Your pseudo-code is pretty much correct. For this example, suppose we had a method call foo.bar()
where foo: T
. I’m going to use the fully qualified syntax (FQS) to be unambiguous about what type the method is being called with, e.g. A::bar(foo)
or A::bar(&***foo)
. I’m just going to write a pile of random capital letters, each one is just some arbitrary type/trait, except T
is always the type of the original variable foo
that the method is called on.
The core of the algorithm is:
- For each “dereference step”
U
(that is, setU = T
and thenU = *T
, …)- if there’s a method
bar
where the receiver type (the type ofself
in the method) matchesU
exactly , use it (a “by value method”) - otherwise, add one auto-ref (take
&
or&mut
of the receiver), and, if some method’s receiver matches&U
, use it (an “autorefd method”)
- if there’s a method
Notably, everything considers the “receiver type” of the method, not the Self
type of the trait, i.e. impl ... for Foo { fn method(&self) {} }
thinks about &Foo
when matching the method, and fn method2(&mut self)
would think about &mut Foo
when matching.
It is an error if there’s ever multiple trait methods valid in the inner steps (that is, there can be only be zero or one trait methods valid in each of 1. or 2., but there can be one valid for each: the one from 1 will be taken first), and inherent methods take precedence over trait ones. It’s also an error if we get to the end of the loop without finding anything that matches. It is also an error to have recursive Deref
implementations, which make the loop infinite (they’ll hit the “recursion limit”).
These rules seem to do-what-I-mean in most circumstances, although having the ability to write the unambiguous FQS form is very useful in some edge cases, and for sensible error messages for macro-generated code.
Only one auto-reference is added because
- if there was no bound, things get bad/slow, since every type can have an arbitrary number of references taken
- taking one reference
&foo
retains a strong connection tofoo
(it is the address offoo
itself), but taking more starts to lose it:&&foo
is the address of some temporary variable on the stack that stores&foo
.
Examples
Suppose we have a call foo.refm()
, if foo
has type:
X
, then we start withU = X
,refm
has receiver type&...
, so step 1 doesn’t match, taking an auto-ref gives us&X
, and this does match (withSelf = X
), so the call isRefM::refm(&foo)
&X
, starts withU = &X
, which matches&self
in the first step (withSelf = X
), and so the call isRefM::refm(foo)
&&&&&X
, this doesn’t match either step (the trait isn’t implemented for&&&&X
or&&&&&X
), so we dereference once to getU = &&&&X
, which matches 1 (withSelf = &&&X
) and the call isRefM::refm(*foo)
Z
, doesn’t match either step so it is dereferenced once, to getY
, which also doesn’t match, so it’s dereferenced again, to getX
, which doesn’t match 1, but does match after autorefing, so the call isRefM::refm(&**foo)
.&&A
, the 1. doesn’t match and neither does 2. since the trait is not implemented for&A
(for 1) or&&A
(for 2), so it is dereferenced to&A
, which matches 1., withSelf = A
Suppose we have foo.m()
, and that A
isn’t Copy
, if foo
has type:
A
, thenU = A
matchesself
directly so the call isM::m(foo)
withSelf = A
&A
, then 1. doesn’t match, and neither does 2. (neither&A
nor&&A
implement the trait), so it is dereferenced toA
, which does match, butM::m(*foo)
requires takingA
by value and hence moving out offoo
, hence the error.&&A
, 1. doesn’t match, but autorefing gives&&&A
, which does match, so the call isM::m(&foo)
withSelf = &&&A
.
(This answer is based on the code, and is reasonably close to the (slightly outdated) README. Niko Matsakis, the main author of this part of the compiler/language, also glanced over this answer.)