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, 3 insertions, 5 deletions

diff --git a/xtask/src/codegen.rs b/xtask/src/codegen.rs
index 770b55a9a..53f524f42 100644
--- a/xtask/src/codegen.rs
+++ b/xtask/src/codegen.rs
@@ -52,7 +52,7 @@ fn update(path: &Path, contents: &str, mode: Mode) -> Result<()> {
         _ => (),
     }
     if mode == Mode::Verify {
-        Err(format!("`{}` is not up-to-date", path.display()))?;
+        anyhow::bail!("`{}` is not up-to-date", path.display());
     }
     eprintln!("updating {}", path.display());
     fs::write(path, contents)?;
@@ -101,10 +101,8 @@ fn do_extract_comment_blocks(text: &str, allow_blocks_with_empty_lins: bool) ->
         let is_comment = line.starts_with(prefix);
         if is_comment {
             block.push(line[prefix.len()..].to_string());
-        } else {
-            if !block.is_empty() {
-                res.push(mem::replace(&mut block, Vec::new()))
-            }
+        } else if !block.is_empty() {
+            res.push(mem::replace(&mut block, Vec::new()));
         }
     }
     if !block.is_empty() {