lang: add locals and assignments

[zerbina]

Summary

• implement declarations for local variables (Decl)
• implement assignments (Asgn)

Details

Language

• scopes can now be nested
• declarations for locals are introduced
• assignments are introduced

The specification for assignments already covers the copy/move/ destruction behaviour, although there's current no way to test it.

Current Limitations

For the initial version, the additions are kept as simple as possible. Notably:

• all locals are mutable (there's no way to declare them as being immutable)
• a local declaration requires an initial value for the local
• the type cannot be specified explicitly (it's always inferred)
• an assignment with an l-value RHS is always a copy assignment
• names cannot be shadowed (i.e., a local cannot use the same name as an already declared procedure)

The plan is to lift/remove all of the above limitations/restrictions/ shortcomings at a later date.

Source2IL

• scope management is changed to accommodate nesting
• exprToIL now returns the ExprType type, which carries additional expression attributes. At the moment, this is only the Lvalue attribute
• the convenience + operator is added, for constructing ExprType values
• all result assignments in exprToIL are updated
• the convenience procedure scopedExprToIL is added, for analyzing an expression within a new nested scope

Tests are added to ensure the declarations and assignments work as specified.

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To-Do

• [x] ~look into removing the "local declaration can be used as expression" rule~
• [x] merge main into the PR once #46 and #47 are merged
• [x] add tests:
  • [x] declaration with void initializer expression
  • [x] declaration with preexisting name
  • [x] declaration with self-reference
  • [x] assignment with l- and r-value lhs
  • [x] assignment with l- and r-value rhs
  • [x] assignment with incompatible type
  • [x] assignment with different but compatible type
  • [x] evaluation order in assignments
  • [x] evaluation order in expression lists
• [x] write a proper commit message

[zerbina]

One thing that's also not present, when compared to NimSkull, is the style-insensitivity for identifiers. Personally, I like NimSkull's style-insensitivity rules, and they're something I would also add to Phy, but it's not something that should happen as part of this PR.

[zerbina]

In order to not treat declarations as expression, the language grammar grammar needs to be extended with groups, so that the Exprs grammar can be changed to:

(Exprs (<local_decl>* <expr>)+)

* matches greedily, which is why (Exprs <expr_or_decl>* <expr>) cannot work.

~I'll make a separate PR for this extension.~

Edit: For the moment, it's simpler to treat declarations as expressions, so I'm not going to change the current behaviour. Treating declarations as expression might also be the better long-term direction.