regression: hang with -Zsave-analysis

Code

I tried this code:

fn next_letter_position(&self, x_index: f32, y_index: f32, width: f32) -> (f32, f32) {
    let vwidth = &self.viewport.x / 2.0;

    match x_index * width {
        0.0 ... vwidth => {
            (x_index + 1.0, y_index)
        },
        _ => (0.0, y_index - 1.0)
    }
}

with rustc rustc 1.65.0-nightly (c07a8b4e0 2022-08-26) this runs in a couple ms:
rustc ./a.rs --crate-type=lib -Zsave-analysis -Ztime=yes -Ztime-passes=yes

with rustc rustc 1.65.0-nightly (3c7278846 2022-09-06) this seems to hang somewhere in save-analysis

  26,77%  librustc_driver-0f3b2583c1b011ce.so  [.] <rustc_middle::hir::map::Map>::get
  24,00%  librustc_driver-0f3b2583c1b011ce.so  [.] <rustc_middle::hir::map::Map>::get_parent_node
   5,76%  librustc_driver-0f3b2583c1b011ce.so  [.] <rustc_save_analysis::SaveContext>::get_path_res

( I killed the process after waiting for 5 minutes 🙃 )

reduced:

fn a(self) {}

RIP -Zsave-analysis

@nnethercote

I think this is caused by #101228 cc @nnethercote

Clearly a well tested feature 😛 I'll take a look.

@petrochenkov

The old code looked like this:

Node::PathSegment(seg) => match seg.res {
    Some(res) if res != Res::Err => res,
    _ => { 
        let parent_node = self.tcx.hir().get_parent_node(hir_id);
        self.get_path_res(parent_node)
    }
},

The new code looks like this:

Node::PathSegment(seg) => {
    if seg.res != Res::Err {
        seg.res
    } else {
        let parent_node = self.tcx.hir().get_parent_node(hir_id);
        self.get_path_res(parent_node)
    }   
}

because seg.res is no longer optional.

The infinite recursion happens because the get_parent_node call returns a parent_node that is the same as the hir_id. When I printed that ID out it looks like this:

HirId { owner: DefId(0:4 ~ s[5740]::a), local_id: 5 }

An easy fix is to check if parent_node is the same as hir_id and return Res:Err if so. But I don't know if that's just papering over a deeper problem.

@petrochenkov: Is it an underlying bug that get_parent_node can return the same node it is given?

Auto merge of rust-lang#101688 - cjgillot:verify-hir-parent, r=petroc…

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regression: hang with -Zsave-analysis #101505

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regression: hang with -Zsave-analysis #101505

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matthiaskrgr commented on Sep 6, 2022

matthiaskrgr commented on Sep 6, 2022

nnethercote commented on Sep 7, 2022

nnethercote commented on Sep 7, 2022

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