mandelbrot: fix overlapping buffers
stability lint: ignore code from macro expansion

This small patch causes the stability lint to bail out when traversing
any AST produced via a macro expansion. Ultimately, we would like to
lint the contents of the macro at the place where the macro is defined,
but regardless we should not be linting it at the use site.

Closes #15703
btree: use pattern matching instead of unwrap
syntax: Add ToTokens for Option<T>
syntax: Add ToTokens for Attribute_
syntax: Generalize ToTokens impl for Vec<T>

It will now `flat_map` over the elements of a `Vec<T>` if
`T: ToTokens`
Disallow importing from types when reexport is involved
syntax: Add quote_method!
lint: add method to get level of a specific lint.

This allows lint traversals to emit more information (when a lint is
non-allow), or avoid doing expensive computations (when a lint is
allow).
syntax: Add ToTokens impl for Method
Rename functions in the CloneableVector trait

* Deprecated `to_owned` in favor of `to_vec`
* Deprecated `into_owned` in favor of `into_vec`

[breaking-change]
str: Add better tests for string slice's Chars iterator

Test using for ch s.chars() { black_box(ch) } to have a test that should
force the iterator to run its full decoding computations.
core::str: Implement Chars iterator using slice::Items

Re-use the vector iterator to implement the chars iterator.

The iterator uses our guarantee that the string contains valid UTF-8,
but its only unsafe code is transmuting the decoded u32 into char.
Add more tests for str Chars iterator

Test iterating (decoding) every codepoint.
Add examples for DList methods rotate_forward, rotate_backward, append, prepend and insert_when
auto merge of #15725 : aochagavia/rust/vec, r=alexcrichton

* Deprecated `to_owned` in favor of `to_vec`
* Deprecated `into_owned` in favor of `into_vec`

[breaking-change]
auto merge of #15726 : aturon/rust/macro-stability, r=alexcrichton

This small patch causes the stability lint to bail out when traversing
any AST produced via a macro expansion. Ultimately, we would like to
lint the contents of the macro at the place where the macro is defined,
but regardless we should not be linting it at the use site.

Closes #15703
auto merge of #15727 : fhahn/rust/remove-some-unwraps, r=alexcrichton

When looking through the `btree` code, I stumbled over a couple of `unwraps` that could be avoided.
auto merge of #15732 : bgamari/rust/to-tokens, r=alexcrichton

Here I add a `ToTokens` impl for `Attribute_` and `Option<T>`, as well as generalize the impl for `Vec<T>`
Removed `index_to_bitset` from the dataflow context.

Part of addressing 15019.