Write inline snapshots with atomic rename (#373)

cargo-insta/Cargo.toml

@@ -21,3 +21,4 @@ proc-macro2 = { version = "1.0.24", features = ["span-locations"] }
 syn = { version = "1.0.50", features = ["full", "visit", "extra-traits"] }
 ignore = "0.4.17"
 uuid = { version = "1.0.0", features = ["v4"] }
+tempfile = "3.5.0"

cargo-insta/src/inline.rs

@@ -6,7 +6,7 @@ use std::path::{Path, PathBuf};
 
 use insta::_cargo_insta_support::SnapshotContents;
 use proc_macro2::TokenTree;
-use syn;
+
 use syn::spanned::Spanned;
 
 #[derive(Debug)]
@@ -47,10 +47,19 @@ impl FilePatcher {
     }
 
     pub fn save(&self) -> Result<(), Box<dyn Error>> {
-        let mut f = fs::File::create(&self.filename)?;
+        // We use a temp file and then atomically rename to prevent a
+        // file watcher restarting the process midway through the write.
+        let mut temp_file = tempfile::Builder::new()
+            .suffix(".snap.tmp")
+            .tempfile_in(self.filename.parent().ok_or("Parent directory not found")?)?;
+
         for line in &self.lines {
-            writeln!(&mut f, "{}", line)?;
+            writeln!(temp_file, "{}", line)?;
         }
+
+        temp_file.flush()?;
+        fs::rename(temp_file.path(), &self.filename)?;
+
         Ok(())
     }