When replacing `.Self` in a facet type we don't want to destroy the
structure of designators in rewrite constraints so that rewrite
constraint resolution and other similar code can still find them. We
were doing this only for the LHS, and for the RHS if it was a standalone
designator, like `.X = .Y` but we want to do this more robustly to also
avoid rewrite the `.Self` in `.Y` in the expression `.X = C(.Y)`. This
allows resolution, and ImplWitnessAccess to recognize the designator and
replace the `.Y` with the RHS value of another rewrite constraint
assigning to `.Y`.
A nested designator like `.(X.X1).(Y.Y1)` results in nested
ImplWitnessAccess instructions, which can produce cycles in the
toolchain easily when replacing `.Self`.
First, when constructing a facet type like `V:! Z where .Z1 impls (Y
where .Y1 = U)` we substitute replace `.Self` in the nested facet type,
and in this case we replace `.Self` with `.Z1` which contains a `.Self`
of its own. This was coming from us being lazy about replacing `.Self`
in an `impl as` declaration, such as `impl C as Z where .Z1 = .Self`.
The self type is known there, so we can more eagerly replace `.Self` as
we do in a `require impls` declaration. Then the replacement for `.Self`
never comes with a `.Self` that needs to also be replaced. Any resulting
`.Self` would always be the top-level one.
Second, when evaluating ImplWitnessAccess, we were replacing .Self in
the LHS of rewrite constraints, but the `.Self` may itself have a type
that contains rewrite constraints. If one of those rewrite constraints
has nested ImplWitnessAccess instructions, we evaluate the new
ImplWitnessAccess, which again finds rewrite constraints to replace
`.Self` in, and we repeat forever. For this one we just stop replacing
.Self in the LHS of rewrite constraints. Since they are always against
.Self, we can always look in the access facet's type for a value.
While fixing ImplWitness access, also correct the lookup to search
through the types of nested ImplWitnessAccess instructions to find a
rewrite value, since it may find it at any level up to the eventual
`.Self`.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
This lets us stop eliding it in textual semir tests with dump ranges.
Previously it would always get elided, even though it was part of the
range being dumped, and was referred to by other instructions in the
dump range.
Since each `.Self` is unique (can change its type if not its value) in a
facet type, having each one distinct by location also aids
understanding.
A `where` expression nested inside a `T impls X` constraint makes
`.Self` ambiguous on the right-hand side of the `where` if `T` is
anything other than `.Self`. After the `where`, the value of a `.Self`
could be `T` or could be the value of `.Self` before the `impls`
constraint: the so-called top-level value of `.Self`.
Implicit use of `.Self` in designators is always allowed, and they are
bound (and replaced by a reference) to the inner-most possible value of
`.Self`. On the right-hand side of the nested `where` above, they have
the value `T as X`.
`.Self impls ...` is also always allowed, since it acts more as a
keyword here, and it always refers to the inner-most possible value of
`.Self`.
Any other explicit use of `.Self` is diagnosed when ambiguous, in any
kind of constraint. This is done in the handling of `WhereExpr` since it
has enough context to allow `.Self impls` (which is an explicit use)
while disallowing other explicit uses. And because it has non-canonical
instructions to work with, so it is able to diagnose errors with precise
locations.
Since `.Self` is no longer going to be marked with depth modifiers, the
eval of `WhereExpr` does not need an input facet value instruction
representing `.Self` to compare with, as they are now going to all be
equivalent. So revert it back to just looking for the `PeriodSelf` name
id, through a shared helper being introduced as `IsPeriodSelf`. And drop
the period self InstId from the `WhereExpr` instruction. This causes
most of the formatted SemIR changes.
Move helpers for working with and replacing `.Self` to their own file,
out of the `facet_type.h` header/cpp files. These are working with
`.Self` facet values more than facet types, though `.Self` is a name
that only exists inside the scope of a facet type.
Dana Jansens
