More informative diagnotic from x.py test attempt atop beta checkout

[pnkfelix]

Make bootstrap be more informative when one does x.py test on a beta checkout without other mods.

To be clear, by default running x.py test on a checkout of the beta branch
currently fails, and with this change will continue to fail, because x.py tests runs x.py test src/tools/tidy which tries to run rustfmt and that
will fail because the rustfmt binary is pinned to the current nighlty and we
do not attempt to distribute one for the beta builds.

This change gives a better error message than the current message, which is just
"./x.py fmt is not supported on this channel" without providing any hint about
what one might do about that problem.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[pnkfelix]

Before:

./x.py fmt is not supported on this channel
failed to run: /media/pnkfelix/Rust/rust.git/objdir-clean/build/bootstrap/debug/bootstrap test --stage 1 src/tools/tidy                                                                                                   

After:

error: no `rustfmt` binary found in /media/pnkfelix/Rust/rust.git/objdir-clean/build/x86_64-unknown-linux-gnu/stage0/bin                                         
info: `rust.channel` is currently set to "dev"
help: if you are testing a beta branch, set `rust.channel` to "beta" in the `config.toml` file 
help: to skip test's attempt to check tidiness, pass `--exclude src/tools/tidy` to `x.py test`
failed to run: /media/pnkfelix/Rust/rust.git/objdir-clean/build/bootstrap/debug/bootstrap test --stage 1 src/tools/tidy

[pnkfelix]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/src/bootstrap/test.rs at line 792:
         if builder.config.channel == "dev" || builder.config.channel == "nightly" {
             builder.info("fmt check");
             if builder.config.initial_rustfmt.is_none() {
-                let inferred_rustfmt_dir =
-                    builder.config.initial_rustc.parent().unwrap();
-                eprintln!("\
+                let inferred_rustfmt_dir = builder.config.initial_rustc.parent().unwrap();
+                eprintln!(
+                    "\
 error: no `rustfmt` binary found in {PATH}
 info: `rust.channel` is currently set to \"{CHAN}\"
 help: if you are testing a beta branch, set `rust.channel` to \"beta\" in the `config.toml` file
Diff in /checkout/src/bootstrap/test.rs at line 801:
 help: to skip test's attempt to check tidiness, pass `--exclude src/tools/tidy` to `x.py test`",
-                          PATH=inferred_rustfmt_dir.display(),
-                          CHAN=builder.config.channel,
+                    PATH = inferred_rustfmt_dir.display(),
+                    CHAN = builder.config.channel,
                 );
                 std::process::exit(1);
             }
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/src/bootstrap/test.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
Build completed unsuccessfully in 0:00:16

oh irony, failing to run the tidy check on this thing that makes it easier to make progress by skipping the tidy check.

Oh wait, that's not irony at all. Or rather, its Alanis Morrisette's notion of irony...

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit d6de60f has been approved by Mark-Simulacrum