Make compile_error! lazy and emit a diagnostic

crates/hir_def/src/body/tests.rs

@@ -84,6 +84,9 @@ macro_rules! env {}
 #[rustc_builtin_macro]
 macro_rules! include {}
 
+#[rustc_builtin_macro]
+macro_rules! compile_error {}
+
 #[rustc_builtin_macro]
 macro_rules! format_args {
     () => {}
@@ -103,6 +106,9 @@ fn f() {
     env!("OUT_DIR");
   //^^^^^^^^^^^^^^^ `OUT_DIR` not set, enable "load out dirs from check" to fix
 
+    compile_error!("compile_error works");
+  //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `compile_error!` called: compile_error works
+
     // Lazy:
 
     format_args!();

crates/hir_expand/src/builtin_macro.rs

@@ -86,7 +86,6 @@ pub fn find_builtin_macro(
 register_builtin! {
     LAZY:
     (column, Column) => column_expand,
-    (compile_error, CompileError) => compile_error_expand,
     (file, File) => file_expand,
     (line, Line) => line_expand,
     (assert, Assert) => assert_expand,
@@ -97,6 +96,7 @@ register_builtin! {
     (format_args_nl, FormatArgsNl) => format_args_expand,
 
     EAGER:
+    (compile_error, CompileError) => compile_error_expand,
     (concat, Concat) => concat_expand,
     (include, Include) => include_expand,
     (include_bytes, IncludeBytes) => include_bytes_expand,
@@ -213,25 +213,6 @@ fn file_expand(
     ExpandResult::ok(expanded)
 }
 
-fn compile_error_expand(
-    _db: &dyn AstDatabase,
-    _id: LazyMacroId,
-    tt: &tt::Subtree,
-) -> ExpandResult<tt::Subtree> {
-    if tt.count() == 1 {
-        if let tt::TokenTree::Leaf(tt::Leaf::Literal(it)) = &tt.token_trees[0] {
-            let s = it.text.as_str();
-            if s.contains('"') {
-                return ExpandResult::ok(quote! { loop { #it }});
-            }
-        };
-    }
-
-    ExpandResult::only_err(mbe::ExpandError::BindingError(
-        "`compile_error!` argument be a string".into(),
-    ))
-}
-
 fn format_args_expand(
     _db: &dyn AstDatabase,
     _id: LazyMacroId,
@@ -280,6 +261,30 @@ fn unquote_str(lit: &tt::Literal) -> Option<String> {
     token.value().map(|it| it.into_owned())
 }
 
+fn compile_error_expand(
+    _db: &dyn AstDatabase,
+    _id: EagerMacroId,
+    tt: &tt::Subtree,
+) -> ExpandResult<Option<(tt::Subtree, FragmentKind)>> {
+    let err = match &*tt.token_trees {
+        [tt::TokenTree::Leaf(tt::Leaf::Literal(it))] => {
+            let text = it.text.as_str();
+            if text.starts_with('"') && text.ends_with('"') {
+                // FIXME: does not handle raw strings
+                mbe::ExpandError::Other(format!(
+                    "`compile_error!` called: {}",
+                    &text[1..text.len() - 1]
+                ))
+            } else {
+                mbe::ExpandError::BindingError("`compile_error!` argument must be a string".into())
+            }
+        }
+        _ => mbe::ExpandError::BindingError("`compile_error!` argument must be a string".into()),
+    };
+
+    ExpandResult { value: Some((quote! {}, FragmentKind::Items)), err: Some(err) }
+}
+
 fn concat_expand(
     _db: &dyn AstDatabase,
     _arg_id: EagerMacroId,
@@ -646,7 +651,8 @@ mod tests {
             "#,
         );
 
-        assert_eq!(expanded, r#"loop{"error!"}"#);
+        // This expands to nothing (since it's in item position), but emits an error.
+        assert_eq!(expanded, "");
     }
 
     #[test]

crates/hir_expand/src/db.rs

@@ -207,6 +207,7 @@ fn macro_expand_with_arg(
             } else {
                 return ExpandResult {
                     value: Some(db.lookup_intern_eager_expansion(id).subtree),
+                    // FIXME: There could be errors here!
                     err: None,
                 };
             }