I've got some traits set up to allow writing some text, as well as some parameters to a sink. The goal is to allow the types that write to the sink (Expressions) to write parameters as borrows rather than requiring cloning. The parameters are stored in the sink until all of the expressions have been written.
That all works fine as long as the types are static, but when I try to use a trait object things fall apart completely. [1] Since Expression has a lifetime on the trait, I believe the compiler is being forced to pick a lifetime earlier when the type is coerced to a trait object, but I'm not sure what I can do to resolve the problem.
Code
use std::{fmt::Display, ops::Deref};
/// A type containing a trait object, which can have a blanket impl for types implementing that trait.
pub trait ParameterRef<'p, T> {
fn from_param_ref(param: &'p T) -> Self;
}
/// A parameter for types implementing [Display]
pub struct DisplayParam<'a>(&'a dyn Display);
impl<'a> std::fmt::Debug for DisplayParam<'a> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_tuple("TestParam")
.field(&self.0.to_string())
.finish()
}
}
impl<'a, T> ParameterRef<'a, T> for DisplayParam<'a>
where
T: Display,
{
fn from_param_ref(param: &'a T) -> Self {
Self(param)
}
}
pub trait Meta {
type Param;
}
/// A sink for strings and parameters. Parameters can be borrowed.
pub trait Write<'param>: Meta + Sized
where
<Self as Meta>::Param: 'param,
{
fn param_ref<T: 'param>(&mut self, param: &'param T)
where
Self::Param: ParameterRef<'param, T>;
fn write(&mut self, sql: impl AsRef<str>);
fn expr<'e: 'param, E>(&mut self, expr: &'e E)
where
E: Expression<'param, Self> + 'e + ?Sized,
{
Expression::to_writer(expr, self)
}
}
#[derive(Debug)]
pub struct TestWriter<Param> {
sql: String,
params: Vec<Param>,
}
impl<Param> Default for TestWriter<Param> {
fn default() -> Self {
Self::new()
}
}
impl<Param> TestWriter<Param> {
pub fn new() -> Self {
Self {
sql: String::new(),
params: Vec::new(),
}
}
}
impl<Param> Meta for TestWriter<Param> {
type Param = Param;
}
impl<'param, Param: 'param> Write<'param> for TestWriter<Param> {
fn param_ref<T: 'param>(&mut self, param: &'param T)
where
Self::Param: ParameterRef<'param, T>,
{
self.params.push(Self::Param::from_param_ref(param));
}
fn write(&mut self, sql: impl AsRef<str>) {
self.sql.push_str(sql.as_ref())
}
}
/// A type which can write to a writer, potentially borrowing parameters from itself to place in the writer.
pub trait Expression<'param, Writer>
where
Writer: ?Sized,
{
fn to_writer<'e: 'param>(&'e self, writer: &mut Writer)
where
Writer: Write<'param>,
Writer::Param: 'param;
fn boxed<'a>(self) -> Box<dyn Expression<'param, Writer> + 'a>
where
Self: Sized + 'a,
{
Box::new(self)
}
}
impl<'param, T, Writer> Expression<'param, Writer> for Box<T>
where
T: ?Sized + Expression<'param, Writer>,
Writer: ?Sized,
{
fn to_writer<'e: 'param>(&'e self, writer: &mut Writer)
where
Writer: Write<'param>,
Writer::Param: 'param,
{
Box::deref(self).to_writer(writer)
}
}
/// A sample expression which writes a static string and a reference to its inner parameter.
struct Test(usize);
impl<'param, W> Expression<'param, W> for Test
where
W: Meta,
W::Param: ParameterRef<'param, usize>,
{
fn to_writer<'e: 'param>(&'e self, writer: &mut W)
where
W: Write<'param>,
W::Param: 'param,
{
writer.write("TEST ");
writer.write(self.0.to_string());
writer.param_ref(&self.0);
}
}
impl<'param, E, W, const N: usize> Expression<'param, W> for [E; N]
where
E: Expression<'param, W>,
{
fn to_writer<'e: 'param>(&'e self, writer: &mut W)
where
W: Write<'param>,
W::Param: 'param,
{
let mut first = false;
for e in self {
if first {
first = true;
} else {
writer.write(", ");
}
e.to_writer(writer)
}
}
}
fn main() {
static_ty();
dyn_ty();
opaque_ty();
}
fn static_ty() {
let mut writer = TestWriter::<DisplayParam>::new();
let expr = [Test(1), Test(2), Test(3)];
writer.expr(&expr);
println!("{:?}", writer);
}
fn dyn_ty() {
let mut writer = TestWriter::<DisplayParam>::new();
let expr = [Test(1).boxed(), Test(2).boxed(), Test(3).boxed()];
// error: `expr` does not live long enough
writer.expr(&expr);
println!("{:?}", writer);
drop(writer);
}
fn opaque_ty() {
fn opaque<'p, E, W>(e: E) -> impl Expression<'p, W>
where
E: Expression<'p, W>,
W: Write<'p>,
W::Param: 'p,
{
e
}
let mut writer = TestWriter::<DisplayParam>::new();
let expr = [opaque(Test(1)), opaque(Test(2)), opaque(Test(3))];
// error: `expr` does not live long enough
writer.expr(&expr);
println!("{:?}", writer);
drop(writer);
}
The errors are
error[E0597]: `expr` does not live long enough
--> src/main.rs:183:17
|
183 | writer.expr(&expr);
| ^^^^^ borrowed value does not live long enough
...
188 | }
| -
| |
| `expr` dropped here while still borrowed
| borrow might be used here, when `expr` is dropped and runs the destructor for type `[Box<dyn Expression<'_, TestWriter<DisplayParam<'_>>>>; 3]`
error[E0597]: `expr` does not live long enough
--> src/main.rs:204:17
|
204 | writer.expr(&expr);
| ^^^^^ borrowed value does not live long enough
...
209 | }
| -
| |
| `expr` dropped here while still borrowed
| borrow might be used here, when `expr` is dropped and runs the destructor for type `[impl Expression<'_, TestWriter<DisplayParam<'_>>>; 3]`
For more information about this error, try `rustc --explain E0597`.
error: could not compile `playground` due to 2 previous errors
I've tried
- Using HRTB's on the trait object. That actually works! As long as you don't actually want to borrow anything from the expression.
- Removing the lifetime from
Expression. I think this is essentially impossible for this use case. Not having a life time makes specifying a bound on what parameters an expression contains on anExpressionimpl basically impossible (again it actually does work as long as you don't need borrowing)
Obviously the simple solution is to just use Clone, but it's does work as long as trait objects aren't involved. That has me curious if there's something less extreme that might make this setup work that I've missed.
Thanks for reading!
-
This turns out to also be true for opaque types implementing Expression âŠī¸
.