[SR-10506] Formalize getType closure pattern

[beccadax]

Previous ID	SR-10506
Radar	None
Original Reporter	@beccadax
Type	Task

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Task, StarterBug, TypeChecker
Assignee	None
Priority	Medium

md5: dd815550dade98eb89026bfbc9fc1d2f

Issue Description:

Various methods in the compiler from swiftAST up take one or more closure parameters which are used to override how the types of AST nodes (mainly expressions) are accessed. In practice, these are mainly used to make a function look up types in a ConstraintSystem during type checking instead of using the values in the nodes themselves. A non-exhaustive list of examples:

• AbstractClosureExpr::getResultType() and CallExpr::create() take a "getType" closure.

• TypeExpr::getInstanceType() takes two closures for "hasType" and "getType".

• LiteralExpr::shallowClone() takes two closures for "getType" and "setType".

• Expr::dump() takes three closures for "getType", "getTypeOfTypeLoc", and "getTypeOfKeyPathComponent". These are plumbed through to various internal types in ASTDumper.cpp.

Rather than continuing to pass around ad-hoc closures, we should formalize this interface into a class that we can pass around. For example, we might declare an "ExprTypeMap" class in AST which declares all of the hasType()/getType()/setType() methods from ConstraintSystem and simply calls through to the equivalent methods on the AST nodes; then we can have ConstraintSystem inherit from this class and override these methods.

[slavapestov]

I like the idea of defining a data type in lib/AST/ that stores an expression type map. In fact, Expr::getType() doesn’t need to exist at all; CSApply can walk the type map given to it by the solver and simplify each type to eliminate variables, and SILGen can simply read from this map when generating SIL from expressions.