Changed `extern crate foo as bar;` error message

Closes #17709
Remove `virtual` structs from the language
Remove `virtual` struct tests
Remove `virtual` structs from the reference
auto merge of #17952 : jakub-/rust/remove-virtual-structs, r=eddyb

Closes #17861.
Never expand specialized columns that only contain wild patterns in them

Doing so would incur deeply nested expansion of the tree with no useful
side effects. This is problematic for "wide" data types such as structs
with dozens of fields but where only a few are actually being matched or bound.
Most notably, matching a fixed slice would use a number of stack frames that
grows with the number of elements in the slice.

Fixes #17877.
libtime: `strftime` and `strptime` return type changed to `Result<String, ParseError>`.

`strftime` currently returns a `String`. This does not indicate that
this function may return an error due to to a malformed format string.
This change introduces a `ParseError` enum which indicates the type of
error that occurred. The return type of `strptime` was also changed to
use this new enum instead of returning `Result<String, String>`. Now,
all instances where `strftime` was used need to have their return value
checked to see if it were valid or not.

[breaking-change]
auto merge of #17944 : jakub-/rust/issue-17877, r=alexcrichton

Fixes #17877.
Update a test error message

 #17709
auto merge of #17467 : hatahet/rust/mybranch, r=alexcrichton



[breaking-change]
auto merge of #17757 : gamazeps/rust/issue17709, r=alexcrichton

I did not put the crate name in the error note, if that's necessary I'll look into it.

Closes #17709
Clean up rustc warnings.

compiletest: compact "linux" "macos" etc.as "unix".
liballoc: remove a superfluous "use".
libcollections: remove invocations of deprecated methods in favor of
    their suggested replacements and use "_" for a loop counter.
libcoretest: remove invocations of deprecated methods;  also add
    "allow(deprecated)" for testing a deprecated method itself.
libglob: use "cfg_attr".
libgraphviz: add a test for one of data constructors.
libgreen: remove a superfluous "use".
libnum: "allow(type_overflow)" for type cast into u8 in a test code.
librustc: names of static variables should be in upper case.
libserialize: v[i] instead of get().
libstd/ascii: to_lowercase() instead of to_lower().
libstd/bitflags: modify AnotherSetOfFlags to use i8 as its backend.
    It will serve better for testing various aspects of bitflags!.
libstd/collections: "allow(deprecated)" for testing a deprecated
    method itself.
libstd/io: remove invocations of deprecated methods and superfluous "use".
    Also add #[test] where it was missing.
libstd/num: introduce a helper function to effectively remove
    invocations of a deprecated method.
libstd/path and rand: remove invocations of deprecated methods and
    superfluous "use".
libstd/task and libsync/comm: "allow(deprecated)" for testing
    a deprecated method itself.
libsync/deque: remove superfluous "unsafe".
libsync/mutex and once: names of static variables should be in upper case.
libterm: introduce a helper function to effectively remove
    invocations of a deprecated method.

We still see a few warnings about using obsoleted native::task::spawn()
in the test modules for libsync.  I'm not sure how I should replace them
with std::task::TaksBuilder and native::task::NativeTaskBuilder
(dependency to libstd?)

Signed-off-by: NODA, Kai <nodakai@gmail.com>