Add matrix based test for documenting the let / let else temporary drop order
The drop order of let and let else is supposed to be the same, and in order to ensure this, the test checks that this holds for the given list of cases. The test also ensures that we drop the temporaries of the condition before executing the else block. We made the test matrix based so it can check all the possible combinations and find out possible edge cases.
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src/test/ui/let-else/let-else-drop-order.rs
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270
src/test/ui/let-else/let-else-drop-order.rs
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// run-pass
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// edition:2021
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// check-run-results
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//
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// Drop order tests for let else
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//
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// Mostly this ensures two things:
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// 1. That let and let else temporary drop order is the same.
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// This is a specific design request: https://github.com/rust-lang/rust/pull/93628#issuecomment-1047140316
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// 2. That the else block truly only runs after the
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// temporaries have dropped.
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//
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// We also print some nice tables for an overview by humans.
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// Changes in those tables are considered breakages, but the
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// important properties 1 and 2 are also enforced by the code.
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// This is important as it's easy to update the stdout file
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// with a --bless and miss the impact of that change.
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#![feature(let_else)]
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#![allow(irrefutable_let_patterns)]
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use std::cell::RefCell;
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use std::rc::Rc;
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#[derive(Clone)]
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struct DropAccountant(Rc<RefCell<Vec<Vec<String>>>>);
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impl DropAccountant {
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fn new() -> Self {
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Self(Default::default())
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}
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fn build_droppy(&self, v: u32) -> Droppy<u32> {
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Droppy(self.clone(), v)
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}
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fn build_droppy_enum_none(&self, _v: u32) -> ((), DroppyEnum<u32>) {
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((), DroppyEnum::None(self.clone()))
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}
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fn new_list(&self, s: impl ToString) {
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self.0.borrow_mut().push(vec![s.to_string()]);
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}
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fn push(&self, s: impl ToString) {
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let s = s.to_string();
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let mut accounts = self.0.borrow_mut();
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accounts.last_mut().unwrap().push(s);
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}
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fn print_table(&self) {
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println!();
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let accounts = self.0.borrow();
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let before_last = &accounts[accounts.len() - 2];
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let last = &accounts[accounts.len() - 1];
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let before_last = get_comma_list(before_last);
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let last = get_comma_list(last);
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const LINES: &[&str] = &[
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"vanilla",
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"&",
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"&mut",
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"move",
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"fn(this)",
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"tuple",
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"array",
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"ref &",
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"ref mut &mut",
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];
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let max_len = LINES.iter().map(|v| v.len()).max().unwrap();
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let max_len_before = before_last.iter().map(|v| v.len()).max().unwrap();
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let max_len_last = last.iter().map(|v| v.len()).max().unwrap();
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println!(
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"| {: <max_len$} | {: <max_len_before$} | {: <max_len_last$} |",
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"construct", before_last[0], last[0]
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);
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println!("| {:-<max_len$} | {:-<max_len_before$} | {:-<max_len_last$} |", "", "", "");
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for ((l, l_before), l_last) in
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LINES.iter().zip(before_last[1..].iter()).zip(last[1..].iter())
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{
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println!(
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"| {: <max_len$} | {: <max_len_before$} | {: <max_len_last$} |",
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l, l_before, l_last,
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);
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}
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}
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#[track_caller]
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fn assert_all_equal_to(&self, st: &str) {
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let accounts = self.0.borrow();
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let last = &accounts[accounts.len() - 1];
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let last = get_comma_list(last);
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for line in last[1..].iter() {
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assert_eq!(line.trim(), st.trim());
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}
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}
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#[track_caller]
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fn assert_equality_last_two_lists(&self) {
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let accounts = self.0.borrow();
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let last = &accounts[accounts.len() - 1];
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let before_last = &accounts[accounts.len() - 2];
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for (l, b) in last[1..].iter().zip(before_last[1..].iter()) {
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if !(l == b || l == "n/a" || b == "n/a") {
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panic!("not equal: '{last:?}' != '{before_last:?}'");
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}
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}
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}
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}
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fn get_comma_list(sl: &[String]) -> Vec<String> {
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std::iter::once(sl[0].clone())
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.chain(sl[1..].chunks(2).map(|c| c.join(",")))
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.collect::<Vec<String>>()
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}
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struct Droppy<T>(DropAccountant, T);
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impl<T> Drop for Droppy<T> {
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fn drop(&mut self) {
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self.0.push("drop");
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}
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}
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#[allow(dead_code)]
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enum DroppyEnum<T> {
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Some(DropAccountant, T),
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None(DropAccountant),
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}
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impl<T> Drop for DroppyEnum<T> {
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fn drop(&mut self) {
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match self {
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DroppyEnum::Some(acc, _inner) => acc,
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DroppyEnum::None(acc) => acc,
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}
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.push("drop");
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}
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}
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macro_rules! nestings_with {
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($construct:ident, $binding:pat, $exp:expr) => {
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// vanilla:
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$construct!($binding, $exp.1);
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// &:
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$construct!(&$binding, &$exp.1);
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// &mut:
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$construct!(&mut $binding, &mut ($exp.1));
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{
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// move:
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let w = $exp;
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$construct!(
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$binding,
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{
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let w = w;
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w
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}
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.1
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);
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}
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// fn(this):
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$construct!($binding, std::convert::identity($exp).1);
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};
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}
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macro_rules! nestings {
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($construct:ident, $binding:pat, $exp:expr) => {
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nestings_with!($construct, $binding, $exp);
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// tuple:
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$construct!(($binding, 77), ($exp.1, 77));
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// array:
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$construct!([$binding], [$exp.1]);
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};
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}
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macro_rules! let_else {
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($acc:expr, $v:expr, $binding:pat, $build:ident) => {
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let acc = $acc;
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let v = $v;
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macro_rules! let_else_construct {
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($arg:pat, $exp:expr) => {
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loop {
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let $arg = $exp else {
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acc.push("else");
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break;
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};
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acc.push("body");
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break;
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}
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};
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}
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nestings!(let_else_construct, $binding, acc.$build(v));
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// ref &:
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let_else_construct!($binding, &acc.$build(v).1);
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// ref mut &mut:
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let_else_construct!($binding, &mut acc.$build(v).1);
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};
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}
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macro_rules! let_ {
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($acc:expr, $binding:tt) => {
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let acc = $acc;
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macro_rules! let_construct {
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($arg:pat, $exp:expr) => {{
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let $arg = $exp;
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acc.push("body");
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}};
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}
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let v = 0;
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{
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nestings_with!(let_construct, $binding, acc.build_droppy(v));
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}
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acc.push("n/a");
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acc.push("n/a");
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acc.push("n/a");
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acc.push("n/a");
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// ref &:
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let_construct!($binding, &acc.build_droppy(v).1);
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// ref mut &mut:
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let_construct!($binding, &mut acc.build_droppy(v).1);
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};
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}
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fn main() {
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let acc = DropAccountant::new();
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println!(" --- matching cases ---");
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// Ensure that let and let else have the same behaviour
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acc.new_list("let _");
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let_!(&acc, _);
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acc.new_list("let else _");
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let_else!(&acc, 0, _, build_droppy);
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acc.assert_equality_last_two_lists();
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acc.print_table();
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// Ensure that let and let else have the same behaviour
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acc.new_list("let _v");
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let_!(&acc, _v);
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acc.new_list("let else _v");
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let_else!(&acc, 0, _v, build_droppy);
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acc.assert_equality_last_two_lists();
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acc.print_table();
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println!();
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println!(" --- mismatching cases ---");
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acc.new_list("let else _ mismatch");
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let_else!(&acc, 1, DroppyEnum::Some(_, _), build_droppy_enum_none);
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acc.new_list("let else _v mismatch");
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let_else!(&acc, 1, DroppyEnum::Some(_, _v), build_droppy_enum_none);
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acc.print_table();
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// This ensures that we always drop before visiting the else case
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acc.assert_all_equal_to("drop,else");
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acc.new_list("let else 0 mismatch");
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let_else!(&acc, 1, 0, build_droppy);
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acc.new_list("let else 0 mismatch");
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let_else!(&acc, 1, 0, build_droppy);
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acc.print_table();
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// This ensures that we always drop before visiting the else case
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acc.assert_all_equal_to("drop,else");
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}
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51
src/test/ui/let-else/let-else-drop-order.run.stdout
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51
src/test/ui/let-else/let-else-drop-order.run.stdout
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@ -0,0 +1,51 @@
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--- matching cases ---
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| construct | let _ | let else _ |
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| ------------ | --------- | ---------- |
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| vanilla | drop,body | drop,body |
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| & | body,drop | body,drop |
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| &mut | body,drop | body,drop |
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| move | drop,body | drop,body |
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| fn(this) | drop,body | drop,body |
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| tuple | n/a,n/a | drop,body |
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| array | n/a,n/a | drop,body |
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| ref & | body,drop | body,drop |
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| ref mut &mut | body,drop | body,drop |
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| construct | let _v | let else _v |
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| ------------ | --------- | ----------- |
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| vanilla | drop,body | drop,body |
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| & | body,drop | body,drop |
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| &mut | body,drop | body,drop |
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| move | drop,body | drop,body |
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| fn(this) | drop,body | drop,body |
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| tuple | n/a,n/a | drop,body |
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| array | n/a,n/a | drop,body |
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| ref & | body,drop | body,drop |
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| ref mut &mut | body,drop | body,drop |
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--- mismatching cases ---
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| construct | let else _ mismatch | let else _v mismatch |
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| ------------ | ------------------- | -------------------- |
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| vanilla | drop,else | drop,else |
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| & | drop,else | drop,else |
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| &mut | drop,else | drop,else |
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| move | drop,else | drop,else |
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| fn(this) | drop,else | drop,else |
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| tuple | drop,else | drop,else |
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| array | drop,else | drop,else |
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| ref & | drop,else | drop,else |
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| ref mut &mut | drop,else | drop,else |
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| construct | let else 0 mismatch | let else 0 mismatch |
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| ------------ | ------------------- | ------------------- |
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| vanilla | drop,else | drop,else |
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| & | drop,else | drop,else |
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| &mut | drop,else | drop,else |
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| move | drop,else | drop,else |
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| fn(this) | drop,else | drop,else |
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| tuple | drop,else | drop,else |
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| array | drop,else | drop,else |
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| ref & | drop,else | drop,else |
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| ref mut &mut | drop,else | drop,else |
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