Soft casting with `if/is` #959

This commit introduces a new feature into the parser, typechecker, and formatter. The work for code gen will be in the next commit. I was able to leverage some existing infrastructure by making using of `AssignmentPattern`. A new field `is` was introduced into `IfBranch`. This field holds a generic `Option<Is>` meaning a new generic has to be introduced into `IfBranch`. When used in `UntypedExpr`, `IfBranch` must use `AssignmentPattern`. When used in `TypedExpr`, `IfBranch` must use `TypedPattern`. The parser was updated such that we can support this kind of psuedo grammar: `if <expr:condition> [is [<pattern>: ]<annotation>]` This can be read as, when parsing an `if` expression, always expect an expression after the keyword `if`. And then optionally there may be this `is` stuff, and within that you may optionally expect a pattern followed by a colon. We will always expect an annotation. This first expression is still saved as the field `condition` in `IfBranch`. If `pattern` is not there AND `expr:condition` is `UntypedExpr::Var` we can set the pattern to be `Pattern::Var` with the same name. From there shadowing should allow this syntax sugar to feel kinda magical within the `IfBranch` block that follow. The typechecker doesn't need to be aware of the sugar described above. The typechecker looks at `branch.is` and if it's `Some(is)` then it'll use `infer_assignment` for some help. Because of the way that `is` can inject variables into the scope of the branch's block and since it's basically just like how `expect` works minus the error we get to re-use that helper method. It's important to note that in the typechecker, if `is` is `Some(_)` then we do not enforce that `condition` is of type `Bool`. This is because the bool itself will be whether or not the `is` itself holds true given a PlutusData payload. When `is` is None, we do exactly what was being done previously so that plain `if` expressions remain unaffected with no semantic changes. The formatter had to be made aware of the new changes with some simple changes that need no further explanation.

Co-authored-by: Kasey White <kwhitemsg@gmail.com>

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Soft casting with `if/is` #959

Soft casting with `if/is` #959

Commits on Jun 25, 2024

Soft casting with if/is #959

Soft casting with if/is #959

Commits on Jun 25, 2024

Soft casting with `if/is` #959

Soft casting with `if/is` #959