Replace uses of fuchsia-zircon with fuchsia-cprng

As in #706, fuchsia is replacing users of fuchsia-zircon with
fuchsia-cprng, since that reduces exposure to our changing
syscalls.

If this is accepted, could a new version be cut?

Author: erickt

Cargo.toml

@@ -20,7 +20,7 @@ appveyor = { repository = "alexcrichton/rand" }
 [features]
 default = ["std" ] # without "std" rand uses libcore
 nightly = ["i128_support"] # enables all features requiring nightly rust
-std = ["rand_core/std", "alloc", "libc", "winapi", "cloudabi", "fuchsia-zircon"]
+std = ["rand_core/std", "alloc", "libc", "winapi", "cloudabi", "fuchsia-cprng"]
 alloc = ["rand_core/alloc"]  # enables Vec and Box support (without std)
 i128_support = [] # enables i128 and u128 support
 serde1 = ["serde", "serde_derive", "rand_core/serde1"] # enables serialization for PRNGs
@@ -44,7 +44,7 @@ winapi = { version = "0.3", features = ["minwindef", "ntsecapi", "profileapi", "
 cloudabi = { version = "0.0.3", optional = true }
 
 [target.'cfg(target_os = "fuchsia")'.dependencies]
-fuchsia-zircon = { version = "0.3.2", optional = true }
+fuchsia-cprng = { version = "0.1.0", optional = true }
 
 [target.wasm32-unknown-unknown.dependencies]
 # use with `--target wasm32-unknown-unknown --features=stdweb`

src/rngs/os.rs

@@ -924,9 +924,9 @@ mod imp {
 
 #[cfg(target_os = "fuchsia")]
 mod imp {
-    extern crate fuchsia_zircon;
+    extern crate fuchsia_cprng;
 
-    use {Error, ErrorKind};
+    use Error;
     use super::OsRngImpl;
 
     #[derive(Clone, Debug)]
@@ -936,25 +936,10 @@ mod imp {
         fn new() -> Result<OsRng, Error> { Ok(OsRng) }
 
         fn fill_chunk(&mut self, dest: &mut [u8]) -> Result<(), Error> {
-            let mut read = 0;
-            while read < dest.len() {
-                match fuchsia_zircon::cprng_draw(&mut dest[read..]) {
-                    Ok(actual) => read += actual,
-                    Err(e) => {
-                        return Err(Error::with_cause(
-                            ErrorKind::Unavailable,
-                            "cprng_draw failed",
-                            e.into_io_error()));
-                    }
-                };
-            }
+            fuchsia_cprng::cprng_draw(dest);
             Ok(())
         }
 
-        fn max_chunk_size(&self) -> usize {
-            fuchsia_zircon::sys::ZX_CPRNG_DRAW_MAX_LEN
-        }
-
         fn method_str(&self) -> &'static str { "cprng_draw" }
     }
 }