Rename assert_eq arguments to left and right.

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.
doc: grammar fix
Update sample code to use `getopts::usage` in `print_usage`
Fix an ICE when trying to resolve a struct variant

Unlike a tuple variant constructor which can be called as a function, a
struct variant constructor is not a function, so cannot be called.

If the user tries to assign the constructor to a variable, an ICE
occurs, because there is no way to use it later. So we should stop the
constructor from being used like that.

A similar mechanism already exists for a normal struct, as it prohibits
a struct from being resolved. This commit does the same for a struct
variant.

This commit also includes some changes to the existing tests.

Fixes #19452.
Fix po4a conf file

This line was declared twice, which causes the build of i10n docs to
fail.
Fix inappropriate ## headings

Fixes #15499.
Fix up some {ignore} and {notrust}s

These should be properly annotated instead.

Fixes #16219.
Change default installation directory to %SYSTEMDRIVE%\Rust.
Modify user PATH, rather than system PATH.
Update intro.md
gdb < RUST_BACKTRACE

Fixes #19717
reference: type definition -> type alias

Fixes #9266
Add comments with type annotations.

This will hopefully help people with their first steps in Rust.

Fixes #16143.
Remove mentions of sundown.
Add LLVM ExecutionEngine API
Perform compiler state reset in phase_1
Use rust-installer for installation

This is just a refactoring of the current installer so that Rust and Cargo
use the same codebase.

cc #16456
serialize: Avoid stray nul characters when auto-serializing char.

Fixes #19719.
Add coercions from *mut to *const and from &mut to *const.
Add doctests for `iter` and `into_iter` for BTreeSet.
rustdoc: Properly inline const items

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821