Adds a new runtime unwinding function that encapsulates the printing of the words "explicit failure" when `fail!()` is called w/o arguments.
The before/after optimized assembly:
```
leaq "str\"str\"(1412)"(%rip), %rax
movq %rax, 24(%rsp)
movq $16, 32(%rsp)
leaq "str\"str\"(1413)"(%rip), %rax
movq %rax, 8(%rsp)
movq $19, 16(%rsp)
leaq 24(%rsp), %rdi
leaq 8(%rsp), %rsi
movl $11, %edx
callq _ZN6unwind12begin_unwind21h15836560661922107792E
```
```
leaq "str\"str\"(1369)"(%rip), %rax
movq %rax, 8(%rsp)
movq $19, 16(%rsp)
leaq 8(%rsp), %rdi
movl $11, %esi
callq _ZN6unwind31begin_unwind_no_time_to_explain20hd1c720cdde6a116480dE@PLT
```
Before/after filesizes:
rwxrwxr-x 1 brian brian 21479503 Jul 20 22:09 stage2-old/lib/librustc-4e7c5e5c.so
rwxrwxr-x 1 brian brian 21475415 Jul 20 22:30 x86_64-unknown-linux-gnu/stage2/lib/librustc-4e7c5e5c.so
This is the lowest-hanging fruit in the fail-bloat wars. Further fixes are going to require harder tradeoffs.
r? @pcwalton
- Made custom syntax extensions capable of expanding custom macros by moving `SyntaxEnv` into `ExtCtx`
- Added convenience method on `ExtCtx` for getting a macro expander.
- Made a few things private to force only a single way to use them (through `ExtCtx`)
- Removed some ancient commented-out code
Closes#14946
Closes#15690 (Guide: improve error handling)
Closes#15729 (Guide: guessing game)
Closes#15751 (repair macro docs)
Closes#15766 (rustc: Print a smaller hash on -v)
Closes#15815 (Add unit test for rlibc)
Closes#15820 (Minor refactoring and features in rustc driver for embedders)
Closes#15822 (rustdoc: Add an --extern flag analagous to rustc's)
Closes#15824 (Document Deque trait and bitv.)
Closes#15832 (syntax: Join consecutive string literals in format strings together)
Closes#15837 (Update LLVM to include NullCheckElimination pass)
Closes#15841 (Rename to_str to to_string)
Closes#15847 (Purge #[!resolve_unexported] from the compiler)
Closes#15848 (privacy: Add publically-reexported foreign item to exported item set)
Closes#15849 (fix string in from_utf8_lossy_100_multibyte benchmark)
Closes#15850 (Get rid of few warnings in tests)
Closes#15852 (Clarify the std::vec::Vec::with_capacity docs)
Emit a single rt::Piece per consecutive string literals. String literals
are split on {{ or }} escapes.
Saves a small amount of static storage and emitted code size.
