This lets rustc users tweak whether the linker should relax ELF relocations,
namely whether it should emit R_X86_64_GOTPCRELX relocations instead of
R_X86_64_GOTPCREL, as the former is allowed by the ABI to be further
optimised. The default value is whatever the target defines.

I found the description as it stands a bit confusing. I've added a bit more explanation to make it clear that a trailing line ending does not produce a final empty line.

Added fixes to documentation of `BitAnd`, `BitOr`, `BitXor` and
`BitAndAssign`, where the documentation for implementation on
`Vector<bool>` was using logical operators in place of the bitwise
operators.

r? @steveklabnik
cc rust-lang#78619

Partial comparison and total_cmp are not equal. This helps
preventing the mistake of creating float wrappers that
base their Ord impl on total_cmp and their PartialOrd impl on
the PartialOrd impl of the float type. PartialOrd and Ord
are required to agree with each other.

These methods were stabilized for the integer atomics in rust-lang#71843, but the methods
were not added for the non-integer atomics `AtomicBool` and `AtomicPtr`.

Add support for SHA256 source file hashing

Adds support for `-Z src-hash-algorithm sha256`, which became available in LLVM 11.

Using an older version of LLVM will cause an error `invalid checksum kind` if the hash algorithm is set to sha256.

r? @eddyb
cc rust-lang#70401 @est31

…ron1011

Add a test for compiletest rustc-env & unset-rustc-env directives

... and move compiletest meta tests into a separate directory.

Implement -Z relax-elf-relocations=yes|no

This lets rustc users tweak whether the linker should relax ELF relocations without recompiling a whole new target with its own libcore etc.

Clarify handling of final line ending in str::lines()

I found the description as it stands a bit confusing. I've added a bit more explanation to make it clear that a trailing line ending does not produce a final empty line.

Do not remove tokens before AST json serialization

`TokenStripper` is error-prone and introduces one more use of `MutVisitor`.
It's much simpler to treat serialization as just one more place that wants lazy token stream to turn into a real token stream.
Also, no code is better than more code, in general.
r? @Aaron1011

(I also merged tests for `TokenStripper` ICEs into one.)

Trivial fixes to bitwise operator documentation

Added fixes to documentation of `BitAnd`, `BitOr`, `BitXor` and
`BitAndAssign`, where the documentation for implementation on
`Vector<bool>` was using logical operators in place of the bitwise
operators.

r? @steveklabnik
cc rust-lang#78619

Point out that total_cmp is no strict superset of partial comparison

Partial comparison and total_cmp are not equal. This helps
preventing the mistake of creating float wrappers that
base their Ord impl on total_cmp and their PartialOrd impl on
the PartialOrd impl of the float type. PartialOrd and Ord
[are required to agree with each other](https://doc.rust-lang.org/std/cmp/trait.Ord.html#how-can-i-implement-ord).

Add fetch_update methods to AtomicBool and AtomicPtr

These methods were stabilized for the integer atomics in rust-lang#71843, but the methods were not added for the non-integer atomics `AtomicBool` and `AtomicPtr`.