Show a better error on wrong struct size

bilge-impl/src/bitsize.rs

@@ -145,14 +145,8 @@ fn generate_struct(item: &ItemStruct, declared_bitsize: u8) -> TokenStream {
     quote! {
         #vis struct #ident #fields_def
 
-        // constness: when we get const blocks evaluated at compile time, add a const computed_bitsize
-        const _: () = assert!(
-            (#computed_bitsize) == (#declared_bitsize),
-            concat!("struct size and declared bit size differ: ",
-            // stringify!(#computed_bitsize),
-            " != ",
-            stringify!(#declared_bitsize))
-        );
+        // constness: assert_eq!, format! are not const
+        const _: () = ::bilge::AssertEquals::<{#computed_bitsize}, {#declared_bitsize}>::EQUAL;
     }
 }
 

src/lib.rs

@@ -47,6 +47,14 @@ pub fn give_me_error() -> BitsError {
     BitsError
 }
 
+/// This is used in `bitsize.rs` gen, for showing a compile error.
+/// Only needed because const contexts can't use `format!` and `COMPUTED_SIZE` is not a plain number.
+/// `evaluation of `bilge::AssertEquals::<19, 18>::EQUAL` failed` will tell you what's wrong.
+pub struct AssertEquals<const COMPUTED_SIZE: usize, const DECLARED_SIZE: usize>;
+impl<const COMPUTED_SIZE: usize, const DECLARED_SIZE: usize> AssertEquals<COMPUTED_SIZE, DECLARED_SIZE> {
+    pub const EQUAL: () = assert!(COMPUTED_SIZE == DECLARED_SIZE, "computed bitsize and declared bitsize differ");
+}
+
 /// Only basing this on Number did not work, as bool and others are not Number.
 /// We could remove the whole macro_rules thing if it worked, though.
 /// Maybe there is some way to do this, I'm not deep into types.

tests/ui/attr-value-does-not-match-size.rs

@@ -0,0 +1,9 @@
+use bilge::prelude::*;
+
+#[bitsize(18)]
+struct Wrong {
+    field1: u6,
+    field2: u13,
+}
+
+fn main() {}

tests/ui/attr-value-does-not-match-size.stderr

@@ -0,0 +1,15 @@
+error[E0080]: evaluation of `bilge::AssertEquals::<19, 18>::EQUAL` failed
+ --> src/lib.rs
+  |
+  |     pub const EQUAL: () = assert!(COMPUTED_SIZE == DECLARED_SIZE, "computed bitsize and declared bitsize differ");
+  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'computed bitsize and declared bitsize differ', $DIR/src/lib.rs:52:27
+  |
+  = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0080]: evaluation of constant value failed
+ --> tests/ui/attr-value-does-not-match-size.rs:3:1
+  |
+3 | #[bitsize(18)]
+  | ^^^^^^^^^^^^^^ referenced constant has errors
+  |
+  = note: this error originates in the attribute macro `bitsize` (in Nightly builds, run with -Z macro-backtrace for more info)