Improve f-string syntax. Fixes RustPython#1099

Author: windelbouwman

parser/src/python.lalrpop

@@ -19,7 +19,7 @@ grammar;
 pub Top: ast::Top = {
     StartProgram <p:Program> => ast::Top::Program(p),
     StartStatement <s:Statement> => ast::Top::Statement(s),
-    StartExpression <e:Expression> => ast::Top::Expression(e),
+    StartExpression <e:Test> => ast::Top::Expression(e),
 };
 
 Program: ast::Program = {

tests/snippets/fstrings.py

@@ -39,3 +39,9 @@ def __str__(self):
 assert f'{v}' == 'foo'
 assert f'{v!r}' == 'bar'
 assert f'{v!s}' == 'baz'
+
+# advanced expressions:
+
+assert f'{True or True}' == 'True'
+assert f'{1 == 1}' == 'True'
+assert f'{"0" if True else "1"}' == '0'

vm/src/obj/objbool.rs

@@ -5,6 +5,7 @@ use crate::pyobject::{IntoPyObject, PyContext, PyObjectRef, PyResult, TryFromObj
 use crate::vm::VirtualMachine;
 
 use super::objint::PyInt;
+use super::objstr::PyStringRef;
 use super::objtype;
 
 impl IntoPyObject for bool {
@@ -47,6 +48,7 @@ The class bool is a subclass of the class int, and cannot be subclassed.";
     extend_class!(context, bool_type, {
         "__new__" => context.new_rustfunc(bool_new),
         "__repr__" => context.new_rustfunc(bool_repr),
+        "__format__" => context.new_rustfunc(bool_format),
         "__or__" => context.new_rustfunc(bool_or),
         "__ror__" => context.new_rustfunc(bool_ror),
         "__and__" => context.new_rustfunc(bool_and),
@@ -82,6 +84,18 @@ fn bool_repr(vm: &VirtualMachine, args: PyFuncArgs) -> Result<PyObjectRef, PyObj
     Ok(vm.new_str(s))
 }
 
+fn bool_format(
+    obj: PyObjectRef,
+    format_spec: PyStringRef,
+    vm: &VirtualMachine,
+) -> PyResult<PyStringRef> {
+    if format_spec.value.is_empty() {
+        vm.to_str(&obj)
+    } else {
+        Err(vm.new_type_error("unsupported format string passed to bool.__format__".to_string()))
+    }
+}
+
 fn do_bool_or(vm: &VirtualMachine, lhs: &PyObjectRef, rhs: &PyObjectRef) -> PyResult {
     if objtype::isinstance(lhs, &vm.ctx.bool_type())
         && objtype::isinstance(rhs, &vm.ctx.bool_type())