Add E0442 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -3047,6 +3047,38 @@ extern "platform-intrinsic" {
 ```
 "##,
 
+E0442: r##"
+Intrinsic argument(s) and/or return value have the wrong length.
+Erroneous code example:
+
+```
+#[repr(simd)]
+struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
+             i8, i8, i8, i8, i8, i8, i8, i8);
+#[repr(simd)]
+struct i32x4(i32, i32, i32, i32);
+#[repr(simd)]
+struct i64x2(i64, i64);
+
+extern "platform-intrinsic" {
+    fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
+    // error: intrinsic arguments have wrong length
+}
+```
+
+To fix this error, please refer to the function declaration to give
+it the awaited types with the awaited length. Example:
+
+```
+#[repr(simd)]
+struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
+
+extern "platform-intrinsic" {
+    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
+}
+```
+"##
+
 }
 
 register_diagnostics! {
@@ -3131,7 +3163,6 @@ register_diagnostics! {
     E0439, // invalid `simd_shuffle`, needs length: `{}`
     E0440, // platform-specific intrinsic has wrong number of type parameters
     E0441, // unrecognized platform-specific intrinsic function
-    E0442, // intrinsic {} has wrong type: found {}, expected {}
     E0443, // intrinsic {} has wrong type: found `{}`, expected `{}` which
            // was used for this vector type previously in this signature
 }