Infrastructure for testing the command line tool.

Note: generates tests into fresh temp dir, which is deleted after testing is
done (regardless of success or failure). You can change the
`which_temp::WhichTempDir` type to revise this behavior.

This infrastructure includes two tests: `tests/cli.rs` and `tests/ice.rs`. Each
uses very different strategies for testing cargo-bisect-rustc.

  1. `tests/cli.rs` uses a so-called meta-build strategy: the test inspects the
     `rustc` version, then generates build files that will inject (or remove,
     e.g. when testing `--regress=success`) `#[rustc_error]` from the source
     code based on the `rustc` version. This way, we get the effect of an error
     that will come or go based solely on the `rustc` version, without any
     dependence on the actual behavior of `rustc` itself (beyond its version
     string format remaining parsable).

     * This strategy should remain usable for the foreseeable future, without
       any need for intervention from `cargo-bisect-rustc` developers.

  2. `tests/ice.rs` uses a totally different strategy: It embeds an ICE that we
     know originated at a certain version of the compiler. The ICE is embedded
     in the file `src/ice/included_main.rs`. The injection point associated with
     the ICE is encoded in the constant `INJECTION_COMMIT`.

     * Over time, since we only keep a certain number of builds associated with
       PR merge commits available to download, the embedded ICE, the
       `INJECTION_COMMIT` definition, and the search bounds defined in
       `INJECTION_LOWER_BOUND` and `INJECTION_UPPER_BOUND` will all have to be
       updated as soon as the commit for `INJECTION_COMMIT` is no longer
       available for download.

     * Thus, this testing strategy requires regular maintenance from the
       `cargo-bisect-rustc` developers. (However, it is more flexible than the
       meta-build strategy, in that you can embed arbitrary failures from the
       recent past using this approach. The meta-build approach can only embed
       things that can be expressed via features like `#[rustc_error]`, which
       cannot currently express ICE's.

----

Includes suggestions from code review

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

----

Includes some coments explaining the `WhichTempDir` type. (That type maybe
should just be an enum rather than a trait you implement... not sure why I made
it so general...)

----

Includes workaround for rustfmt issue.

Specifically, workaround rust-lang/rustfmt#3794 which
was causing CI's attempt to run `cargo fmt -- --check` to erroneously report:

```
% cargo fmt -- --check
error[E0583]: file not found for module `meta_build`
  --> /private/tmp/cbr/tests/cli.rs:11:20
   |
11 |     pub(crate) mod meta_build;
   |                    ^^^^^^^^^^
   |
   = help: name the file either meta_build.rs or meta_build/mod.rs inside the directory "/private/tmp/cbr/tests/cli/cli"

error[E0583]: file not found for module `command_invocation`
  --> /private/tmp/cbr/tests/ice.rs:34:20
   |
34 |     pub(crate) mod command_invocation;
   |                    ^^^^^^^^^^^^^^^^^^
   |
   = help: name the file either command_invocation.rs or command_invocation/mod.rs inside the directory "/private/tmp/cbr/tests/ice/common"

```

----

Includes fix for oversight in my cli test system: it needed to lookup target
binary, not our PATH.

(This functionality is also available via other means, such as
`$CARGO_BIN_EXE_<name>` and https://crates.io/crates/assert_cmd. I opted not to
use the builtin env variable because that is only available in very recent cargo
versions, and I would prefer our test suite to work peven on older versions of
cargo, if that is feasible...)

----

Includes applications of rustfmt suggestions, as well as an expansion of a
comment in a manner compatible with rustfmt.

(Namely, that replaced an inline comment which is erroneously deleted by rustfmt
(see rust-lang/rustfmt#2781 ) with an additional note
in the comment above the definition.)

Cargo.toml

@@ -36,3 +36,4 @@ colored="1.9"
 [dev-dependencies]
 quickcheck = "0.9.2"
 tempfile = "3.1.0"
+test_bin=  "0.3.0"

tests/cli/meta_build.rs

@@ -0,0 +1,84 @@
+use std::fs::{DirBuilder};
+use std::path::{Path};
+
+pub struct InjectionPoint {
+    pub date: YearMonthDay,
+    pub associated_sha: &'static str,
+}
+
+pub struct Test<'a> {
+    pub crate_name: &'a str,
+    pub cli_params: &'a [&'a str],
+    pub delta_date: InjectionPoint,
+    pub delta_kind: DeltaKind,
+}
+
+impl<'a> Test<'a> {
+    pub fn expected_sha(&self) -> &str {
+        self.delta_date.associated_sha
+    }
+}
+
+pub fn make_crate_files(
+    dir_builder: &DirBuilder,
+    dir: &Path,
+    test: &Test)
+    -> Result<(), failure::Error>
+{
+    (crate::make_a_crate::Crate {
+        dir,
+        name: test.crate_name,
+        build_rs: Some(meta_build(test).into()),
+        cargo_toml: format!(r##"
+[package]
+name = "{NAME}"
+version = "0.1.0"
+authors = ["Felix S. Klock II <pnkfelix@pnkfx.org>"]
+"##, NAME=test.crate_name).into(),
+        main_rs: MAIN_RS.into(),
+    }).make_files(dir_builder)?;
+
+    Ok(())
+}
+
+// A test crate to exercise `cargo-bisect-rustc` has three basic components: a
+// Cargo.toml file, a build.rs script that inspects the current version of Rust
+// and injects an error for the appropriate versions into a build-time generated
+// version.rs file, and a main.rs file that include!'s the version.rs file
+//
+// We only inject errors based on YYYY-MM-DD date comparison (<, <=, >=, >), and
+// having that conditonally add a `#[rustc_error]` to the (injected) `fn main()`
+// function.
+
+const MAIN_RS: &'static str = std::include_str!("meta_build/included_main.rs");
+
+#[derive(Copy, Clone)]
+pub struct YearMonthDay(pub u32, pub u32, pub u32);
+
+#[derive(Copy, Clone)]
+pub enum DeltaKind { Fix, Err }
+
+fn meta_build(test: &Test) -> String {
+    let YearMonthDay(year, month, day) = test.delta_date.date;
+    let delta_kind = test.delta_kind;
+    let date_item = format!(r##"
+/// `DELTA_DATE` identfies nightly where simulated change was injected.
+const DELTA_DATE: YearMonthDay = YearMonthDay({YEAR}, {MONTH}, {DAY});
+"##,
+                            YEAR=year, MONTH=month, DAY=day);
+
+    let kind_variant = match delta_kind {
+        DeltaKind::Fix => "Fix",
+        DeltaKind::Err => "Err",
+    };
+    let kind_item = format!(r##"
+/// `DELTA_KIND` identfies whether simulated change is new error, or a fix to ancient error.
+const DELTA_KIND: DeltaKind = DeltaKind::{VARIANT};
+"##,
+                            VARIANT=kind_variant);
+
+    format!("{DATE_ITEM}{KIND_ITEM}{SUFFIX}",
+            DATE_ITEM=date_item, KIND_ITEM=kind_item, SUFFIX=BUILD_SUFFIX)
+}
+
+const BUILD_SUFFIX: &'static str = std::include_str!("meta_build/included_build_suffix.rs");

tests/cli/meta_build/included_build_suffix.rs

@@ -0,0 +1,80 @@
+// Strategy inspired by dtolnay/rustversion: run `rustc --version` at build time
+// to observe version info.
+//
+// (The dtolnay/rustversion is dual-licensed under APACHE/MIT as of January 2020.)
+
+use std::env;
+use std::ffi::OsString;
+use std::fs;
+use std::path::Path;
+use std::process::{self, Command};
+
+#[derive(PartialOrd, Ord, PartialEq, Eq, Debug)]
+struct YearMonthDay(u32, u32, u32);
+
+enum DeltaKind { Fix, Err }
+
+fn main() {
+    let mut context = Context::introspect();
+    context.generate();
+}
+
+struct Context {
+    commit: Commit,
+    rustc_date: YearMonthDay,
+}
+
+#[derive(PartialOrd, Ord, PartialEq, Eq, Debug)]
+struct Commit(String);
+
+impl Context {
+    fn introspect() -> Context {
+        let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+        let output = Command::new(&rustc).arg("--version").output().unwrap_or_else(|e| {
+            let rustc = rustc.to_string_lossy();
+            eprintln!("Error: failed to run `{} --version`: {}", rustc, e);
+            process::exit(1);
+        });
+        let output = String::from_utf8(output.stdout).unwrap();
+        let mut tokens = output.split(' ');
+
+        let _rustc = tokens.next().unwrap();
+        let _version = tokens.next().unwrap();
+        let open_paren_commit = tokens.next().unwrap();
+        let date_close_paren = tokens.next().unwrap();
+
+        let commit = Commit(open_paren_commit[1..].to_string());
+
+        let date_str: String =
+            date_close_paren.matches(|c: char| c.is_numeric() || c == '-').collect();
+        let mut date_parts = date_str.split('-');
+        let year: u32 = date_parts.next().unwrap().parse().unwrap();
+        let month: u32 = date_parts.next().unwrap().parse().unwrap();
+        let day: u32 = date_parts.next().unwrap().parse().unwrap();
+
+        Context { commit, rustc_date: YearMonthDay(year, month, day) }
+    }
+
+    fn generate(&mut self) {
+        let inject_with_error = match DELTA_KIND {
+            DeltaKind::Err => self.rustc_date >= DELTA_DATE,
+            DeltaKind::Fix => self.rustc_date < DELTA_DATE,
+        };
+        let prefix = if inject_with_error { "#[rustc_error] " } else { "" };
+        let maybe_static_error = format!("{PREFIX}{ITEM}", PREFIX=prefix, ITEM="fn main() { }");
+
+        let content = format!(r#"{MAIN}
+pub const COMMIT: &'static str = "{COMMIT}";
+pub const DATE: &'static str = "{Y:04}-{M:02}-{D:02}";
+"#,
+                              MAIN=maybe_static_error,
+                              COMMIT=self.commit.0,
+                              Y=self.rustc_date.0,
+                              M=self.rustc_date.1,
+                              D=self.rustc_date.2);
+
+        let out_dir = env::var_os("OUT_DIR").expect("OUT_DIR not set");
+        let out_file = Path::new(&out_dir).join("version.rs");
+        fs::write(out_file, content).expect("failed to write version.rs");
+    }
+}

tests/cli/meta_build/included_main.rs

@@ -0,0 +1,2 @@
+#![feature(rustc_attrs)]
+include!(concat!(env!("OUT_DIR"), "/version.rs"));

tests/cli/mod.rs

@@ -0,0 +1,92 @@
+const INJECTION_COMMIT: &'static str = "f8fd4624474a68bd26694eff3536b9f3a127b2d3";
+const INJECTION_LOWER_BOUND: &'static str = "2020-02-06";
+const INJECTION_UPPER_BOUND: &'static str = "2020-02-08";
+
+const INJECTION_POINT: InjectionPoint = InjectionPoint {
+    date: YearMonthDay(2020, 02, 07),
+    associated_sha: INJECTION_COMMIT,
+};
+
+mod cli {
+    pub(crate) mod meta_build;
+}
+
+pub(crate) use self::cli::meta_build;
+
+mod common {
+    pub(crate) mod command_invocation;
+    pub(crate) mod make_a_crate;
+    pub(crate) mod which_temp;
+}
+
+pub(crate) use self::common::command_invocation;
+pub(crate) use self::common::make_a_crate;
+pub(crate) use self::common::which_temp;
+
+use self::meta_build::{DeltaKind, InjectionPoint, Test, YearMonthDay};
+use self::which_temp::{WhichTempDir, WhichTempDirectory};
+
+// These tests pass `--preserve` and `--access=github` because that is the best
+// way to try to ensure that the tests complete as quickly as possible.
+
+pub const BASIC_TEST: Test = Test {
+    crate_name: "cbr_test_cli_basic",
+    cli_params: &["--preserve", "--access=github",
+                  "--start", INJECTION_LOWER_BOUND, "--end", INJECTION_UPPER_BOUND],
+    delta_date: INJECTION_POINT,
+    delta_kind: DeltaKind::Err,
+};
+
+pub const FIXED_TEST: Test = Test {
+    crate_name: "cbr_test_cli_fixed",
+    cli_params: &["--regress=success",
+                  "--preserve", "--access=github",
+                  "--start", INJECTION_LOWER_BOUND, "--end", INJECTION_UPPER_BOUND],
+    delta_date: INJECTION_POINT,
+    delta_kind: DeltaKind::Fix,
+};
+
+// Ordinarily, I would put both of these tests into separate `#[test]` methods.
+// However, if you do that, then `cargo test` will run them in parallel, and you
+// end up with `cargo-bisect-rustc` racing to install the toolchains it
+// downloads.
+//
+// (It is arguably a bug that we do not gracefully handle this situation.)
+//
+// In any case, the simplest fix for the test infrastructure is to ensure that
+// no tests overlap in the range of dates they search for a regression.
+#[test]
+fn cli_test() -> Result<(), failure::Error> {
+    test_cli_core::<WhichTempDir>(&BASIC_TEST)?;
+    test_cli_core::<WhichTempDir>(&FIXED_TEST)?;
+    Ok(())
+}
+
+fn test_cli_core<WhichTemp>(test: &meta_build::Test) -> Result<(), failure::Error>
+where WhichTemp: WhichTempDirectory
+{
+    let root = WhichTemp::root()?;
+    let tmp_dir = WhichTemp::target(&root);
+    let dir = tmp_dir.join(test.crate_name);
+
+    let dir_builder = WhichTemp::dir_builder();
+    meta_build::make_crate_files(&dir_builder, &dir, test)?;
+
+    let mut cmd = command_invocation::Context {
+        cli_params: test.cli_params,
+        dir: dir.as_path(),
+    };
+
+    let command_invocation::Output { status: _, stderr, stdout } = cmd.run()?;
+
+    println!("Command output stdout for {}: \n```\n{}\n```", test.crate_name, stdout);
+    println!("Command output stderr for {}: \n```\n{}\n```", test.crate_name, stderr);
+
+    // The most basic check: does the output actually tell us about the
+    // "regressing" commit.
+    let needle = format!("Regression in {}", test.expected_sha());
+    // println!("searching for {:?} in stdout: {:?} stderr: {:?}", needle, stdout, stderr);
+    assert!(stderr.contains(&needle));
+
+    Ok(())
+}

tests/common/command_invocation.rs

@@ -0,0 +1,57 @@
+use std::path::Path;
+use std::process::ExitStatus;
+
+pub(crate) struct Context<'a> {
+    pub cli_params: &'a [&'a str],
+    pub dir: &'a Path,
+}
+
+pub struct Output {
+    pub status: ExitStatus,
+    pub stdout: String,
+    pub stderr: String,
+}
+
+impl<'a> Context<'a> {
+    pub fn run(&mut self) -> Result<Output, failure::Error> {
+        let mut command = test_bin::get_test_bin("cargo-bisect-rustc");
+        for param in self.cli_params {
+            command.arg(param);
+        }
+        let dir = self.dir;
+        println!(
+            "running `{:?} {}` in {:?}",
+            command,
+            self.cli_params.join(" "),
+            dir.display()
+        );
+        assert!(dir.exists());
+        let output = command.current_dir(dir).output()?;
+
+        let stderr = String::from_utf8_lossy(&output.stderr);
+
+        // prepass over the captured stdout, which by default emits a lot of
+        // progressive info about downlaods that is fine in interactive settings
+        // but just makes for a lot of junk when you want to understand the
+        // final apparent output.
+        let mut stdout = String::with_capacity(output.stdout.len());
+        let mut line = String::new();
+        for c in &output.stdout {
+            match *c as char {
+                '\r' => line.clear(),
+                '\n' => {
+                    stdout.push_str(&line);
+                    line.clear();
+                }
+                c => line.push(c),
+            }
+        }
+        stdout.push_str(&line);
+
+        Ok(Output {
+            status: output.status,
+            stderr: stderr.to_string(),
+            stdout,
+        })
+    }
+}

tests/common/make_a_crate.rs

@@ -0,0 +1,43 @@
+use std::borrow::Cow;
+use std::fs::{DirBuilder, File};
+use std::io::{Write};
+use std::path::{Path};
+
+type Text<'a> = Cow<'a, str>;
+
+pub struct Crate<'a> {
+    pub dir: &'a Path,
+    pub name: &'a str,
+    pub build_rs: Option<Text<'a>>,
+    pub cargo_toml: Text<'a>,
+    pub main_rs: Text<'a>,
+}
+
+impl<'a> Crate<'a> {
+    pub fn make_files(&self, dir_builder: &DirBuilder) -> Result<(), failure::Error> {
+        let dir = self.dir;
+        let cargo_toml_path = dir.join("Cargo.toml");
+        let build_path = dir.join("build.rs");
+        let src_path = dir.join("src");
+        let main_path = src_path.join("main.rs");
+
+        dir_builder.create(&dir)?;
+        dir_builder.create(src_path)?;
+
+        if let Some(build_rs) = &self.build_rs {
+            let mut build_file = File::create(build_path)?;
+            writeln!(build_file, "{}", build_rs)?;
+            build_file.sync_data()?;
+        }
+
+        let mut cargo_toml_file = File::create(cargo_toml_path)?;
+        writeln!(cargo_toml_file, "{}", self.cargo_toml)?;
+        cargo_toml_file.sync_data()?;
+
+        let mut main_file = File::create(main_path)?;
+        writeln!(main_file, "{}", self.main_rs)?;
+        main_file.sync_data()?;
+
+        Ok(())
+    }
+}