Carbon gives the C++ declaration of an exported Carbon thunk an asm
label that
includes the target's global symbol prefix (`_` on Mach-O). Clang names
a
declaration with an asm label `\01<label>` in the IR, so that LLVM uses
the
label verbatim as the symbol name, while Carbon lowered the thunk
definition
under its unprefixed name. Both name the same symbol, but they were
distinct
globals in the IR, so LLVM couldn't see that the C++ calls refer to the
Carbon
definition. That only works while the definition is never discarded.
Mark the Carbon thunk as a `CppThunk` (as described in
`toolchain/docs/check/cpp/thunks.md`), record its C++ declaration in
`clang_decls`, and when lowering the thunk, ask Clang for its function
(creating it if necessary) and define the thunk in that function. This
means
that the thunk and the C++ calls to it always use the same IR global,
with
Clang's name and signature.
The thunk takes all of its parameters by reference, so Clang lowers its
parameters the same way Carbon does, but some C++ ABIs (such as Apple
arm64)
return `this` from constructors. In that case, return the parameter that
Clang
marks `returned`.
On targets without a global prefix, the IR names already matched, so the
only
change is that the thunk definitions now carry Clang's parameter and
function
attributes.
Determining that a type has trivial destruction requires checking
whether all of its subobjects have trivial destruction and whether its
type has a custom witness for `Destroy.Op`. This isn't possible when
`Destroy` is the custodian for `Destroy.Op` and `SubobjectDestroy.Op`,
so we need to make `SubobjectDestroy` its own interface.
`Destroy.SelfDestruct` becomes a standalone function because it depends
on `SubobjectDestroy` and `Destroy` being implemented, and is also
responsible for generating any custom witnesses for both interfaces.
Making `SelfDestruct` an interface was experimented with, but this
required a blanket impl that caused issues for facet types.
`Destroy` depends on `SubobjectDestroy` to be implemented. We'd ideally
use `require impls SubobjectDestroy`, but it isn't complete yet, and it
also causes problems when `Destroy` is implemented inside the class
definition. We work around this by temporarily renaming the `Destroy`
interface to `DestroyImpl`, and introducing a constraint called
`Destroy` so that user code doesn't need to write `Core.SubobjectDestroy
& Core.Destroy`.
When exporting `X` from Carbon package `Foo`, expose it to C++ with
fully-qualified name `Carbon::Foo::X`, not simply `Carbon::X`. This
ensures that C++ mangled names properly distinguish between same-named
Carbon entities in different packages.
Name *lookup* into `Carbon::` still uses the same rules as before: it
finds what a `package.` lookup would find in Carbon. So `X` is named
from C++ as `Carbon::X` still, and a lookup for `Carbon::Foo` fails,
even though `X`'s "true name" is now `Carbon::Foo::X`.
Assisted-by: Gemini via Antigravity
When identifying a facet type, defer evaluation of LookupImplWitnesses
by redirecting them to evaluate into IdentifiedWitness. Then the caller
of identify will need to substitute the IdentifiedWitnesses, either with
witnesses of their own, or with LookupImplWitness to cause a lookup
operation.
This allows:
- An `impl as` on a named constraint can subst the witnesses in
designators from the named constraint with a witness to the impl itself.
This prevents them from looking for and finding some other overlapping
impl. (This PR)
- Impl lookup to collect rewrite/equality requirements from all named
constraints and then use the impls it has already found to evaluate any
designators in those requirements instead of performing impl lookup
again (creating a cycle). (Done in
https://github.com/carbon-language/carbon-lang/pull/7944)
- Similarly, an `impl as` can collect impls and equality requirements
from all named constraints and verify them correctly by replacing
witnesses that should refer to the impl with the correct witness instead
of performing a lookup and finding an overlapping impl. (Done in
https://github.com/carbon-language/carbon-lang/pull/7937)
- ImplWitnessAccess to be able to find rewrites from a named constraint,
not just from the initial DeclaredFacetType. (TBD)
To do this we need to introduce the concept of "identified" specifics.
In order to identify a facet type, we must construct a specific for each
require decl in a named constraint. These specifics will have
IdentifiedWitnesses inside them, but their args would otherwise match a
specific for the require decl constructed in another context. To avoid
polluting the canonical specific namespace, we introduce a second
namespace of specifics that are "identified" since they are created by
identifying.
As we have introduced a new global state that modifies eval behaviour,
we must disable that state when resolving a specific (as per
https://github.com/carbon-language/carbon-lang/pull/7923). But inside an
"identified" specific, we preserve the behaviour so that the witnesses
modified by identify (through SubstPeriodSelf or through evaluating a
require specific) have their evaluation deferred.
Base classes will be destroyed in a dedicated change, so we can
trivially confirm that the base is being destroyed.
This is a partial implementation of #7362.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
This change partially implements [PR #7362], which revises how objects
are destroyed. It is a partial implementation for two reasons:
1. This change moves `Destroy.Op`'s current behaviour into
`Destroy.SubobjectDestroy`, but it doesn't add support for objects with
non-trivial destruction.
2. `Destroy.SubobjectDestroy` is a workaround for `require impls
SubobjectDestroy`. We aren't able to use the latter until the dependents
add their requirements' implementations to their own witness tables.
[PR #7362]: https://github.com/carbon-language/carbon-lang/pulls/7362
Use a single `SemIR::Function` per `Core` interface method, whether it's
generated locally or imported. This prevents generating duplicate
functions, which lead to different types when the witness appears in a
`FacetValue` as part of a specific for a class.
We use a `CanonicalValueStore` of `GeneratedFunction` objects that allow
finding an existing FunctionId for a `Generated` special function before
(re-)generating it. Mangling for `Generated` functions is also moved to
use the values from the `GeneratedFunction`'s canonicalization key, so
that we have a consistent source of truth for the unique ID of a
`Generated` function across all files.
New tests are in
`toolchain/check/testdata/impl/custom_witness/destroy.carbon`.
Fix a bunch of cases where we use the same external name to mean
multiple different things in the same test. We've historically gotten
away with this, but under `--share-cpp-ast`, it becomes an error, at
least if the entity is either defined in, or used from, C++ code.
Assisted-by: Gemini via Antigravity (original change) and Claude Code
(suggested edits in review)
---------
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Instead of creating a CodeGenerator per CppDomain, and then crashing in
lowering when we try to consume the same llvm Module multiple times,
create a CodeGenerator for each CppFile within the domain.
For now, we mulitplex all of Clang's ASTConsumer output to all code
generators, which means that any strong external definitions within a
Carbon file (for example, in an inline `Cpp` fragment) will be emitted
to all output files in the same `CppDomain`, resulting in link errors
due to symbol redefinitions. This will be addressed later. But this
should be sufficient for Carbon compilations in which such symbols are
not defined.
We also don't yet attempt to classify which compilations will need C++
code generation, and instead create a clang `CodeGenerator` for every
Carbon file that has C++ imports. For `carbom compile`, only one Carbon
file will need code generation, and yet we still build multiple
`CodeGenerator` objects in general. Fixing this requires more plumbing
from the driver, and this will also be handled in a follow-up.
Assisted-by: Gemini via Antigravity
A method declared with `self` does not modify the object, but it was
exported to C++ as a non-const member function, so calling it on a const
reference would fail.
```carbon
class C {
fn Get(self);
}
inline Cpp '''
void F(const Carbon::C& c) {
c.Get();
}
''';
```
```
error: 'this' argument to member function 'Get' has type 'const Carbon::C', but function is not marked const
```
Import already maps `f() const` to `fn f(self)`, and this PR implements
the same behavior for exporting. No ref-qualifier is added, since that
maps to `ref self`, so that is unchanged.
`GetThisArg()` now builds `this` from the method instead of the parent
record, so that it picks up the method's const-qualifier.
We can end up emitting the same thunk from multiple compilations in some
cases -- in particular, when the thunk is wrapping a function that is
either synthesized by the compiler or imported from C++. When this
happens, we will have multiple-definition link errors unless we allow
redefinitions across multiple files.
It'd be nice to detect when we need to do this and when we don't, but
that's a bit tricky to do in practice. Ideally, in fact, we would use a
different strategy, and emit the thunks as discardable definitions in
each compilation that *uses* them. But for now emitting them with
weak_odr linkage seems like a good way to make progress.
Fix a few API issues. There's also a newly-added file in compiler-rt
that is not supposed to be built by default but is not being excluded
properly by a glob. Added a patch to exclude that and sent
https://github.com/llvm/llvm-project/pull/208861 upstream.
Each file dump now starts with a `; ---` comment and ends with a blank
line. This makes it easier to visually scan the dump for a file of
interest. The comment format is somewhat arbitrary; I chose `---` to
align with the `--- filename.carbon` separator in SemIR dumps, but
without the filename, because that appears on each of the next two lines
already.
Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.
Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.
An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.
Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.
Assisted-by: Claude Code with Claude Opus 4.7
---------
Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
David Blaikie
Christopher Di Bella
Richard Smith
Dana Jansens
Geoff Romer
Nicholas Bishop
arhwx
