Provide common infrastructure for writing Sway code analyzers and generators

[ironcev]

A need for common infrastructure for writing Sway code analyzers and generators, or in general, tools built on top of the Sway compiler, was first pointed out by @tritao in this comment.

Description

Static analyzers and code generators built on top of Sway can have some, or all of these common needs:

• finding elements in the lexed or typed trees.
• navigating between the lexed and typed trees. E.g., finding a trait impl in the lexed tree and asking for the typed declaration of the trait.
• navigating between related elements. E.g., finding usages of a symbol.
• modifying lexed or typed trees.
• generating source code out of lexed or typed trees.

A typical example of a tool having all these needs would be the Sway equivalent of Rust's Clippy.

Current Examples

sway-analyzer implements higher level abstractions, Context structs, that wrap sway_ast types and decorates them with additional contextual information, like, e.g., access to the module in which they are defined. Finding elements of interest is done via combination of the Visitor pattern and numerous helper functions found in the util module. The analyzer does not modify code.

Charcoal, a Solidity-to-Sway transpiler, implements a higher level abstraction that represents parts of the Sway AST. While transpiling, it creates instances of those higher level abstractions based on Solidity code. The resulting tree is in the end formatted to text in what resembles a simplified version of the swayfmt.

forc-migrate provides a simple abstraction for matching parts of lexed or typed trees, locating equivalent elements across the trees, as well as an abstraction for modifying lexed trees. All the abstractions operate directly on sway_ast types. It generates code out of modified lexed trees by using swayfmt. The major constraint in implementing the API for matching, locating, and modifying was invested effort. The goal was to come up with a simple yet extensible API, that will simplify writing code migrations.

Prior Art

• ast-grep - a polyglot tool for code structural search, lint, and rewriting.
• Clang LibTooling - a library to support writing standalone tools based on Clang.
• Clang Transformer - a framework for writing C++ diagnostics and program transformations.
• Clang LibASTMatchers - a domain-specific language to create predicates on Clang's AST.
• Clippy Syntax Tree Patterns - a domain-specific language to describe Clippy lints using syntax tree patterns.
• Roslyn - C# and Visual Basic compilers with an API surface for building code analysis tools.