error in using rust's rand crate

Cargo.lock

[root]
name = "hello_world"
version = "0.0.1"
dependencies = [
 "rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)",
]

[[package]]
name = "libc"
version = "0.2.10"
source = "registry+https://github.com/rust-lang/crates.io-index"

[[package]]
name = "rand"
version = "0.3.14"
source = "registry+https://github.com/rust-lang/crates.io-index"
dependencies = [
 "libc 0.2.10 (registry+https://github.com/rust-lang/crates.io-index)",
]

Cargo.toml

[package]

name = "hello_world"
version = "0.0.1"
authors = [ "Deepak Kannan <[email protected]>" ]

[dependencies]

rand = "0.3.14"

main.rs

extern crate rand;

use std::io;
use rand::Rng;

fn main() {
    println!("Guess the number!");

    let secret_number = rand::thread_rng().gen_range(1, 101);

    println!("The secret number is: {}", secret_number);

    println!("Please input your guess.");

    let mut guess = String::new();

    io::stdin().read_line(&mut guess)
        .expect("Failed to read line");

    println!("You guessed: {}", guess);
}

output.txt

# why does `rustc` throw a error
# but cargo build and run works
# using `rust-mode` in emacs and it flags the same error as `rustc`

$ rustc src/main.rs
src/main.rs:9:25: 9:41 error: unresolved name `rand::thread_rng` [E0425]
src/main.rs:9     let secret_number = rand::thread_rng().gen_range(1, 101);
                                      ^~~~~~~~~~~~~~~~
src/main.rs:9:25: 9:41 help: run `rustc --explain E0425` to see a detailed explanation
error: aborting due to previous error

$ cargo build
   Compiling hello_world v0.0.1 (file:///Users/deepak/Code/ScratchPad/hello-rust)

$ cargo run 
Running `target/debug/hello_world`
Guess the number!
The secret number is: 2
Please input your guess.
1
You guessed: 1

$ rustc --explain E0425
An unresolved name was used. Example of erroneous codes:

```
something_that_doesnt_exist::foo;
// error: unresolved name `something_that_doesnt_exist::foo`

// or:
trait Foo {
    fn bar() {
        Self; // error: unresolved name `Self`
    }
}

// or:
let x = unknown_variable;  // error: unresolved name `unknown_variable`
```

Please verify that the name wasn't misspelled and ensure that the
identifier being referred to is valid for the given situation. Example:

```
enum something_that_does_exist {
    foo
}

// or:
mod something_that_does_exist {
    pub static foo : i32 = 0i32;
}

something_that_does_exist::foo; // ok!

// or:
let unknown_variable = 12u32;
let x = unknown_variable; // ok!