Add a --check option to wit-bindgen. (#486)

* Add a `--check` option to wit-bindgen.

Add a `--check` option to wit-bindgen for testing whether generated
files are already up to date. With this flag, if the generated files
differ from what would be generated, the wit-bindgen command exits
with an error.

* Use `read` instead of `read_to_string` to handle binary data.

* Issue a specific error message if two files differ only in line ending.

Author: sunfishcode

src/bin/wit-bindgen.rs

@@ -1,6 +1,7 @@
 use anyhow::{anyhow, bail, Context, Result};
 use clap::Parser;
 use std::path::PathBuf;
+use std::str;
 use wit_bindgen_core::{wit_parser, Files, WorldGenerator};
 use wit_parser::{Resolve, UnresolvedPackage};
 
@@ -64,6 +65,11 @@ struct Common {
     /// it's `foo.bar` which is the world named `bar` within document `foo`.
     #[clap(short, long)]
     world: Option<String>,
+
+    /// Indicates that no files are written and instead files are checked if
+    /// they're up-to-date with the source files.
+    #[clap(long)]
+    check: bool,
 }
 
 fn main() -> Result<()> {
@@ -87,6 +93,31 @@ fn main() -> Result<()> {
             None => name.into(),
         };
         println!("Generating {:?}", dst);
+
+        if opt.check {
+            let prev = std::fs::read(&dst).with_context(|| format!("failed to read {:?}", dst))?;
+            if prev != contents {
+                // The contents differ. If it looks like textual contents, do a
+                // line-by-line comparison so that we can tell users what the
+                // problem is directly.
+                if let (Ok(utf8_prev), Ok(utf8_contents)) =
+                    (str::from_utf8(&prev), str::from_utf8(contents))
+                {
+                    if !utf8_prev
+                        .chars()
+                        .any(|c| c.is_control() && !matches!(c, '\n' | '\r' | '\t'))
+                        && utf8_prev.lines().eq(utf8_contents.lines())
+                    {
+                        bail!("{} differs only in line endings (CRLF vs. LF). If this is a text file, configure git to mark the file as `text eol=lf`.", dst.display());
+                    }
+                }
+                // The contents are binary or there are other differences; just
+                // issue a generic error.
+                bail!("not up to date: {}", dst.display());
+            }
+            continue;
+        }
+
         if let Some(parent) = dst.parent() {
             std::fs::create_dir_all(parent)
                 .with_context(|| format!("failed to create {:?}", parent))?;