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Rollup merge of rust-lang#99291 - est31:let_else_tests, r=joshtriplett
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Add let else drop order tests

Add a systematic matrix based test that checks temporary drop order in various settings, `let-else-drop-order.rs`, as requested [here](rust-lang#93628 (comment)).

The drop order of let and let else is supposed to be the and in order to ensure this, the test checks that this holds for a number of cases.

The test also ensures that we drop the temporaries of the condition before executing the else block.

cc rust-lang#87335 tracking issue for `let else`
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matthiaskrgr authored Sep 5, 2022
2 parents 6e4a9ab + 2d8460e commit f9709a7
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270 changes: 270 additions & 0 deletions src/test/ui/let-else/let-else-drop-order.rs
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// run-pass
// edition:2021
// check-run-results
//
// Drop order tests for let else
//
// Mostly this ensures two things:
// 1. That let and let else temporary drop order is the same.
// This is a specific design request: https://github.com/rust-lang/rust/pull/93628#issuecomment-1047140316
// 2. That the else block truly only runs after the
// temporaries have dropped.
//
// We also print some nice tables for an overview by humans.
// Changes in those tables are considered breakages, but the
// important properties 1 and 2 are also enforced by the code.
// This is important as it's easy to update the stdout file
// with a --bless and miss the impact of that change.

#![feature(let_else)]
#![allow(irrefutable_let_patterns)]

use std::cell::RefCell;
use std::rc::Rc;

#[derive(Clone)]
struct DropAccountant(Rc<RefCell<Vec<Vec<String>>>>);

impl DropAccountant {
fn new() -> Self {
Self(Default::default())
}
fn build_droppy(&self, v: u32) -> Droppy<u32> {
Droppy(self.clone(), v)
}
fn build_droppy_enum_none(&self, _v: u32) -> ((), DroppyEnum<u32>) {
((), DroppyEnum::None(self.clone()))
}
fn new_list(&self, s: impl ToString) {
self.0.borrow_mut().push(vec![s.to_string()]);
}
fn push(&self, s: impl ToString) {
let s = s.to_string();
let mut accounts = self.0.borrow_mut();
accounts.last_mut().unwrap().push(s);
}
fn print_table(&self) {
println!();

let accounts = self.0.borrow();
let before_last = &accounts[accounts.len() - 2];
let last = &accounts[accounts.len() - 1];
let before_last = get_comma_list(before_last);
let last = get_comma_list(last);
const LINES: &[&str] = &[
"vanilla",
"&",
"&mut",
"move",
"fn(this)",
"tuple",
"array",
"ref &",
"ref mut &mut",
];
let max_len = LINES.iter().map(|v| v.len()).max().unwrap();
let max_len_before = before_last.iter().map(|v| v.len()).max().unwrap();
let max_len_last = last.iter().map(|v| v.len()).max().unwrap();

println!(
"| {: <max_len$} | {: <max_len_before$} | {: <max_len_last$} |",
"construct", before_last[0], last[0]
);
println!("| {:-<max_len$} | {:-<max_len_before$} | {:-<max_len_last$} |", "", "", "");

for ((l, l_before), l_last) in
LINES.iter().zip(before_last[1..].iter()).zip(last[1..].iter())
{
println!(
"| {: <max_len$} | {: <max_len_before$} | {: <max_len_last$} |",
l, l_before, l_last,
);
}
}
#[track_caller]
fn assert_all_equal_to(&self, st: &str) {
let accounts = self.0.borrow();
let last = &accounts[accounts.len() - 1];
let last = get_comma_list(last);
for line in last[1..].iter() {
assert_eq!(line.trim(), st.trim());
}
}
#[track_caller]
fn assert_equality_last_two_lists(&self) {
let accounts = self.0.borrow();
let last = &accounts[accounts.len() - 1];
let before_last = &accounts[accounts.len() - 2];
for (l, b) in last[1..].iter().zip(before_last[1..].iter()) {
if !(l == b || l == "n/a" || b == "n/a") {
panic!("not equal: '{last:?}' != '{before_last:?}'");
}
}
}
}

fn get_comma_list(sl: &[String]) -> Vec<String> {
std::iter::once(sl[0].clone())
.chain(sl[1..].chunks(2).map(|c| c.join(",")))
.collect::<Vec<String>>()
}

struct Droppy<T>(DropAccountant, T);

impl<T> Drop for Droppy<T> {
fn drop(&mut self) {
self.0.push("drop");
}
}

#[allow(dead_code)]
enum DroppyEnum<T> {
Some(DropAccountant, T),
None(DropAccountant),
}

impl<T> Drop for DroppyEnum<T> {
fn drop(&mut self) {
match self {
DroppyEnum::Some(acc, _inner) => acc,
DroppyEnum::None(acc) => acc,
}
.push("drop");
}
}

macro_rules! nestings_with {
($construct:ident, $binding:pat, $exp:expr) => {
// vanilla:
$construct!($binding, $exp.1);

// &:
$construct!(&$binding, &$exp.1);

// &mut:
$construct!(&mut $binding, &mut ($exp.1));

{
// move:
let w = $exp;
$construct!(
$binding,
{
let w = w;
w
}
.1
);
}

// fn(this):
$construct!($binding, std::convert::identity($exp).1);
};
}

macro_rules! nestings {
($construct:ident, $binding:pat, $exp:expr) => {
nestings_with!($construct, $binding, $exp);

// tuple:
$construct!(($binding, 77), ($exp.1, 77));

// array:
$construct!([$binding], [$exp.1]);
};
}

macro_rules! let_else {
($acc:expr, $v:expr, $binding:pat, $build:ident) => {
let acc = $acc;
let v = $v;

macro_rules! let_else_construct {
($arg:pat, $exp:expr) => {
loop {
let $arg = $exp else {
acc.push("else");
break;
};
acc.push("body");
break;
}
};
}
nestings!(let_else_construct, $binding, acc.$build(v));
// ref &:
let_else_construct!($binding, &acc.$build(v).1);

// ref mut &mut:
let_else_construct!($binding, &mut acc.$build(v).1);
};
}

macro_rules! let_ {
($acc:expr, $binding:tt) => {
let acc = $acc;

macro_rules! let_construct {
($arg:pat, $exp:expr) => {{
let $arg = $exp;
acc.push("body");
}};
}
let v = 0;
{
nestings_with!(let_construct, $binding, acc.build_droppy(v));
}
acc.push("n/a");
acc.push("n/a");
acc.push("n/a");
acc.push("n/a");

// ref &:
let_construct!($binding, &acc.build_droppy(v).1);

// ref mut &mut:
let_construct!($binding, &mut acc.build_droppy(v).1);
};
}

fn main() {
let acc = DropAccountant::new();

println!(" --- matching cases ---");

// Ensure that let and let else have the same behaviour
acc.new_list("let _");
let_!(&acc, _);
acc.new_list("let else _");
let_else!(&acc, 0, _, build_droppy);
acc.assert_equality_last_two_lists();
acc.print_table();

// Ensure that let and let else have the same behaviour
acc.new_list("let _v");
let_!(&acc, _v);
acc.new_list("let else _v");
let_else!(&acc, 0, _v, build_droppy);
acc.assert_equality_last_two_lists();
acc.print_table();

println!();

println!(" --- mismatching cases ---");

acc.new_list("let else _ mismatch");
let_else!(&acc, 1, DroppyEnum::Some(_, _), build_droppy_enum_none);
acc.new_list("let else _v mismatch");
let_else!(&acc, 1, DroppyEnum::Some(_, _v), build_droppy_enum_none);
acc.print_table();
// This ensures that we always drop before visiting the else case
acc.assert_all_equal_to("drop,else");

acc.new_list("let else 0 mismatch");
let_else!(&acc, 1, 0, build_droppy);
acc.new_list("let else 0 mismatch");
let_else!(&acc, 1, 0, build_droppy);
acc.print_table();
// This ensures that we always drop before visiting the else case
acc.assert_all_equal_to("drop,else");
}
51 changes: 51 additions & 0 deletions src/test/ui/let-else/let-else-drop-order.run.stdout
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--- matching cases ---

| construct | let _ | let else _ |
| ------------ | --------- | ---------- |
| vanilla | drop,body | drop,body |
| & | body,drop | body,drop |
| &mut | body,drop | body,drop |
| move | drop,body | drop,body |
| fn(this) | drop,body | drop,body |
| tuple | n/a,n/a | drop,body |
| array | n/a,n/a | drop,body |
| ref & | body,drop | body,drop |
| ref mut &mut | body,drop | body,drop |

| construct | let _v | let else _v |
| ------------ | --------- | ----------- |
| vanilla | drop,body | drop,body |
| & | body,drop | body,drop |
| &mut | body,drop | body,drop |
| move | drop,body | drop,body |
| fn(this) | drop,body | drop,body |
| tuple | n/a,n/a | drop,body |
| array | n/a,n/a | drop,body |
| ref & | body,drop | body,drop |
| ref mut &mut | body,drop | body,drop |

--- mismatching cases ---

| construct | let else _ mismatch | let else _v mismatch |
| ------------ | ------------------- | -------------------- |
| vanilla | drop,else | drop,else |
| & | drop,else | drop,else |
| &mut | drop,else | drop,else |
| move | drop,else | drop,else |
| fn(this) | drop,else | drop,else |
| tuple | drop,else | drop,else |
| array | drop,else | drop,else |
| ref & | drop,else | drop,else |
| ref mut &mut | drop,else | drop,else |

| construct | let else 0 mismatch | let else 0 mismatch |
| ------------ | ------------------- | ------------------- |
| vanilla | drop,else | drop,else |
| & | drop,else | drop,else |
| &mut | drop,else | drop,else |
| move | drop,else | drop,else |
| fn(this) | drop,else | drop,else |
| tuple | drop,else | drop,else |
| array | drop,else | drop,else |
| ref & | drop,else | drop,else |
| ref mut &mut | drop,else | drop,else |

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