…t()) (clippy::mem_replace_with_default)

This fixes rust-lang/wg-cargo-std-aware#53

Now users will be able to do:
```
cargo build -Zbuild-std=core -Zbuild-std-features=compiler-builtins-mem
```
and correctly get the Rust implemenations for `memcpy` and friends.

Signed-off-by: Joe Richey <[email protected]>

…k() in std::sys::unix::process::Command::spawn()

`assert!` panics on failure, which is not signal-safe.

mem::ManuallyDrop::new -> ManuallyDrop::new

No functional changes intended.

more tiny clippy cleanups

commits stand alone and can be reviewed one by one

BTreeMap: keep an eye out on the size of the main components

r? @Mark-Simulacrum

Ensure that all LLVM components requested by tests are available on CI

Addresses #75064 (comment)

I used an environment variable because passing a command line option all the way from CI to compiletest would be just too much hassle for this task.
I added a new variable, but any of the already existing ones defined by CI could be used instead.
r? @Mark-Simulacrum

library: Forward compiler-builtins "mem" feature

This fixes rust-lang/wg-cargo-std-aware#53

Now users will be able to do:
```
cargo build -Zbuild-std=core -Zbuild-std-features=compiler-builtins-mem
```
and correctly get the Rust implemenations for `memcpy` and friends.

Signed-off-by: Joe Richey <[email protected]>

liveness: Use Option::None to represent absent live nodes

No functional changes intended.

Add unstable book docs for `-Zunsound-mir-opts`

The `-Zunsound-mir-opts` flag was added in #76899.

Use `rtassert!` instead of `assert!` from the child process after fork() in std::sys::unix::process::Command::spawn()

As discussed in #73894, `assert!` panics on failure, which is not signal-safe, and `rtassert!` is a suitable replacement.

Fixes #73894.

r? @Amanieu @cuviper @joshtriplett

Add test for async/await combined with const-generics.

Fixes #74906.

Fix typo in alloc vec comment

cc @the8472

Alloc vec use imported path

mem::ManuallyDrop::new -> ManuallyDrop::new

cc @the8472

Add test for issue #74761

Adds test for and closes #74761 which previously crashed compiler.

Update books

## rust-by-example

5 commits in 19f0a0372af497b34369cf182d9d16156cab2969..7d3ff1c12db08a847a57a054be4a7951ce532d2d
2020-08-26 09:38:48 -0300 to 2020-09-28 15:54:25 -0300
- Prevent duplicated error information in "wrapping errors" example (rust-lang/rust-by-example#1375)
- Avoid lifetime annotation in struct example (rust-lang/rust-by-example#1378)
- Clone.md comment and variable name change (rust-lang/rust-by-example#1379)
- Replace panic example with a simpler version (rust-lang/rust-by-example#1380)
- Prefer `const` over `static` (rust-lang/rust-by-example#1381)

## embedded-book

1 commits in 0cd2ca116274b915924c3a7e07c1e046b6f19b77..dd310616308e01f6cf227f46347b744aa56b77d9
2020-08-19 10:33:15 +0000 to 2020-09-26 08:54:08 +0000
- Add instructions for ARM v8 instruction sets  (rust-embedded/book#265)