This lint is IMO inherently ill-conceived.
I have looked for the reasons why this might be thought to be a good
idea and there were basically two (and they are sort of contradictory):
I. "Calling ‘.clone()` on an Rc, Arc, or Weak can obscure the fact
that only the pointer is being cloned, not the underlying data."
This is the wording from
https://rust-lang.github.io/rust-clippy/v0.0.212/#clone_on_ref_ptr
It is a bit terse; we are left to infer why it is a bad idea to
obscure this fact. It seems to me that if it is bad to obscure some
fact, that must be because the fact is a hazard. But why would it be
a hazard to not copy the underlying data ?
In other languages, faliing to copy the underlying data is a serious
correctness hazard. There is a whose class of bugs where things were
not copied, and then mutated and/or reused in multiple places in ways
that were not what the programmer intended. In my experience, this is
a very common bug when writing Python and Javascript. I'm told it's
common in golang too.
But in Rust this bug is much much harder to write. The data inside an
Arc is immutable. To have this bug you'd have use interior mutability
- ie mess around with Mutex or RefCell. That provides a good barrier
to these kind of accidents.
II. "The reason for writing Rc::clone and Arc::clone [is] to make it
clear that only the pointer is being cloned, as opposed to the
underlying data. The former is always fast, while the latter can
be very expensive depending on what is being cloned."
This is the reasoning found here
https://github.com/rust-lang/rust-clippy/issues/2048
This is saying that *not* using Arc::clone is hazardous.
Specifically, that a deep clone is a performance hazard.
But for this argument, the lint is precisely backwards. It's linting
the "good" case and asking for it to be written in a more explicit
way; while the supposedly bad case can be written conveniently.
Also, many objects (in our codebase, and in all the libraries we use)
that are Clone are in fact simply handles. They contain Arc(s) (or
similar) and are cheap to clone. Indeed, that is the usual case.
It does not make sense to distinguish in the syntax we use to clone
such a handle, whether the handle is a transparent Arc, or an opaque
struct containing one or more other handles.
Forcing Arc::clone to be written as such makes for code churn when a
type is changed from Arc<Something> to Something: Clone, or vice
versa.
We had no function to infallibly convert BoundedInt32<{0 or 1},H>
into a u32, even though we could have. Because of that, we were
treating weight_scale as an i32 when logically it's a u32 or a
NonZeroU32.
Moreover, it turns out we were using an incorrect minimum for the
bwweightscale param, which would in theory have allowed the
authorities to make us divide by zero.
This patch introduces the necessary From<> implementation and uses
it. It corrects the binimum bwweightscale, and prevents a
division-by-zero issue in case weight_scale is zero.
This is mostly a finger exercise, and an experiment in "what does
grcov consider to be coverage". Here's what I've found out...
* In grcov's eyes, most #[derive(Foo)] lines count as containing code;
but calling any one derived function counts as calling those lines.
* Unlike with tarpaulin, it is actually possible to reach 100% grcov
line coverage. (Tarpaulin likes to pick "}" lines and tell you that
you never reached them; or sometimes it picks expression
statements that have the effect of a return, and tells you that
they're unreached. Even with these tests, tarpaulin claims that
the line coverage of tor-units is only 97.3%.)
* In rust, it may be a bit hopeless trying to get high function
coverage. Even though we've hit every line of the tor-units crate,
the function coverage from its own tests is only 9.38% (55.41%
from other crates). I think this is probably due to derived
functions, or maybe due to generics getting instantiated?
I've got no idea; the denominator for the function coverage
lines fluctuates oddly.
There are some missing parts here (like persistence and tests)
and some incorrect parts (I am 90% sure that the "exploratory
circuit" flag is bogus). Also it is not integrated with the circuit
manager code.
Previously, we'd have to declare the field for a parameter in one
place, its default in a second, and its consensus key in a third.
That's error-prone and not so fun! This patch changes the
way we declare parameters so that we declare a structure once,
and macros expand it to all do the right thing.
This required a few new traits and implementations to ensure
uniformity across the types that can go in parameters: We need every
parameter type to implement TryFrom<i32> and to implement
SaturatingFromInt32.
Eventually we might want SaturatingFromInt32 to be a more generic
SaturatingFrom, but that's not for now.
Ian Jackson
Nick Mathewson
Daniel Eades