5b2e8693e4
See Issue 8142 for discussion.
This makes it illegal for a Drop impl to be more specialized than the
original item.
So for example, all of the following are now rejected (when they would
have been blindly accepted before):
```rust
struct S<A> { ... };
impl Drop for S<i8> { ... } // error: specialized to concrete type
struct T<'a> { ... };
impl Drop for T<'static> { ... } // error: specialized to concrete region
struct U<A> { ... };
impl<A:Clone> Drop for U<A> { ... } // error: added extra type requirement
struct V<'a,'b>;
impl<'a,'b:a> Drop for V<'a,'b> { ... } // error: added extra region requirement
```
Due to examples like the above, this is a [breaking-change].
(The fix is to either remove the specialization from the `Drop` impl,
or to transcribe the requirements into the struct/enum definition;
examples of both are shown in the PR's fixed to `libstd`.)
----
This is likely to be the last thing blocking the removal of the
`#[unsafe_destructor]` attribute.
Includes two new error codes for the new dropck check.
Update run-pass tests to accommodate new dropck pass.
Update tests and docs to reflect new destructor restriction.
----
Implementation notes:
We identify Drop impl specialization by not being as parametric as the
struct/enum definition via unification.
More specifically:
1. Attempt unification of a skolemized instance of the struct/enum
with an instance of the Drop impl's type expression where all of
the impl's generics (i.e. the free variables of the type
expression) have been replaced with unification variables.
2. If unification fails, then reject Drop impl as specialized.
3. If unification succeeds, check if any of the skolemized
variables "leaked" into the constraint set for the inference
context; if so, then reject Drop impl as specialized.
4. Otherwise, unification succeeded without leaking skolemized
variables: accept the Drop impl.
We identify whether a Drop impl is injecting new predicates by simply
looking whether the predicate, after an appropriate substitution,
appears on the struct/enum definition.
% The Rust Programming Language
Welcome! This book will teach you about the Rust Programming Language. Rust is a modern systems programming language focusing on safety and speed. It accomplishes these goals by being memory safe without using garbage collection.
"The Rust Programming Language" is split into three sections, which you can navigate through the menu on the left.
Basics
This section is a linear introduction to the basic syntax and semantics of Rust. It has individual sections on each part of Rust's syntax, and culminates in a small project: a guessing game.
After reading "Basics," you will have a good foundation to learn more about Rust, and can write very simple programs.
Intermediate
This section contains individual chapters, which are self-contained. They focus on specific topics, and can be read in any order.
After reading "Intermediate," you will have a solid understanding of Rust, and will be able to understand most Rust code and write more complex programs.
Advanced
In a similar fashion to "Intermediate," this section is full of individual, deep-dive chapters, which stand alone and can be read in any order. These chapters focus on the most complex features, as well as some things that are only available in upcoming versions of Rust.
After reading "Advanced," you'll be a Rust expert!