librustc: Apply null pointer optimization to slices, closures and trait
objects.
librustc: Remove match arm since we don't allow enum to float casts.
librustc: Don't create &[T] slices with NULL as the ptr.
Add tests for null pointer opt.
libcore: Fix Items iterator for zero sized types.
librustc: Disallow modules and types from having the same name.

This will break code that looks like:

    struct Foo {
        ...
    }

    mod Foo {
        ...
    }

Change this code to:

    struct Foo {
        ...
    }

    impl Foo {
        ...
    }

Or rename the module.

Closes #15205.

[breaking-change]
Improve non-exhaustive pattern witnesses for structs with multiple fields
librustc: Update debuginfo.
auto merge of #15406 : luqmana/rust/nop, r=pcwalton

Extend the null ptr optimization to work with slices, closures, procs, & trait objects by using the internal pointers as the discriminant.

This decreases the size of `Option<&[int]>` (and similar) by one word.
mk: Run tidy after tests in 'make check'

Tidy takes like forever to run.
auto merge of #15443 : pcwalton/rust/module-and-type-with-same-name, r=nick29581

This will break code that looks like:

    struct Foo {
        ...
    }

    mod Foo {
        ...
    }

Change this code to:

    struct Foo {
        ...
    }

    impl Foo {
        ...
    }

Or rename the module.

Closes #15205.

[breaking-change]

r? @nick29581
auto merge of #15508 : jakub-/rust/struct-pattern-witness, r=alexcrichton
auto merge of #15516 : brson/rust/tidy, r=alexcrichton

Tidy takes like forever to run. How many times have I wished I didn't have to sit through tidy before seeing my tests fail?
rustc: Fix naming output files with --crate-name

The output file was only being renamed based off #[crate_name], not #[crate_id]
or --crate-name. Both of these behaviors have been restored now.
auto merge of #15518 : alexcrichton/rust/fix-some-crate-names, r=sfackler

The output file was only being renamed based off #[crate_name], not #[crate_id]
or --crate-name. Both of these behaviors have been restored now.
std: Rename the `ToStr` trait to `ToString`, and `to_str` to `to_string`.

[breaking-change]