### Context & Motivation
The error handling utilities in `//base/error.h` are very useful for
writing code with strong safety guarantees. While hardening the `Dump`
debug utilities (from review in #4866), I encountered a rough edge with
references and pointers. After a [brief Discord discussion in
#contributing-help](https://discord.com/channels/655572317891461132/1052653651895779359/1334675462877610038),
it was suggested that adding support for references to `ErrorOr` would
be a good candidate to move forward.
Using a reference type with the `ErrorOr` class (e.g. `ErrorOr<Node&>`)
produces two errors:
<ol>
<li><strong><code>variant can not have a reference type as an
alternative</code></strong>
<ul><li>From private field: <code>std::variant<Error, T>
val_;</code></li></ul>
</li>
<li><strong><code>'operator->' declared as a pointer to a
reference</code></strong>
<ul><li>From member function: <code>auto operator->() ->
T*</code></li></ul>
</li>
</ol>
### Changes
To support reference types, both errors are resolved:
1. `std::reference_wrapper` is conditionally used for storage when `T`
is a reference type
2. type trait aliases like `using ValueT = std::remove_reference_t<T>`
are used to produce compatible types for methods like `auto operator->()
-> ValueT*`
This adds a RawStringOstream. Versus TestRawOstream, which is
consolidated over to RawStringOstream, it uses a string for storage
instead of a vector, mainly to support move-to-string semantics. Versus
llvm::raw_string_ostream, it owns the string and supports pwrite (which
is needed for driver and its fd_ostream compatibility requirement).
This converts most uses of llvm::raw_string_ostream, leaving behind a
few in InstNamer that explicitly cannot own the string, such as:
```
llvm::raw_string_ostream(name)
<< "_" << tree.tokens().GetColumnNumber(token);
```
I have this as its own library so that it can use CHECK.
Yes this doesn't save much code, but it's code we repeatedly write.
---------
Co-authored-by: Geoff Romer <gromer@google.com>
This changes to an `Error` return to let the driver do the "error: "
prefix, except for one case with `help` that needs more work to change
(I'm not planning on picking up that TODO). It also changes
capitalization, backtick use, and a few minor punctuation things to try
to better match the diagnostic style.
This also adds `Error` matchers so that the changes to command line
testing are clearer.
Rationale: this convention avoids forcing closely-related code to be far
apart in the namespace hierarchy, and vice versa. By the same token, it
makes the namespace hierarchy more consistent with the directory
hierarchy.
Replacing direct raw_string_ostream uses. I figure the wrapper should be used more consistently.
There are still remaining raw_string_ostream uses that weren't compatible -- I'm continuing to look at those, but felt it was cleaner to have this on its own.
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.
Calvin
Jon Ross-PerkinsandGeoff Romer
CanftIn
