnose unexpected runtime values and produce errors in the semir
from which they would be imported so that runtime values are never
imported by another semir.
If we had an instruction where you had an error value from the first
operand, and runtime from the second, and we imported it:
- Before https://github.com/carbon-language/carbon-lang/pull/5728 we
would crash in import, but only because we treated errors as runtime
- After https://github.com/carbon-language/carbon-lang/pull/5728 we
would import ErrorInst because we propagate errors. This is desirable
for cases with compile-time values and errors present only.
- After this PR, we would crash again, cuz you're importing a runtime
thing.
This change means that instructions containing an
`InstConstantKind::Never` instruction like`ValueParam` will consistently
evaluate to a runtime value, even if there are errors present. This is
visible in the `BindName` instructions changing in the semir, where they
became constant `ErrorInst` values previously but no longer do.
This changes `Destroy` to use an interface for its implementation.
Note that this change includes a lot of test updates. Even when
`Destroy` is a no-op, it still causes code generation as part of
determining that.
Originally I was trying to use ranges to cut down the scope of this, and
to a degree I think they have. But a flipside here is that cases where
no destructors should be generated -- particularly globals -- would be
needed to completely remove destructor calls. Even for ranges, the range
can often include the destructor placement. So I've shifted
frame-of-thought a little: accept a bunch of destructor churn, because
destructors are needed and will be prevalent. The verbosity is a feature
of the design to make desugaring apparent in IR, not a bug.
If convert fails after applying a substitution in deduce, fail deduction
rather than succeeding deduction with an ErrorInst in the deduced
argument.
For example, in
`toolchain/check/testdata/facet/fail_convert_class_type_to_generic_facet_value.carbon`
the `WrongGenericParam` is deduced for the first argument, then
substituted into the second parameter, but the argument can not convert
to the parameter after substitution. In this case, deduction fails
instead of producig a call with an error in the second argument. The
resulting semir drops the call with an error argument:
```
-// CHECK:STDOUT: %CallGenericMethod.specific_fn: <specific function> = specific_function %CallGenericMethod.ref, @CallGenericMethod(constants.%WrongGenericParam, <error>) [concrete = <error>]
-// CHECK:STDOUT: %CallGenericMethod.call: init %empty_tuple.type = call %CallGenericMethod.specific_fn() [concrete = <error>]
```
When building a FacetType from an existing FacetType, don't diagnose
rewrite constraints that are compatible with the existing FacetType.
To do this, we consider two RHS as identical[1] if they have the same
constant value after substituting from available rewrite constraints in
the being-constructed FacetType, since the syntactic representation of
the RHS is lost during eval.
[1]
https://docs.google.com/document/d/1Yt-i5AmF76LSvD4TrWRIAE_92kii6j5yFiW-S7ahzlg/edit?tab=t.0#heading=h.qti4vn50zwy
When the binding pattern appears within a `var` pattern, convert to a
reference. Otherwise, convert to a value.
This gets the advent of code examples to produce the right answers again
:)
---------
Co-authored-by: Geoff Romer <gromer@google.com>
The sort and dedupe operations in facet type resolution explicitly work
with `ImplWitnessAccess` instructions as being a reference to an
associated constant on some entity. If only one of the instructions is
an `ImplWitnessAccess`, we still want to consider that one as such, not
get its constant value, which may be some concrete type, and use that
for comparison instead.
This makes the new test fail (which we don't want) in a consistent way
with a similar test of TypeAnd (which we also don't want to fail),
making the system more consistent, while leaving some improvements to be
done.
Avoid inconsistent orderings between instructions, by making the
comparison function into a total order. To do so, we sort
ImplWitnessAccess instructions first, and sort them by their InstId.
Non-ImplWitnessAccess instructions come second, and sort them by their
constant InstId. Thanks to jonmeow for figuring out that the function
was not producing a total order and why.
Since this means the order is no longer relative to source order, we
order the two assignments in the diagnostic by source order(ish) by
putting the lower InstId first in the diagnostic output.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Replace the binary `Operation` interfaces with the `OperationWith(T:!
type)` interfaces described in the design, and add a `Result` associated
type for both unary and binary operations. Update the `impl`s in the
prelude for integer types to use the new form, including supporting
implicit conversion of either operand.
I've tried to split this PR up into commits focused on distinct changes
for review convenience. It may be simplest to review it one commit at a
time.
Given a facet type: `(Z where .X = .Y) where .X =.Y`
The rewrite constraints in the inner facet type are each an
`ImplWitnessAccess` into a witness for the self of type `Z` (which is
the facet type before the `where`). The rewrite constraints in the outer
facet type are each an `ImplWitnessAccess` for the self of type `Z where
.X = .Y`, which is a different self facet type.
This means when deduping in canonicalization, the first `.X` and the
second `.X` are different instructions, and different constant values,
so they both remain in the rewrite constraints, incorrectly. Then if the
outer `.X` is allowed to evaluate to a value from its facet type, it
finds `.Y` resulting in `.Y = .Y` which is also incorrect.
Because of the failure to dedupe the first facet type, that is also
diagnosed as two different assignments to the same `.X`. To resolve
that, we introduce `CompareFacetTypeConstraintValues()` compare values
in facet type constraints, and treat accesses to the same associated
constant in the same facet value as `equivalent` even when through
different witnesses. This allows us to dedupe the two `.X = .Y` rules
into one in the combined facet type.
Given a different facet type: `(Z where .X = ()) where .X = {}`. Here we
want to diagnose that `.X` has been assigned two different values. To do
so, we need to see that the two `.X` values are the same, and we use
`CompareFacetTypeConstraintValues()` to do this comparison. Then we see
two rewrite rules for the same LHS, and we can diagnose that.
We enable evaluating `ImplWitnessAccess` on `.Self` to pull a value from
rewrite constraints in a facet type so that we can see that we are not
incorrect evaluating the LHS of rewrite constraints and producing
cycles. By doing so, also enable generic code to see and use concrete
values in associated constants in facet types.
If the RHS of a rewrite constraint refers to an associated constant,
pull the value for that constant from other rewrite constraints. We
repeat this each time a RHS value is changed until we reach a fixed
point, as per the "Rewrite constraint resolution" rule:
https://docs.carbon-lang.dev/docs/design/generics/appendix-rewrite-constraints.html#rewrite-constraint-resolution
While replacing references to associated constants in the RHS, if the
reference is to the LHS of the same rewrite constraint, we diagnose it
as a cycle which has no fixed point, and replace reference to the
associated constant with `ErrorInst`.
Add a facet type rewrite constraint resolution step that is run every
time a facet type is constructed, in line with the design here:
https://docs.carbon-lang.dev/docs/design/generics/appendix-rewrite-constraints.html#rewrite-constraint-resolution
The resolution has multiple steps, and this PR implements the first of
them, finding and diagnosing any duplicate rewrites to the same
associated constant.
We already diagnosed this for impl construction, now we do so for all
facet types, which includes the one used for impl construction, so this
diagnostic is a superset of the previous.
Now that included files can specify `EXTRA-ARGS`, this uses that to
handle min_prelude files. Also moves `As`/`ImplicitAs` out to a shared
file, partly because we duplicate it a few times over, partly just to
show that it works.
Also removes the `min_prelude/` subdirectories because many of these
files were touched by autoupdate regardless. I noticed one conflict for
`impl_thunk.carbon`, so renaming that one to
`impl_thunk_min_prelude.carbon`.
`GetArgReplacements` I simply noticed was unused, so removing it.
The former signature unintentionally allowed any parameter and result
types, because it only checked that the type of the type was `type`,
which is tautological (for non-error values). Also add missing tests for
the builtin.
Previously we walked the global variables defined by the current file
and emitted an LLVM global variable definition for each of them. Now
instead, when emitting a constant reference to a global variable, we
emit an LLVM global variable declaration, and we then subsequently walk
the global variables defined by the current file and convert each of
them from a declaration to a definition.
In order to make import of names of global variables work, add support
for import of `var`, as well as support for importing `tuple_access` and
`tuple_pattern` in the case where the `var` has a tuple pattern in its
declaration. Also treat `bind_name`s that are reference bindings to
`var`s as having the same constant reference value as their `var` so
that we can properly import and lower them.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Where `--no-dump-sem-ir` is used, change to `--dump-sem-ir-ranges=only`.
Otherwise, add `--dump-sem-ir-ranges=if-present` with a TODO to change
to `only`.
Note, SemIR is affected just because the extra comments change line
numbers in files where splits aren't in use.
- Track the `VarPattern` instruction on the `VarStorage` instruction so
that it's available for name mangling.
- Mangle global variables based on the first binding name within their
pattern.
- Give global variables external rather than internal linkage, except if
they have no bindings whatsoever in their pattern.
- To support lowering references to bindings nested within a global var,
such as for `var (x: i32, b: i32)`, add some basic initial support for
reference constant expressions. Treat a global `var` as a reference
constant, and treat an aggregate access into a reference constant as a
reference constant.
This is making two inter-related changes:
- Change `file` to reuse the formatter logic of `constants` and
`imports`, meaning empty `file` scopes will be omitted
- Mark `<elided>` sections in blocks (not in non-block scopes, because
they're not as sequential)
Dana JansensandRichard Smith
Jon Ross-Perkins
