Prevent copies when initializing value expression from reference
expression. This is based on
https://github.com/carbon-language/carbon-lang/pull/2006, which
introduces expression categories, and how it is possible to convert
to/from those different categories. Continuation of
https://github.com/carbon-language/carbon-lang/pull/2907
## Functional changes
* Initializing a value expression from a reference expression takes its
value without a copy
* Reading from the value expression causes an error if the value changed
from the time it was initialized
* In this situation, prevents a copy both for variable definitions, and
call parameter bindings
## Main implementation changes
* Add new `ExpressionCategoryAction`, which evaluates an expression and
returns an `ExpressionValue` containing its category and address (if
any), in addition to the resulting `Value*`
* `ExpressionAction`s now invokes `ExpressionCategoryAction` and unwraps
the returned `ExpressionValue`
* `RuntimeScope::BindAndPin` method, and corresponding when attempting
to read a `value_node`.
## Next work
* Avoid unnecessary copies from value expression to value expression,
after ensuring that even value expression temporaries are registered for
destruction (https://github.com/Pixep/carbon-lang/pull/9)
To filter based on file contexts, you can use `-trace_file_context=...`
flag along with `--trace_file=...` flag. These are the following options
you can pass to the `-trace_file_context=...` flag (you can also pass
them as list by separating them with comma).
1. `main`: Include trace output for file containing the main function.
2. `prelude`: Include trace output for prelude.
3. `import`: Include trace output for imports.
4. `include`: Include trace output for all.
If the flag isn't used or non of the options are passed, by default only
main file context is traced.
File contexts are distinguished based upon the source location by
passing the source location to `TraceStream::is_enabled(..)` as
arguments. If no arguments are passed, file context is
`FileContext::Unknown` and for now it will add the trace from unknown
file context.
Note: `import` option currently doesn't work as imports aren't supported
yet.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Created `explorer/trace_testdata/full_trace.carbon` to test the whole trace and some changes in `explorer/file_test.cpp` to treat `/trace_testdata/` tests differently.
Adrien Leravat
Prabhat SachdevaandRichard Smith