Use anyhow::Result in xtask, add contexts

This builds on #2231 but was actually done before that. You see, the
cause for #2231 was that I got this error message:

    Error: Error { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }) }

Just switching to `anyhow::Result` got me stack traces (when setting
`RUST_LIB_BACKTRACE=1`) that at least showed

    stack backtrace:
      0: std::backtrace::Backtrace::create
      1: std::backtrace::Backtrace::capture
      2: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
      3: xtask::install_server
      4: xtask::install
      5: xtask::main
      6: std::rt::lang_start::{{closure}}
      7: std::panicking::try::do_call
      8: __rust_maybe_catch_panic
      9: std::rt::lang_start_internal
      10: std::rt::lang_start
      11: main

With the added contexts (not at all exhaustive), the error became

    Error: install server

    Caused by:
        0: build AutoCfg with target directory
        1: No such file or directory (os error 2)

Since anyhow is such a small thing (no new transitive dependencies!),
and in general gives you `Result<T, Box<dyn Error>>` on steroids, I
think this a nice small change. The only slightly annoying thing was to
replace all the `Err(format!(…))?` calls (haven't even looked at whether
we can make it support wrapping strings though), but the `bail!` macro
is shorter anyway :)

1 files changed, 7 insertions, 9 deletions

diff --git a/xtask/src/lib.rs b/xtask/src/lib.rs
index bae4c4650..bfee2f9c8 100644
--- a/xtask/src/lib.rs
+++ b/xtask/src/lib.rs
@@ -2,10 +2,10 @@
 
 pub mod codegen;
 
+use anyhow::Context;
+pub use anyhow::Result;
 use std::{
-    env,
-    error::Error,
-    fs,
+    env, fs,
     io::{Error as IoError, ErrorKind},
     path::{Path, PathBuf},
     process::{Command, Output, Stdio},
@@ -13,8 +13,6 @@ use std::{
 
 use crate::codegen::Mode;
 
-pub type Result<T> = std::result::Result<T, Box<dyn Error>>;
-
 const TOOLCHAIN: &str = "stable";
 
 pub fn project_root() -> PathBuf {
@@ -69,7 +67,7 @@ pub fn run_rustfmt(mode: Mode) -> Result<()> {
         .status()
     {
         Ok(status) if status.success() => (),
-        _ => install_rustfmt()?,
+        _ => install_rustfmt().context("install rustfmt")?,
     };
 
     if mode == Mode::Verify {
@@ -112,7 +110,7 @@ pub fn run_clippy() -> Result<()> {
         .status()
     {
         Ok(status) if status.success() => (),
-        _ => install_clippy()?,
+        _ => install_clippy().context("install clippy")?,
     };
 
     let allowed_lints = [
@@ -162,9 +160,9 @@ where
     let exec = args.next().unwrap();
     let mut cmd = Command::new(exec);
     f(cmd.args(args).current_dir(proj_dir).stderr(Stdio::inherit()));
-    let output = cmd.output()?;
+    let output = cmd.output().with_context(|| format!("running `{}`", cmdline))?;
     if !output.status.success() {
-        Err(format!("`{}` exited with {}", cmdline, output.status))?;
+        anyhow::bail!("`{}` exited with {}", cmdline, output.status);
     }
     Ok(output)
 }