This reverts commit 5a7058c.

In [rust-lang#131059] we found out that `-Zon-broken-pipe=kill` is actually
**load-bearing** [1] for (at least) `rustc` and `rustdoc` to have the
kill-process-on-broken-pipe behavior, e.g. `rustc --print=sysroot |
false` will ICE and `rustdoc --print=sysroot | false` will panic on a
broken pipe.

[rust-lang#131059]: rust-lang#131059
[1]: rust-lang#131059 (comment)

…he error

…li-obk

panic when an interpreter error gets unintentionally discarded

One important invariant of Miri is that when an interpreter error is raised (*in particular* a UB error), those must not be discarded: it's not okay to just check `foo().is_err()` and then continue executing.

This seems to catch new contributors by surprise fairly regularly, so this PR tries to make it so that *if* this ever happens, we get a panic rather than a silent missed UB bug. The interpreter error type now contains a "guard" that panics on drop, and that is explicitly passed to `mem::forget` when an error is deliberately discarded.

Fixes rust-lang/miri#3855

…r-ozkan

Revert rust-lang#131060 "Drop conditionally applied cargo `-Zon-broken-pipe=kill` flags"

In [rust-lang#131059] we found out that `-Zon-broken-pipe=kill` is actually **load-bearing**[^1] for
(at least) `rustc` and `rustdoc` to have the kill-process-on-broken-pipe behavior, e.g. `rustc
--print=sysroot | false` will ICE and `rustdoc --print=sysroot | false` will panic on a broken pipe.

This PR reverts 5a7058c (reverts PR rust-lang#131060) in favor of a future
fix to *unconditionally* apply `-Zon-broken-pipe=kill` to tool builds and also not drop the
`-Zon-broken-pipe=kill` flag for rustc binary builds.

I could not figure out how to write a regression test for the `rustc --print=sysroot | false`
behavior on Unix, so this is a plain revert for now.

This revert will unfortunately reintroduce rust-lang#130980 until we fix it again with the different approach.

See more details at <rust-lang#131059 (comment)> and in the timeline below.

### Timeline of kill-process-on-broken-pipe behavior changes

See [`unix_sigpipe` tracking issue rust-lang#97889][rust-lang#97889] for more context around unix sigpipe handling.

- From the very beginning since 2014, Rust binaries by default use `sig_ign`. This meant that if
  output pipe is broken yet the program tries to use `println!` and such, there will be a broken
  pipe panic from std. This lead to ICEs from e.g. `rustc --help | false` [rust-lang#34376].
- [rust-lang#49606] mitigated [rust-lang#34376] by adding an explicit signal handler to `rustc_driver` register a
  sigpipe handler with `SIG_DFL` which will cause the binary using `rustc_driver` to terminate if
  `rustc_driver::set_sigpipe_handler()` is called. `rustc`'s main binary wrapper uses
  `rustc_driver::set_sigpipe_handler()`, and so does `rustdoc`.
- A more universal way to set sigpipe behavior for Unix was introduced as part of [rust-lang#97889], i.e. `#
  [unix_sigpipe = "sig_dfl"]` attribute.
- [rust-lang#102587] migrated `rustc` to use `#[unix_sigpipe = "sig_dfl"]` instead of
  `rustc_driver::set_sigpipe_handler`.
- [rust-lang#103495] migrated `rustdoc` to use `#[unix_sigpipe = "sig_dfl"]` instead of
  `rustc_driver::set_sigpipe_handler`. `rustc_driver::set_sigpipe_handler` was removed.
- Following concerns about sigpipe setting UI in [rust-lang#97889], the UI for specifying sigpipe behavior
  was changed in [rust-lang#124480] from `#[unix_sigpipe = "sig_dfl"]` attribute to the commmand line flag
  `-Zon-broken-pipe=kill`.
    - In the same PR, `#[unix_sigpipe = "sig_dfl"]` were removed from `rustc` and `rustdoc` main
      binary crate entry points in favor of the command line flag. Kill-process-on-broken-pipe
      behavior was preserved by adding `-Zon-broken-pipe=kill` for `rustdoc` tool build step and
      `rustc` during compile steps.
- [rust-lang#126934] added `-Zon-broken-pipe=kill` for tool builds *except* for cargo to help with some miri
  tests because at the time the PR was written, this would lead to a couple of cargo test failures.
  Conditionally setting `RUSTFLAGS` can lead to tool build invalidation, e.g. building `cargo`
  without `-Zon-broken-pipe=kill` but `clippy` with the flag can lead to invalidation of the tool
  build cache. This is not a problem at the time, because nothing (not even miri) tests built stage
  1 cargo (all used initial cargo).
- In [rust-lang#130634] we found out that `run-make` tests like `compiler-builtins` needed stage 1 cargo, not
  just beta bootstrap cargo, because there can be changes that are present in stage 1 cargo but
  absent in beta cargo, which was blocking a beta backport.
- [rust-lang#130642] and later [rust-lang#130739] now build stage 1 cargo. And as previously mentioned, since
  `-Zon-broken-pipe=kill` was specifically *not* set for cargo, this caused tool build cache
  invalidation meaning rebuilds of stage 1 even if nothing in source was changed due to differing
  `RUSTFLAGS` since `run-make` also builds `rustdoc` and such [rust-lang#130980].

[rust-lang#34376]: rust-lang#34376
[rust-lang#49606]: rust-lang#49606
[rust-lang#97889]: rust-lang#97889
[rust-lang#102587]: rust-lang#102587
[rust-lang#103495]: rust-lang#103495
[rust-lang#124480]: rust-lang#124480
[rust-lang#130634]: rust-lang#130634
[rust-lang#130642]: rust-lang#130642
[rust-lang#130739]: rust-lang#130739
[rust-lang#130980]: rust-lang#130980
[rust-lang#131059]: rust-lang#131059
[^1]: rust-lang#131059 (comment)

r? ``@onur-ozkan`` (or bootstrap)

A couple of fixes for dataflow graphviz dumps

A couple of trivial drive-by fixes to issues I noticed while debugging my buggy borrowck code:

One is a fix of the `-Zdump-mir-dataflow` file extensions, the dataflow graphviz files are currently dumped  as `..dot`.

<details>

```console
-rw-rw-r-- 1 lqd lqd 13051 Oct  1 23:21 mir_dump/issue_47680.main.-------.borrows.borrowck..dot
-rw-rw-r-- 1 lqd lqd 13383 Oct  1 23:21 mir_dump/issue_47680.main.-------.ever_init.borrowck..dot
-rw-rw-r-- 1 lqd lqd 13591 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_init.borrowck..dot
-rw-rw-r-- 1 lqd lqd  9257 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_init.elaborate_drops..dot
-rw-rw-r-- 1 lqd lqd 14086 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_uninit.borrowck..dot
-rw-rw-r-- 1 lqd lqd  9257 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_uninit.elaborate_drops..dot
```

<summary>Some examples on nightly</summary>

</details>

And the other is for the specific `Borrows` dataflow analysis, whose domain is loans but shows locations when dumped (the location where the loan is introduced). It's not a huge deal but we didn't even print these locations in MIR dumps, and in general cross-referencing loan data (like loan liveness) is more annoying without this change.

<details>

![Untitled](https://github.com/user-attachments/assets/b325a6e9-1aee-4655-8441-d3b1b55ded3c)

<summary>Here's how it'll look in case inquisitive minds want to know</summary>

</details>

The visualization state diff display is still suboptimal in loops for some of the effects escaping a block, e.g. a gen that's not dominated/postdominated by a kill will not show up in statement diffs. (This happens in the previous screenshot, there's no `+bw1` anywhere). We can fix that in the future.