e3e7446305 Add lifetimebound to attributes for general-purpose usage (Cory Fields)
1d58cc7cb0 span: add lifetimebound attribute (Cory Fields)
62733fee87 span: (almost) match std::span's constructor behavior (Cory Fields)
Pull request description:
Replaces #19382 with a different approach. See [this comment](https://github.com/bitcoin/bitcoin/pull/19382#discussion_r446332852) for the reasoning behind the switch.
--
Description from #19382:
See [here](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf) for more detail on lifetimebound.
This is implemented using preprocesor macros rather than configure checks in order to keep span.h self-contained.
The ```[[clang::lifetimebound]]``` syntax was chosen over ```__attribute__((lifetimebound))``` because the former is more flexible and works to guard ```this``` as well as function parameters, and also because at least for now, it's available only in clang.
There are currently no violations in our codebase, but this can easily be tested by inserting one like this somewhere and compiling with a modern clang:
```c++
Span<const int> bad(std::vector<int>{1,2,3});
```
The result:
> warning: temporary whose address is used as value of local variable 'bad' will be destroyed at the end of the full-expression [-Wdangling]
Span<const int> bad(std::vector<int>{1,2,3});
```
ACKs for top commit:
sipa:
ACK e3e7446305
ajtowns:
ACK e3e7446305 (drive by; only a quick skim of code and some basic sanity checks)
MarcoFalke:
review ACK e3e7446305🔗
jonatack:
ACK e3e7446 change since last review is adding `[[clang::lifetimebound]]` as `LIFETIMEBOUND` to src/attributes.h as suggested in https://github.com/bitcoin/bitcoin/pull/19387#issuecomment-650752959.
Tree-SHA512: 05a3440ee595ef0e8d693a2820b360707695c016a68e15df47c20cd8d053646cc6c8cca8addd7db40e72b3fce208879a41c8102ba7ae9223e4366e5de1175211
This prevents constructing a Span<A> given two pointers into an array
of B (where B is a subclass of A), at least without explicit cast to
pointers to A.
c31cbe7cfe Add C++17 test to Travis (Pieter Wuille)
7829685e27 Add configure option for c++17 (Pieter Wuille)
0fbde488b2 Support conversion between Spans of compatible types (Pieter Wuille)
7cbfebbf3d Update ax_cxx_compile_stdcxx.m4 (Pieter Wuille)
Pull request description:
This adds a `--enable-c++17` option to the configure script, fixes the only C++17 incompatibility (with a commit taken from #18468), and adds a Travis test for it.
This is all off by default, and release builds remain C++11.
It implements the first step of the plan in https://github.com/bitcoin/bitcoin/issues/16684.
ACKs for top commit:
elichai:
tACK c31cbe7cfe
practicalswift:
Tested ACK c31cbe7cfe
hebasto:
ACK c31cbe7cfe, tested on Linux Mint 19.3 both C++11 and C++17 modes. Compiled and passed tests locally.
Tree-SHA512: a4b00776dbceef9c12abbb404c6bcd48f7916ce24c8c7a14116355f64e817578b7fcddbedd5ce435322319d1e4de43429b68553f4d96d970c308fe3e3e59b9d1
MarcoFalke