WIP: Make tests and examples compile with Rust 1.10

- Avoid using attributes on statements; use them on items instead.

- Change a test to use std::env::VarError instead of std::fmt::Error,
  because the latter didn't impl Error until Rust 1.11.

- Add an empty block after invocations of `error_chain!` inside
  functions, to work around rust-lang/rust#34436
  (fixed in Rust 1.11).

- Replace a single instance of `?` with `try!`.

This still fails to work with 1.10 due to
rust-lang/rust#22250 .  Because of that issue,
an invocation of `#[derive(Debug)]` inside a macro doesn't take cfg(...)
attributes into account, and fails to compile due to referencing enum
variants that don't exist.  Posted for reference only.

examples/quickstart.rs

@@ -53,8 +53,7 @@ fn run() -> Result<()> {
     use std::fs::File;
 
     // This operation will fail
-    File::open("tretrete")
-        .chain_err(|| "unable to open tretrete file")?;
+    try!(File::open("tretrete").chain_err(|| "unable to open tretrete file"));
 
     Ok(())
 }

examples/size.rs

@@ -21,20 +21,14 @@ fn main() {
     println!("      ErrorKind::Msg: {}", size_of_val(&msg));
     println!("        String: {}", size_of::<String>());
     println!("    State: {}", size_of::<error_chain::State>());
+    let state = error_chain::State::default();
+    println!("      State.next_error: {}", size_of_val(&state.next_error));
     #[cfg(feature = "backtrace")]
-    {
-        let state = error_chain::State {
-            next_error: None,
-            backtrace: None,
-        };
-        println!("      State.next_error: {}", size_of_val(&state.next_error));
+    fn size_of_backtrace() {
+        let state = error_chain::State::default();
         println!("      State.backtrace: {}", size_of_val(&state.backtrace));
     }
     #[cfg(not(feature = "backtrace"))]
-    {
-        let state = error_chain::State {
-            next_error: None,
-        };
-        println!("      State.next_error: {}", size_of_val(&state.next_error));
-    }
+    fn size_of_backtrace() {}
+    size_of_backtrace();
 }

tests/tests.rs

@@ -236,18 +236,18 @@ fn has_backtrace_depending_on_env() {
 
 #[test]
 fn chain_err() {
-    use std::fmt;
+    use std::env;
 
     error_chain! {
         foreign_links {
-            Fmt(fmt::Error);
+            Var(env::VarError);
         }
         errors {
             Test
         }
     }
 
-    let _: Result<()> = Err(fmt::Error).chain_err(|| "");
+    let _: Result<()> = Err(env::VarError::NotPresent).chain_err(|| "");
     let _: Result<()> = Err(Error::from_kind(ErrorKind::Test)).chain_err(|| "");
 }
 
@@ -262,6 +262,8 @@ fn links() {
             Test(test::Error, test::ErrorKind);
         }
     }
+
+    {}
 }
 
 #[cfg(test)]
@@ -435,6 +437,8 @@ fn documentation() {
             Variant
         }
     }
+
+    {}
 }
 
 #[cfg(test)]
@@ -469,6 +473,8 @@ fn rustup_regression() {
             }
         }
     }
+
+    {}
 }
 
 #[test]
@@ -503,6 +509,8 @@ fn error_first() {
 
         foreign_links { }
     }
+
+    {}
 }
 
 #[test]