Notably this nails down part of the behavior that MaybeUninit assumes,
e.g. that a Option<MaybeUninit<&u8>> does not take advantage of non-zero
optimization, and thus is a safe construct.

These names aren't ever handled by resolve, so there's no reason to
make them gensyms.

Fix the data types indication in basic examples of the Trait std::fmt::LowerExp and std::fmt::UpperExp.
Since there aren’t any type annotation on the let statement using the number 42.0, they are of type f64 according to The Book:
https://doc.rust-lang.org/book/ch03-02-data-types.html#floating-point-types

…r=nikomatsakis,Centril

Test interaction of unions with non-zero/niche-filling optimization

Notably this nails down part of the behavior that MaybeUninit assumes, e.g. that a Option<MaybeUninit<&u8>> does not take advantage of non-zero optimization, and thus is a safe construct.

It also verifies the status quo: that even unions that could theoretically take advantage of niches don't. (relevant: rust-lang#36394)

…tors, r=oli-obk

Perform constant propagation into terminators

Perform constant propagation into MIR `Assert` and `SwitchInt` `Terminator`s which in some cases allows them to be removed by the branch simplification pass.

r? @oli-obk

Enable thumbv7a-pc-windows-msvc target build end to end in rust/master

With this PR, plus another commit rust-lang/compiler-builtins@cf98161, I'm able to build the target thumbv7a-pc-windows-msvc successfully, and I'm able to use the stage2 artifacts to build arm32 projects. The commit in compiler_builtins is in release 0.1.14, the current cargo.lock in rust master still uses 0.1.12, so I bumped the compiler_builtins version in cargo.lock to 0.1.15

The command I used to build rust
```
c:\python27\python.exe x.py build --host x86_64-pc-windows-msvc --build x86_64-pc-windows-msvc --target thumbv7a-pc-windows-msvc --verbose
```

**Changes**
1. update cargolock to use compiler_builtins 0.1.15
2. handle libunwind in libtest for thumv7a the same as what we have for aarch64
3. in llvm codegen add a field in CodegenContext to carry the arch, so later in create_msvc_imps function, the arch can be used to check against "x86", instead of 32 pointer width. Apparently Thumv7a is handled differently than x86.

**Background**
I'm from Microsoft working on enabling Azure IoTEdge on ARM32 Windows IoTCore, Azure IoTEdge has a component called IoTEdged written in rust as a NT service running on Windows, so we need to enable rust on thumbv7a in order to have full IoTEdge. My colleague had made some heavy lifting and we've been using our private toolchain to build IoTEdged in our devops pipeline, because at that time we cannot build thumbv7a target end to end successfully. This change is a followup to enable the end to end build for thumbv7a-pc-windows-msvc target.

**Next step**
I'll submit more changes to have this target built nightly in rust/master, to achieve the same availability for aarch64-pc-windows-msvc, indexed here https://rust-lang.github.io/rustup-components-history/aarch64-pc-windows-msvc.html and can be manually installed. **Please do share what takes to make this happen, is there a formal process I need to follow\?**

Fix lints handling in rustdoc

Part of rust-lang#60664: now lints are handled just like any other lints you would setup in rustc. Still remains to handle `missing code examples` and `missing_docs` as part of the same group.

r? @oli-obk

…m, r=petrochenkov

Stop using gensyms in HIR lowering

These names aren't ever handled by resolve, so there's no reason to
make them gensyms.

Diagnostics wanting to behave differently for these variables should
inspect either the `MatchSource`/`LocalSource` or the `Span`. All
current diagnostics appear to do this.

r? @petrochenkov

…hievink

Fix data types indication

Fix the data types indication in basic examples of the Trait std::fmt::LowerExp and std::fmt::UpperExp.
Since there aren’t any type annotation on the let statement using the number 42.0, they are of type f64 according to The Book:
https://doc.rust-lang.org/book/ch03-02-data-types.html#floating-point-types