This PR implements the `returned var` for the explorer.
- Enabled `returned var` and `return var` syntax in lexer and parser.
- Split `Return` statement into `ReturnVar` and `ReturnExpression` to distinguish the two types of returns.
- Added logic in name resolution, type checking and interpretation to process `returned var` definition and `ReturnVar` statement.
This causes compile-time evaluation to give the same result as runtime
evaluation wherever possible. In particular, referencing a generic
parameter at compile time will resolve to the actual value if we're
evaluating a call to the enclosing function so a value for the
parameter is available in the call frame.
Clean up recently-added support for `SymbolicWitness`es using this.
This renames `Witness` to `ImplWitness` and adds a new form, `SymbolicWitness`, that holds an expression by which a witness can be computed. A common base class `Witness` is provided.
We form the new kind of witness when evaluation of a witness expression fails because the witness is not in scope, as happens when evaluating a subexpression such as a type expression in isolation, and retry evaluation in the larger context when the interpreter performs type instantiation when running the code.
This allows us to properly handle compile-time evaluation of constructs involving witness table lookups when the witness can be statically determined. The intent is that we will eventually also form symbolic witness table references when impl selection finds a non-final witness, in order to support specialization.
Simplify `NominalClassValue`: it can now always store a witness map rather than either a witness map or a witness-or-witness-expression map.
* added documentation for tracing output, changed the printing of scopes to be part of the stack, made other minor changes hoping to enhance readability
* reformat a list in the README
* minor edit
* fix the lists
* updates to the interpreter/README.md
I'm doing this to avoid macro name conflicts, following https://google.github.io/styleguide/cppguide.html#Preprocessor_Macros: "If you do export a macro from a header, it must have a globally unique name. To achieve this, it must be named with a prefix consisting of your project's namespace name (but upper case)."
Commands run:
```
sed -i 's/\(DCHECK\|CHECK\|FATAL\|MAKE_UNIQUE_NAME\|MAKE_UNIQUE_NAME_IMPL\|RAW_EXITING_STREAM\|RETURN_IF_ERROR\|RETURN_IF_ERROR_IMPL\|ASSIGN_OR_RETURN\|ASSIGN_OR_RETURN_IMPL\|DIAGNOSTIC_KIND\|RETURN_IF_STACK_LIMITED\)(/CARBON_\1(/g' $(git ls-files *.cpp *.h *.lpp *.ypp *.def ':!third_party')
sed -i 's/#undef DIAGNOSTIC_KIND/#undef CARBON_DIAGNOSTIC_KIND/' toolchain/diagnostics/diagnostic_registry.def
```
Note this isn't *quite* everything, but it's intended to be a large pass at everything:
```
╚╡git grep '#define ' *.cpp *.h *.lpp *.ypp *.def ':!third_party' | grep -v '#define CARBON' | grep -v _H_
explorer/syntax/lexer.lpp: #define YY_USER_ACTION \
explorer/syntax/lexer.lpp: #define SIMPLE_TOKEN(name) \
explorer/syntax/lexer.lpp: #define ARG_TOKEN(name, arg) \
explorer/syntax/parse_and_lex_context.h:#define YY_DECL \
migrate_cpp/cpp_refactoring/var_decl.cpp:#define ABSTRACT_TYPE(Class, Base)
migrate_cpp/cpp_refactoring/var_decl.cpp:#define TYPE(Class, Base) \
```
We may in particular want to do a pass to clean up #ifdef guards and make them be CARBON_ rooted.
Richard Smith
SlaterLatiao
Jeremy G. Siek
Jon Meow