Consistently use extern "C"

This is what rustfmt favors, so let's favor it too!

Closes #1042

guide/src/contributing/design/describe.md

@@ -39,7 +39,7 @@ In addition to the shims we talked about above which JS generates the macro
 
 ```
 #[no_mangle]
-pub extern fn __wbindgen_describe_greet() {
+pub extern "C" fn __wbindgen_describe_greet() {
     <Fn(&str)>::describe();
 }
 ```

guide/src/contributing/design/exporting-rust-struct.md

@@ -95,13 +95,13 @@ let's take a look at that.
 // original input to `#[wasm_bindgen]` omitted ...
 
 #[export_name = "foo_new"]
-pub extern fn __wasm_bindgen_generated_Foo_new(arg0: i32) -> u32
+pub extern "C" fn __wasm_bindgen_generated_Foo_new(arg0: i32) -> u32
     let ret = Foo::new(arg0);
     Box::into_raw(Box::new(WasmRefCell::new(ret))) as u32
 }
 
 #[export_name = "foo_get"]
-pub extern fn __wasm_bindgen_generated_Foo_get(me: u32) -> i32 {
+pub extern "C" fn __wasm_bindgen_generated_Foo_get(me: u32) -> i32 {
     let me = me as *mut WasmRefCell<Foo>;
     wasm_bindgen::__rt::assert_not_null(me);
     let me = unsafe { &*me };
@@ -109,7 +109,7 @@ pub extern fn __wasm_bindgen_generated_Foo_get(me: u32) -> i32 {
 }
 
 #[export_name = "foo_set"]
-pub extern fn __wasm_bindgen_generated_Foo_set(me: u32, arg1: i32) {
+pub extern "C" fn __wasm_bindgen_generated_Foo_set(me: u32, arg1: i32) {
     let me = me as *mut WasmRefCell<Foo>;
     ::wasm_bindgen::__rt::assert_not_null(me);
     let me = unsafe { &*me };
@@ -117,7 +117,7 @@ pub extern fn __wasm_bindgen_generated_Foo_set(me: u32, arg1: i32) {
 }
 
 #[no_mangle]
-pub unsafe extern fn __wbindgen_foo_free(me: u32) {
+pub unsafe extern "C" fn __wbindgen_foo_free(me: u32) {
     let me = me as *mut WasmRefCell<Foo>;
     wasm_bindgen::__rt::assert_not_null(me);
     (*me).borrow_mut(); // ensure no active borrows

guide/src/contributing/design/exporting-rust.md

@@ -87,12 +87,12 @@ at a mostly abbreviated and/or "simplified" in the sense of this is what it
 compiles down to:
 
 ```rust
-pub extern fn greet(a: &str) -> String {
+pub extern "C" fn greet(a: &str) -> String {
     format!("Hello, {}!", a)
 }
 
 #[export_name = "greet"]
-pub extern fn __wasm_bindgen_generated_greet(
+pub extern "C" fn __wasm_bindgen_generated_greet(
     arg0_ptr: *const u8,
     arg0_len: usize,
 ) -> *mut String {

guide/src/contributing/design/importing-js-struct.md

@@ -10,7 +10,7 @@ As usual though, let's dive into an example!
 
 ```rust
 #[wasm_bindgen(module = "./bar")]
-extern {
+extern "C" {
     type Bar;
 
     #[wasm_bindgen(constructor)]
@@ -125,7 +125,7 @@ pub struct Bar {
 
 impl Bar {
     fn new() -> Bar {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_new() -> u32;
         }
         unsafe {
@@ -135,7 +135,7 @@ impl Bar {
     }
 
     fn another_function() -> i32 {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_another_function() -> i32;
         }
         unsafe {
@@ -144,7 +144,7 @@ impl Bar {
     }
 
     fn get(&self) -> i32 {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_get(ptr: u32) -> i32;
         }
         unsafe {
@@ -155,7 +155,7 @@ impl Bar {
     }
 
     fn set(&self, val: i32) {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_set(ptr: u32, val: i32);
         }
         unsafe {
@@ -165,7 +165,7 @@ impl Bar {
     }
 
     fn property(&self) -> i32 {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_property(ptr: u32) -> i32;
         }
         unsafe {
@@ -176,7 +176,7 @@ impl Bar {
     }
 
     fn set_property(&self, val: i32) {
-        extern {
+        extern "C" {
             fn __wbg_s_Bar_set_property(ptr: u32, val: i32);
         }
         unsafe {

guide/src/contributing/design/importing-js.md

@@ -9,7 +9,7 @@ greetings in JS but call it from Rust. We might have, for example:
 
 ```rust
 #[wasm_bindgen(module = "./greet")]
-extern {
+extern "C" {
     fn greet(a: &str) -> String;
 }
 
@@ -51,8 +51,8 @@ generated Rust as well. Like before this is simplified from what's actually
 generated.
 
 ```rust
-extern fn greet(a: &str) -> String {
-    extern {
+extern "C" fn greet(a: &str) -> String {
+    extern "C" {
         fn __wbg_f_greet(a_ptr: *const u8, a_len: usize, ret_len: *mut usize) -> *mut u8;
     }
     unsafe {

guide/src/contributing/design/js-objects-in-rust.md

@@ -86,7 +86,7 @@ pub fn foo(a: &JsValue) {
 }
 
 #[export_name = "foo"]
-pub extern fn __wasm_bindgen_generated_foo(arg0: u32) {
+pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
     let arg0 = unsafe {
         ManuallyDrop::new(JsValue::__from_idx(arg0))
     };
@@ -189,7 +189,7 @@ pub fn foo(a: JsValue) {
 }
 
 #[export_name = "foo"]
-pub extern fn __wasm_bindgen_generated_foo(arg0: u32) {
+pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
     let arg0 = unsafe {
         JsValue::__from_idx(arg0)
     };