mandelbrot: fix overlapping buffers
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
Add examples for DList methods rotate_forward, rotate_backward, append, prepend and insert_when
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.
Removed the `_frozen` methods from dataflow API.

Calls to methods like `each_bit_on_entry_frozen` and
`each_gen_bit_frozen` now go to the `each_bit_on_entry` and
`each_gen_bit` methods.
auto merge of #15733 : sanxiyn/rust/use-from-type, r=alexcrichton

Importing from types was disallowed in #6462. Flag was set for paths whether it is a module or a type. Type flag was set when impl was seen. The problem is, for cross-crate situations, when reexport is involved, it is possible that impl is seen too late because metadata is loaded lazily.

Fix #15664.
auto merge of #15737 : huonw/rust/lint-level-here, r=pnkfelix

This allows lint traversals to emit more information (when a lint is
non-allow), or avoid doing expensive computations (when a lint is
allow).
rustc: #[crate_name] and --crate-name must match

Part of the original discussions around the `--crate-name` flag brought up that
mass confusion can arise when the flag specifies a different name than is
contained in the crate.

The current primary use case of the `--crate-name` flag is through cargo and
not requiring a `#[crate_name]` attribute, but if the `#[crate_name]` attribute
is specified it will likely go awry when the two names deviate from one another.
This commit requires that if both are provided they both match to prevent this
confusion.
librustc: Implement unboxed closures with mutable receivers
auto merge of #14539 : pcwalton/rust/unboxed-closures, r=pnkfelix

@nikomatsakis r?
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