Why does compiling this program without a main function result in "undefined reference to main"?

Question

I have this code below:

#![no_main]

pub mod struct_example {
    struct Pair {
        a: f64,
        b: f64,
    }

    pub fn example () {
        println!("##### Struct Example #####");
        let pair: Pair = Pair {a: 0.3, b: 0.4};
        println!("{} and {}", pair.a, pair.b);
    }
}

That I am trying to compile by issuing:

rustc src/structures.rs 

What I get is the error below:

$ rustc src/structures.rs         
error: linking with `cc` failed: exit code: 1
note: "cc" "-Wl,--as-needed" "-m64" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "./structures" "./structures.o" "-Wl,--whole-archive" "-lmorestack" "-Wl,--no-whole-archive" "-Wl,--gc-sections" "-pie" "-nodefaultlibs" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunicode-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-4e7c5e5c.rlib" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "/home/lowks/src/rust/gettingrusty/.rust/lib/x86_64-unknown-linux-gnu" "-L" "/home/lowks/src/rust/gettingrusty/lib/x86_64-unknown-linux-gnu" "-Wl,--whole-archive" "-Wl,-Bstatic" "-Wl,--no-whole-archive" "-Wl,-Bdynamic" "-ldl" "-lpthread" "-lrt" "-lgcc_s" "-lpthread" "-lc" "-lm" "-lcompiler-rt"
note: /usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 0 has invalid symbol index 11
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 1 has invalid symbol index 12
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 2 has invalid symbol index 2
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 3 has invalid symbol index 2
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 4 has invalid symbol index 11
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 5 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 6 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 7 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 8 has invalid symbol index 12
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 9 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 10 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 11 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 12 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 13 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 14 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 15 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 16 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 17 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 18 has invalid symbol index 13
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_info): relocation 19 has invalid symbol index 22
/usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/Scrt1.o(.debug_line): relocation 0 has invalid symbol index 2
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

error: aborting due to previous error

Why is this? Is there something I am missing here ?

Answer 1

By default, rustc produces a binary crate, not a library one, which needs a main(). You can pass --crate-type=lib if you're attempting to build a library instead of a binary.