Latest nightly (`rustc 1.64.0-nightly (62b272d25 2022-07-21)`) introduces a new borrowcheck error in previously compiling `mdbook` `0.4.20`

[mitchmindtree]

As of rustc 1.64.0-nightly (62b272d25 2022-07-21), a new lifetime error has appeared when attempting to build mdbook 0.4.20.

Error

error[E0597]: `local_ctx` does not live long enough
   --> src/renderer/html_handlebars/helpers/navigation.rs:154:25
    |
154 |             t.render(r, &local_ctx, &mut local_rc, out)
    |                         ^^^^^^^^^^ borrowed value does not live long enough
155 |         })?;
    |         -
    |         |
    |         `local_ctx` dropped here while still borrowed
    |         borrow might be used here, when `local_rc` is dropped and runs the destructor for type `handlebars::RenderContext<'_, '_>`
    |
    = note: values in a scope are dropped in the opposite order they are defined

For more information about this error, try `rustc --explain E0597`.
error: could not compile `mdbook` due to previous error

Version it worked on

rustc 1.64.0-nightly (d68e7eb 2022-07-20)

Version with regression

rustc 1.64.0-nightly (62b272d 2022-07-21)

Details

You can find more details at the downstream mdbook issue rust-lang/mdBook#1860.

Curiously, the new error can be fixed by re-ordering some seemingly inocuous statements - see this PR rust-lang/mdBook#1861.

Perhaps this is indicative of an error in non-lexical-lifetime handling on the latest nightly? Or alternatively, this could be the result of fixing a soundness / safety issue related to the interop between lifetimes and drop order?

This error was originally discovered in an unrelated CI pass failing in the following PR: FuelLabs/sway#2359.

[5225225]

Ran a bisection in mdBook, version rust-lang/mdBook@92afe9b

Looks to be a rollup, #99567. None of them jump out as obvious candidates.

---

searched nightlies: from nightly-2022-07-20 to nightly-2022-07-22
regressed nightly: nightly-2022-07-22
searched commit range: d68e7eb...62b272d
regressed commit: af7ab34

bisected with cargo-bisect-rustc v0.6.1

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2022-07-20 --end 2022-07-22 --preserve -- check 

[5225225]

This might actually be #99413 (so, a soundness fix.)? A btreemap is used in that bit of code, but it's talking about a RenderContext.

[ehuss]

Confirmed that it is caused by #99413. Here is a slightly smaller repro:

#![allow(unused)]
use std::collections::BTreeMap;
use std::marker::PhantomData;
use std::rc::Rc;

fn main() {
    let mut rc = RenderContext {
        local_helpers: BTreeMap::new(),
    };
    foo(&mut rc);
}

#[derive(Clone)]
struct RenderContext<'rc> {
    local_helpers: BTreeMap<String, Rc<dyn HelperDef + 'rc>>,
}

pub trait HelperDef {}

fn foo(rc: &mut RenderContext<'_>) {
    let mut local_rc = rc.clone();
    let local_ctx: BTreeMap<String, String> = BTreeMap::new();
    render(&local_ctx, &mut local_rc);
}

fn render<'rc>(ctx: &'rc BTreeMap<String, String>, rc: &mut RenderContext<'rc>) {}

I think the error here is correct and #99413 is a valid fix. It's just a consequence of how the lifetimes in the handlebars crate are defined that cause the error.

[danielhenrymantilla]

Until the fix to mdbook is officially released, btw, for those running into this, the workaround is to use the git dev, potentially as a patch:

• for those using cargo install mdbook, instead, do

  cargo install mdbook --git 'https://github.com/rust-lang/mdBook' --rev '8f01d0234f708042ea8d0dc5cac63859b3f14cb1'

• for those depending on mdbook as a library, on the Cargo.toml file of your workspace (where the Cargo.lock file lies / is generated), add:

  [patch.crates-io.mdbook]
  version = "0.4.20"
  git = "https://github.com/rust-lang/mdBook"
  rev = "8f01d0234f708042ea8d0dc5cac63859b3f14cb1"