Fix gcc-shim, apply clippy warning & optimizations to it (#777)

src/bin/gcc-shim.rs

@@ -2,7 +2,7 @@
 
 use std::env;
 use std::fs::File;
-use std::io::prelude::*;
+use std::io::{self, prelude::*};
 use std::path::PathBuf;
 
 fn main() {
@@ -10,39 +10,61 @@ fn main() {
     let program = args.next().expect("Unexpected empty args");
 
     let out_dir = PathBuf::from(
-        env::var_os("GCCTEST_OUT_DIR").expect(&format!("{}: GCCTEST_OUT_DIR not found", program)),
+        env::var_os("GCCTEST_OUT_DIR")
+            .unwrap_or_else(|| panic!("{}: GCCTEST_OUT_DIR not found", program)),
     );
 
     // Find the first nonexistent candidate file to which the program's args can be written.
-    for i in 0.. {
-        let candidate = &out_dir.join(format!("out{}", i));
+    let candidate = (0..).find_map(|i| {
+        let candidate = out_dir.join(format!("out{}", i));
 
-        // If the file exists, commands have already run. Try again.
         if candidate.exists() {
-            continue;
+            // If the file exists, commands have already run. Try again.
+            None
+        } else {
+            Some(candidate)
+        }
+    }).unwrap_or_else(|| panic!("Cannot find the first nonexistent candidate file to which the program's args can be written under out_dir '{}'", out_dir.display()));
+
+    // Create a file and record the args passed to the command.
+    let f = File::create(&candidate).unwrap_or_else(|e| {
+        panic!(
+            "{}: can't create candidate: {}, error: {}",
+            program,
+            candidate.display(),
+            e
+        )
+    });
+    let mut f = io::BufWriter::new(f);
+
+    (|| {
+        for arg in args {
+            writeln!(f, "{}", arg)?;
         }
 
-        // Create a file and record the args passed to the command.
-        let mut f = File::create(candidate).expect(&format!(
-            "{}: can't create candidate: {}",
+        f.flush()?;
+
+        let mut f = f.into_inner()?;
+        f.flush()?;
+        f.sync_all()
+    })()
+    .unwrap_or_else(|e| {
+        panic!(
+            "{}: can't write to candidate: {}, error: {}",
             program,
-            candidate.to_string_lossy()
-        ));
-        for arg in args {
-            writeln!(f, "{}", arg).expect(&format!(
-                "{}: can't write to candidate: {}",
-                program,
-                candidate.to_string_lossy()
-            ));
-        }
-        break;
-    }
+            candidate.display(),
+            e
+        )
+    });
 
     // Create a file used by some tests.
     let path = &out_dir.join("libfoo.a");
-    File::create(path).expect(&format!(
-        "{}: can't create libfoo.a: {}",
-        program,
-        path.to_string_lossy()
-    ));
+    File::create(path).unwrap_or_else(|e| {
+        panic!(
+            "{}: can't create libfoo.a: {}, error: {}",
+            program,
+            path.display(),
+            e
+        )
+    });
 }