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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/check/testdata/function/call/ref.carbon
Vendored
+19
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@@ -190,10 +190,21 @@ fn G() {
// CHECK:STDOUT: %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
// CHECK:STDOUT: %G.type: type = fn_type @G [concrete]
// CHECK:STDOUT: %G: %G.type = struct_value () [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.type: type = facet_type <@DefaultOrUnformed> [concrete]
// CHECK:STDOUT: %T: type = symbolic_binding T, 0 [symbolic]
// CHECK:STDOUT: %T.as.DefaultOrUnformed.impl.Op.type.6c6: type = fn_type @T.as.DefaultOrUnformed.impl.Op, @T.as.DefaultOrUnformed.impl(%T) [symbolic]
// CHECK:STDOUT: %T.as.DefaultOrUnformed.impl.Op.ae1: %T.as.DefaultOrUnformed.impl.Op.type.6c6 = struct_value () [symbolic]
// CHECK:STDOUT: %DefaultOrUnformed.impl_witness.6fd: <witness> = impl_witness imports.%DefaultOrUnformed.impl_witness_table, @T.as.DefaultOrUnformed.impl(%C) [concrete]
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value %C, (%DefaultOrUnformed.impl_witness.6fd) [concrete]
// CHECK:STDOUT: %Destroy.Op.type: type = fn_type @Destroy.Op [concrete]
// CHECK:STDOUT: %Destroy.Op: %Destroy.Op.type = struct_value () [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: imports {
// CHECK:STDOUT: %Core.import_ref.01d: @T.as.DefaultOrUnformed.impl.%T.as.DefaultOrUnformed.impl.Op.type (%T.as.DefaultOrUnformed.impl.Op.type.6c6) = import_ref Core//prelude/parts/default, loc{{\d+_\d+}}, loaded [symbolic = @T.as.DefaultOrUnformed.impl.%T.as.DefaultOrUnformed.impl.Op (constants.%T.as.DefaultOrUnformed.impl.Op.ae1)]
// CHECK:STDOUT: %DefaultOrUnformed.impl_witness_table = impl_witness_table (%Core.import_ref.01d), @T.as.DefaultOrUnformed.impl [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: %C.decl: type = class_decl @C [concrete = constants.%C] {} {}
// CHECK:STDOUT: %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {} {}
@@ -225,6 +236,14 @@ fn G() {
// CHECK:STDOUT: %c.var_patt: %pattern_type.7c7 = var_pattern %c.patt [concrete]
// CHECK:STDOUT: }
// CHECK:STDOUT: %c.var: ref %C = var %c.var_patt
// CHECK:STDOUT: %DefaultOrUnformed.facet: %DefaultOrUnformed.type = facet_value constants.%C, (constants.%DefaultOrUnformed.impl_witness.6fd) [concrete = constants.%DefaultOrUnformed.facet]
// CHECK:STDOUT: %.loc10_11.1: %DefaultOrUnformed.type = converted constants.%C, %DefaultOrUnformed.facet [concrete = constants.%DefaultOrUnformed.facet]
// CHECK:STDOUT: %as_type: type = facet_access_type %.loc10_11.1 [concrete = constants.%C]
// CHECK:STDOUT: %.loc10_11.2: type = converted %.loc10_11.1, %as_type [concrete = constants.%C]
// CHECK:STDOUT: <elided>
// CHECK:STDOUT: %.loc10_3: ref %C = splice_block %c.var {}
// CHECK:STDOUT: %T.as.DefaultOrUnformed.impl.Op.call: init %C to %.loc10_3 = call %T.as.DefaultOrUnformed.impl.Op.specific_fn()
// CHECK:STDOUT: assign %c.var, %T.as.DefaultOrUnformed.impl.Op.call
// CHECK:STDOUT: %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
// CHECK:STDOUT: %c: ref %C = ref_binding c, %c.var
// CHECK:STDOUT: %c.ref: ref %C = name_ref c, %c