Allow (don't diagnose) impls that have the same type structure if they
are associated with the same match_first block. Similarly, allow final
impls (made final by their enclosing match_first block) that overlap in
type structure when they are associated with the same match_first block.
If the type structures are the same, choose the first impl from the
match_first block.same.
If the type structures overlap but are not the same, and share a
match_first block, then we should choose the first overlapping impl from
the match_first block. This is true both for final and non-final impls.
See
https://github.com/carbon-language/carbon-lang/blob/de8b03faa3178ae683d8e7124fbcba81eb88e00c/proposals/p005337-interface-extension-and-final-impl-update.md#impl-selection-algorithm
But in the non-final case if there is an overlapping impl outside the
match_first, it can win if it's more specific.
Previously the last decl had to be the definition. Now we allow a
declaration after a definition, so that the user can write a match_first
block last, and put (re-)declarations of impls in it, after the
definitions have already been written elsewhere.
We track the location of the decl that was associated with a match_first
block so that we can correctly point to it in diagnostics when an impl
is written twice in match_first blocks. Since impls may not be
redeclared across an import boundary, we will never have a `SemIR::Impl`
with a match_first from a different file in a redeclaration, so we don't
need to import the location of a previous decl that was in a match_first
for diagnostics. As such we just store a LocId on the `SemIR::Impl`
struct.
When we check an `impl` decl, find the containing `match_first` block,
if any, and store a connection to it in the `SemIR::Impl` structure,
along with the impl's position in that `match_first` block so that we
can sort/prioritize the `SemIR::Impl`s later.
Also, update the `is_final` flag if the `match_first` block is modified
as `final`. But ensure we diagnose trying to put a `final impl`, or a
redeclaration of one, in a `match_first` block. Also diagnose if an impl
is attached to a `match_first` block more than once - either in two
different blocks or in the same block at different positions.
Putting the `match_first` connection on the `SemIR::Impl` structure
means we have to import it, so implement import and add a smoke test for
that, which ensures nothing explodes.
Drop the `scope_stack` entry for the `match_first` block, as it was not
needed. Once we started tracking the `match_first` size on the `Context`
class, it became more straightforward to just store the `match_first`
decl `InstId` in the same place. The `match_first` block is not supposed
to act like a different scope for the purpose of redecls anyhow, so it's
a bit simpler this way.
It must be in a namespace, function, or class. In particular, it can't
be in another match_first, doing so immediately associates any impl
inside with two different match_first scopes, but we only want them
associated with one for prioritization.
Push a scope_stack entry for match_first blocks, and handle impls being
inside those scope entries. An impl should not use the `match_first`
block as its "enclosing scope" for the purpose of deciding if the impl
is a redecl of another impl. We should look through it to the class or
namespace the impl (and match_first) are located inside.
We don't yet store the relationship between the impl and its match_first
block, nor then can we use it in impl lookup for prioritization.
Dana Jansens