The `fallback` setting for import-std-module is supposed to allow running
expression that require an imported C++ module without causing any regressions
for users (neither in terms of functionality nor performance). This is done by
first trying to normally parse/evaluate an expression and when an error occurred
during this first attempt, we retry with the loaded 'std' module.
When we run into a system with a 'std' module that for some reason doesn't build
or otherwise causes parse errors, then this currently means that the second
parse attempt will overwrite the error diagnostics of the first parse attempt.
Given that the module build errors are outside of the scope of what the user can
influence, it makes more sense to show the errors from the first parse attempt
that are only concerned with the actual user input.
Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D110696
By now LLDB can import the 'std' C++ module to improve expression evaluation,
but there are still a few problems to solve before we can do this by default.
One is that importing the C++ module is slightly slower than normal expression
evaluation (mostly because the disk access and loading the initial lookup data
is quite slow in comparison to the barebone Clang setup the rest of the LLDB
expression evaluator is usually doing). Another problem is that some complicated
types in the standard library aren't fully supported yet by the ASTImporter, so
we end up types that fail to import (which usually appears to the user as if the
type is empty or there is just no result variable).
To still allow people to adopt this mode in their daily debugging, this patch
adds a setting that allows LLDB to automatically retry failed expression with a
loaded C++ module. All success expressions will behave exactly as they would do
before this patch. Failed expressions get a another parse attempt if we find a
usable C++ module in the current execution context. This way we shouldn't have
any performance/parsing regressions in normal debugging workflows, while the
debugging workflows involving STL containers benefit from the C++ module type
info.
This setting is off by default for now with the intention to enable it by
default on macOS soon-ish.
The implementation is mostly just extracting the existing parse logic into its
own function and then calling the parse function again if the first evaluation
failed and we have a C++ module to retry the parsing with.
Reviewed By: shafik, JDevlieghere, aprantl
Differential Revision: https://reviews.llvm.org/D92784
