adding @error directive

Signed-off-by: Jakub Doka <jakub.doka2@gmail.com>

lang/README.md

@@ -427,6 +427,9 @@ $SOME_CONST := 34
 foo := @use("foo.hb")
 
 main := fn(): uint {
+	if false {
+		@error("unexpected '\0", u8, "', wah now?\0")
+	}
 	byte := @as(u8, 10)
 	_ = sum(byte, byte)
 	same_type_as_byte := @as(@TypeOf(byte), 30)
@@ -474,6 +477,7 @@ arbitrary text
 - `@len(<ty>)`: reports a length of the type of indexing purposes or length ot a string constant
 - `@kindof(<ty>)`: gives an u8 integer describing the kind of type as an index to array `[Builtin, Struct, Enum, Union, Ptr, Slice, Opt, Func, Template, Global, Const, Module]`
 - `@Any()`: generic parameter based on inference, TBD: this will ake arguments in the future that restrict what is accepted
+- `@error(...<expr>)`: emit compiler error, if reachable, and use arguments to construct a message, can display strings and types
 
 #### c_strings
 ```hb

lang/src/son.rs

@@ -1482,6 +1482,35 @@ impl<'a> Codegen<'a> {
                 self.assert_ty(expr.pos(), &mut val, ty, "hinted expr");
                 Some(val)
             }
+            Expr::Directive { pos, name: "error", args } => {
+                let mut error_msg = String::new();
+                for arg in args {
+                    let Some(val) = self.expr(arg) else {
+                        self.error(arg.pos(), "unreachable argument");
+                        continue;
+                    };
+                    match self.ci.nodes[val.id].kind {
+                        Kind::Global { global }
+                            if let Ok(str) = core::str::from_utf8(
+                                &self.tys.ins.globals[global].data
+                                    [..self.tys.ins.globals[global].data.len() - 1],
+                            ) =>
+                        {
+                            error_msg.push_str(str)
+                        }
+                        Kind::CInt { value } if val.ty == ty::Id::TYPE => {
+                            _ = write!(
+                                error_msg,
+                                "{}",
+                                ty::Display::new(self.tys, self.files, ty::Id::from(value as u64))
+                            )
+                        }
+                        _ => _ = self.error(arg.pos(), "expression can not (yet) be displayed"),
+                    }
+                }
+                self.error(pos, error_msg);
+                None
+            }
             Expr::Directive { pos, name: "eca", args } => {
                 inference!(ty, ctx, self, pos, "return type", "@as(<return_ty>, @eca(<expr>...))");