This change adds a new compiler flag that can help reduce the size of
ELF binaries that contain many functions.

By default, when enabling function sections (which is the default for most
targets), the LLVM backend will generate different section names for each
function. For example, a function "func" would generate a section called
".text.func". Normally this is fine because the linker will merge all those
sections into a single one in the binary. However, starting with LLVM 12
(llvm/llvm-project@ee5d1a0), the backend will
also generate unique section names for exception handling, resulting in
thousands of ".gcc_except_table.*" sections ending up in the final binary
because some linkers don't currently merge or strip these EH sections.
This can bloat the ELF headers and string table significantly in
binaries that contain many functions.

The new option is analogous to Clang's -fno-unique-section-names, and
instructs LLVM to generate the same ".text" and ".gcc_except_table"
section for each function, resulting in smaller object files and
potentially a smaller final binary.

Fix and extent ControlFlow `traverse_inorder` example

1. The existing example compiles on its own, but any usage fails to be monomorphised and so doesn't compile. Fix that by using Fn trait instead of FnMut.
2. Added an example usage of `traverse_inorder` showing how we can terminate the traversal early.

Fixes rust-lang#90063

1. The existing example compiles on its own, but any usage fails
   to be monomorphised and so doesn't compile. Fix that by using
   a mutable reference as an input argument.
2. Added an example usage of `traverse_inorder` showing how we
   can terminate the traversal early.

Fixes rust-lang#90063

Add -Z no-unique-section-names to reduce ELF header bloat.

This change adds a new compiler flag that can help reduce the size of ELF binaries that contain many functions.

By default, when enabling function sections (which is the default for most targets), the LLVM backend will generate different section names for each function. For example, a function `func` would generate a section called `.text.func`. Normally this is fine because the linker will merge all those sections into a single one in the binary. However, starting with [LLVM 12](llvm/llvm-project@ee5d1a04), the backend will also generate unique section names for exception handling, resulting in thousands of `.gcc_except_table.*` sections ending up in the final binary because some linkers like LLD don't currently merge or strip these EH sections (see discussion [here](https://reviews.llvm.org/D83655)). This can bloat the ELF headers and string table significantly in binaries that contain many functions.

The new option is analogous to Clang's `-fno-unique-section-names`, and instructs LLVM to generate the same `.text` and `.gcc_except_table` section for each function, resulting in a smaller final binary.

The motivation to add this new option was because we have a binary that ended up with so many ELF sections (over 65,000) that it broke some existing ELF tools, which couldn't handle so many sections.

Here's our old binary:

```
$ readelf --sections old.elf | head -1
There are 71746 section headers, starting at offset 0x2a246508:

$ readelf --sections old.elf | grep shstrtab
  [71742] .shstrtab      STRTAB          0000000000000000 2977204c ad44bb 00      0   0  1
```

That's an 11MB+ string table. Here's the new binary using this option:

```
$ readelf --sections new.elf | head -1
There are 43 section headers, starting at offset 0x29143ca8:

$ readelf --sections new.elf | grep shstrtab
  [40] .shstrtab         STRTAB          0000000000000000 29143acc 0001db 00      0   0  1
```

The whole binary size went down by over 20MB, which is quite significant.

Fix and extent ControlFlow `traverse_inorder` example

Fix and extent ControlFlow `traverse_inorder` example

1. The existing example compiles on its own, but any usage fails to be monomorphised and so doesn't compile. Fix that by using Fn trait instead of FnMut.
2. Added an example usage of `traverse_inorder` showing how we can terminate the traversal early.

Fixes rust-lang#90063

…=lnicola

⬆️ rust-analyzer

r? ~~``@ghose~~`` sorry, ``@ghost``

Prevent duplicate caller bounds candidates by exposing default substs in Unevaluated

Fixes rust-lang#89334

The changes introduced in rust-lang#87280 allowed for "duplicate" caller bounds candidates to be assembled that only differed in their default substs having been "exposed" or not and resulted in an ambiguity error during trait selection. To fix this we expose the defaults substs during the creation of the ParamEnv.

r? `@lcnr`