making @len work on strings

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

lang/README.md

@@ -440,6 +440,7 @@ main := fn(): uint {
 	ecall_that_returns_uint := @as(uint, @eca(1, foo.Type.(10, 20), 5, 6))
 	embedded_array := @as([u8; 15], @embed("text.txt"))
 	two_fields := @len(foo.Type)
+	string_length := @len("foo\0")
 	the_struct_kind := @kindof(foo.Type)
 	return @inline(foo.foo)
 }
@@ -466,7 +467,7 @@ arbitrary text
 - `@eca(...<expr>)`: invoke `eca` instruction, where return type is inferred and `<expr>...` are arguments passed to the call in the standard call convention
 - `@embed(<string>)`: include relative file as an array of bytes
 - `@inline(<func>, ...<args>)`: equivalent to `<func>(...<args>)` but function is guaranteed to inline, compiler will otherwise never inline
-- `@len(<ty>)`: reports a length of the type of indexing purposes
+- `@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]`
 
 #### c_strings

lang/src/son.rs

@@ -1309,16 +1309,18 @@ impl<'a> Codegen<'a> {
                 self.gen_inferred_const(ctx, ty::Id::DINT, align)
             }
             Expr::Directive { name: "len", args: [ety], .. } => {
-                let ty = self.ty(ety);
-                let Some(len) = self.tys.len_of(ty) else {
-                    return self.error(
-                        ety.pos(),
-                        fa!(
-                            "'@len' only supports structs and arrays, \
-                            '{}' is neither",
-                            self.ty_display(ty)
-                        ),
-                    );
+                let ty = self.expr(ety)?;
+                let len = match self.ci.nodes[ty.id].kind {
+                    Kind::CInt { value }
+                        if let Some(len) = self.tys.len_of(ty::Id::from(value as u64)) =>
+                    {
+                        len
+                    }
+                    Kind::Global { global } => self.tys.ins.globals[global].data.len() as u32 - 1,
+                    _ => {
+                        return self
+                            .error(ety.pos(), "'@len' only supports structs and arrays or strings")
+                    }
                 };
                 self.gen_inferred_const(ctx, ty::Id::DINT, len)
             }