Vendor dependencies

Let's see how I like this workflow.

vendor/tokio/tests/io_fill_buf.rs

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+#![warn(rust_2018_idioms)]
+#![cfg(all(feature = "full", not(tokio_wasi)))] // Wasi does not support file operations
+
+use tempfile::NamedTempFile;
+use tokio::fs::File;
+use tokio::io::{AsyncBufReadExt, BufReader};
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn fill_buf_file() {
+    let file = NamedTempFile::new().unwrap();
+
+    assert_ok!(std::fs::write(file.path(), b"hello"));
+
+    let file = assert_ok!(File::open(file.path()).await);
+    let mut file = BufReader::new(file);
+
+    let mut contents = Vec::new();
+
+    loop {
+        let consumed = {
+            let buffer = assert_ok!(file.fill_buf().await);
+            if buffer.is_empty() {
+                break;
+            }
+            contents.extend_from_slice(buffer);
+            buffer.len()
+        };
+
+        file.consume(consumed);
+    }
+
+    assert_eq!(contents, b"hello");
+}