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Add an interface for initialization of vars without an explicit initializer (#6934)

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When a `var` is not explicitly given an initializer, initialize it in
one of two ways:

* If its type implements the new interface `Core.Default`, call
`Core.Default.Op` to initialize it.
* Otherwise, if its type implements `UnformedInit`, leave it in an
unformed state. For now, this is always an uninitialized state, but that
will change in the future.
* If neither of those apply, the `var` declaration is ill-formed.

This is a step towards implementing leads decision #6739 and proposals
#257 and #5913.

Assisted-by: Gemini 3.1 Pro via Antigravity

---------

Co-authored-by: Geoff Romer <gromer@google.com>
This commit is contained in:
Richard Smith
2026-03-19 23:46:06 +00:00
committed by GitHub
co-authored by Geoff Romer
parent fd2d210c63
commit ce50f181f1
146 changed files with 5226 additions and 3535 deletions
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toolchain/testing/testdata/min_prelude/parts/as.carbon
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+2
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@@ -2,6 +2,7 @@
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/default.carbon
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/destroy.carbon
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/form.carbon
@@ -9,6 +10,7 @@
package Core library "prelude/parts/as";
export import library "prelude/parts/default";
export import library "prelude/parts/destroy";
export import library "prelude/parts/form";
toolchain/testing/testdata/min_prelude/parts/default.carbon
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+15
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@@ -0,0 +1,15 @@
// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// --- min_prelude/parts/default.carbon
package Core library "prelude/parts/default";
interface DefaultOrUnformed { fn Op() -> Self; }
// In tests, just allow any type to be default-initialized as a no-op.
// TODO: This is a hack to allow existing tests to contiue working.
impl forall [T:! type] T as DefaultOrUnformed {
fn Op() -> Self = "make_uninitialized";
}
toolchain/testing/testdata/min_prelude/parts/optional.carbon
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+2
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@@ -4,6 +4,7 @@
//
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/bool.carbon
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/copy.carbon
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/default.carbon
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/destroy.carbon
// --- min_prelude/parts/optional.carbon
@@ -12,6 +13,7 @@ package Core library "prelude/parts/optional";
import library "prelude/parts/bool";
import library "prelude/parts/copy";
import library "prelude/parts/default";
import library "prelude/parts/destroy";
class Optional(T:! Copy) {