Update wording of diagnostic

Author: oli-obk

compiler/rustc_hir_typeck/messages.ftl

@@ -139,10 +139,10 @@ hir_typeck_option_result_asref = use `{$def_path}::as_ref` to convert `{$expecte
 hir_typeck_option_result_cloned = use `{$def_path}::cloned` to clone the value inside the `{$def_path}`
 hir_typeck_option_result_copied = use `{$def_path}::copied` to copy the value inside the `{$def_path}`
 
-hir_typeck_pass_to_variadic_function = can't pass `{$ty}` to variadic function
+hir_typeck_pass_to_variadic_function = can't pass `{$ty}` into variable arguments
     .suggestion = cast the value to `{$cast_ty}`
     .help = cast the value to `{$cast_ty}`
-    .teach_help = certain types, like `{$ty}`, must be casted before passing them to a variadic function, because of arcane ABI rules dictated by the C standard
+    .teach_help = a `{$ty}` would be cast to `{$cast_ty}` by C's default argument promotion
 
 hir_typeck_remove_semi_for_coerce = you might have meant to return the `match` expression
 hir_typeck_remove_semi_for_coerce_expr = this could be implicitly returned but it is a statement, not a tail expression

tests/ui/c-variadic/issue-32201.rs

@@ -7,7 +7,7 @@ fn bar(_: *const u8) {}
 fn main() {
     unsafe {
         foo(0, bar);
-        //~^ ERROR can't pass `fn(*const u8) {bar}` to variadic function
+        //~^ ERROR can't pass `fn(*const u8) {bar}` into variable arguments
         //~| HELP cast the value to `fn(*const u8)`
     }
 }

tests/ui/c-variadic/issue-32201.stderr

@@ -1,4 +1,4 @@
-error[E0617]: can't pass `fn(*const u8) {bar}` to variadic function
+error[E0617]: can't pass `fn(*const u8) {bar}` into variable arguments
   --> $DIR/issue-32201.rs:9:16
    |
 LL |         foo(0, bar);

tests/ui/c-variadic/variadic-ffi-1.stderr

@@ -58,37 +58,37 @@ LL |         let y: extern "C" fn(f: isize, x: u8, ...) = bar;
    = note: expected fn pointer `extern "C" fn(_, _, ...)`
                  found fn item `extern "C" fn(_, _) {bar}`
 
-error[E0617]: can't pass `f32` to variadic function
+error[E0617]: can't pass `f32` into variable arguments
   --> $DIR/variadic-ffi-1.rs:28:19
    |
 LL |         foo(1, 2, 3f32);
    |                   ^^^^ help: cast the value to `c_double`: `3f32 as c_double`
 
-error[E0617]: can't pass `bool` to variadic function
+error[E0617]: can't pass `bool` into variable arguments
   --> $DIR/variadic-ffi-1.rs:29:19
    |
 LL |         foo(1, 2, true);
    |                   ^^^^ help: cast the value to `c_int`: `true as c_int`
 
-error[E0617]: can't pass `i8` to variadic function
+error[E0617]: can't pass `i8` into variable arguments
   --> $DIR/variadic-ffi-1.rs:30:19
    |
 LL |         foo(1, 2, 1i8);
    |                   ^^^ help: cast the value to `c_int`: `1i8 as c_int`
 
-error[E0617]: can't pass `u8` to variadic function
+error[E0617]: can't pass `u8` into variable arguments
   --> $DIR/variadic-ffi-1.rs:31:19
    |
 LL |         foo(1, 2, 1u8);
    |                   ^^^ help: cast the value to `c_uint`: `1u8 as c_uint`
 
-error[E0617]: can't pass `i16` to variadic function
+error[E0617]: can't pass `i16` into variable arguments
   --> $DIR/variadic-ffi-1.rs:32:19
    |
 LL |         foo(1, 2, 1i16);
    |                   ^^^^ help: cast the value to `c_int`: `1i16 as c_int`
 
-error[E0617]: can't pass `u16` to variadic function
+error[E0617]: can't pass `u16` into variable arguments
   --> $DIR/variadic-ffi-1.rs:33:19
    |
 LL |         foo(1, 2, 1u16);

tests/ui/error-codes/E0617.rs

@@ -5,22 +5,22 @@ extern "C" {
 fn main() {
     unsafe {
         printf(::std::ptr::null(), 0f32);
-        //~^ ERROR can't pass `f32` to variadic function
+        //~^ ERROR can't pass `f32` into variable arguments
         //~| HELP cast the value to `c_double`
         printf(::std::ptr::null(), 0i8);
-        //~^ ERROR can't pass `i8` to variadic function
+        //~^ ERROR can't pass `i8` into variable arguments
         //~| HELP cast the value to `c_int`
         printf(::std::ptr::null(), 0i16);
-        //~^ ERROR can't pass `i16` to variadic function
+        //~^ ERROR can't pass `i16` into variable arguments
         //~| HELP cast the value to `c_int`
         printf(::std::ptr::null(), 0u8);
-        //~^ ERROR can't pass `u8` to variadic function
+        //~^ ERROR can't pass `u8` into variable arguments
         //~| HELP cast the value to `c_uint`
         printf(::std::ptr::null(), 0u16);
-        //~^ ERROR can't pass `u16` to variadic function
+        //~^ ERROR can't pass `u16` into variable arguments
         //~| HELP cast the value to `c_uint`
         printf(::std::ptr::null(), printf);
-        //~^ ERROR can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` to variadic function
+        //~^ ERROR can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` into variable arguments
         //~| HELP cast the value to `unsafe extern "C" fn(*const i8, ...)`
     }
 }

tests/ui/error-codes/E0617.stderr

@@ -1,34 +1,34 @@
-error[E0617]: can't pass `f32` to variadic function
+error[E0617]: can't pass `f32` into variable arguments
   --> $DIR/E0617.rs:7:36
    |
 LL |         printf(::std::ptr::null(), 0f32);
    |                                    ^^^^ help: cast the value to `c_double`: `0f32 as c_double`
 
-error[E0617]: can't pass `i8` to variadic function
+error[E0617]: can't pass `i8` into variable arguments
   --> $DIR/E0617.rs:10:36
    |
 LL |         printf(::std::ptr::null(), 0i8);
    |                                    ^^^ help: cast the value to `c_int`: `0i8 as c_int`
 
-error[E0617]: can't pass `i16` to variadic function
+error[E0617]: can't pass `i16` into variable arguments
   --> $DIR/E0617.rs:13:36
    |
 LL |         printf(::std::ptr::null(), 0i16);
    |                                    ^^^^ help: cast the value to `c_int`: `0i16 as c_int`
 
-error[E0617]: can't pass `u8` to variadic function
+error[E0617]: can't pass `u8` into variable arguments
   --> $DIR/E0617.rs:16:36
    |
 LL |         printf(::std::ptr::null(), 0u8);
    |                                    ^^^ help: cast the value to `c_uint`: `0u8 as c_uint`
 
-error[E0617]: can't pass `u16` to variadic function
+error[E0617]: can't pass `u16` into variable arguments
   --> $DIR/E0617.rs:19:36
    |
 LL |         printf(::std::ptr::null(), 0u16);
    |                                    ^^^^ help: cast the value to `c_uint`: `0u16 as c_uint`
 
-error[E0617]: can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` to variadic function
+error[E0617]: can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` into variable arguments
   --> $DIR/E0617.rs:22:36
    |
 LL |         printf(::std::ptr::null(), printf);