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authorSamuel Tardieu <sam@rfc1149.net>2025-07-16 17:06:43 +0200
committerGitHub <noreply@github.com>2025-07-16 17:06:43 +0200
commit2a8f97910a794f852b893213e059cab2ca12da7e (patch)
treea2c2b3e63722d7fda9d2e06665f59c4b45101f0c /compiler/rustc_resolve/src/rustdoc.rs
parent19470bc9c55b03d774658f457f98d197f3663c4f (diff)
parent14f47009cc5b02cd8c28ca2cc9c1fd58aeeef25b (diff)
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Rollup merge of #143968 - Stypox:tracing-FnAbiOf, r=RalfJung
Add tracing to `InterpCx::fn_abi_of_instance/fn_abi_of_fn_ptr`

This PR adds tracing to the `InterpCx::fn_abi_of_instance`/`::fn_abi_of_fn_ptr` functions by shadowing `FnAbiOf`'s trait methods with inherent methods on `InterpCx`, like done in rust-lang/rust#142721. The reason why I am targeting these two functions is because they are used for Miri interpretation, and they make a `layout_of` query down the line without passing through the `layout_of` that was traced in rust-lang/rust#142721.

There are other places where `layout_of` is called without being traced (see the analysis below), but that's because the `Machine` used there is not `MiriMachine` but rather `CompileTimeMachine` which does not implement `enter_trace_span()`. But after discussing with ```````@RalfJung``````` we agreed that the const-eval part should not be traced together with Miri, that's why I am ignoring the other places where `layout_of` is called.

r? ```````@RalfJung```````

<details><summary>Analysis of the places where <code>layout_of</code> is called</summary>

I did some analysis for https://github.com/rust-lang/rust/pull/142721#discussion_r2171494841, and these are all the places where the query `tcx.layout_of` is called (directly or indirectly) outside of a traced `InterpCx::layout_of` while a program is being interpreted by Miri:

```
adjust_for_rust_scalar  at ./compiler/rustc_ty_utils/src/abi.rs:302:35
{closure#2}             at ./compiler/rustc_ty_utils/src/abi.rs:522:25
eval_body_using_ecx<>   at ./compiler/rustc_const_eval/src/const_eval/eval_queries.rs:49:22
{closure#1}<>           at ./compiler/rustc_const_eval/src/interpret/operand.rs:851:76
{closure#0}<>           at ./compiler/rustc_const_eval/src/interpret/stack.rs:612:18
size_and_align          at ./compiler/rustc_middle/src/mir/interpret/mod.rs:387:38
```

I got these by:
- patching rustc with this patch that adds a span to the `layout_of` query which prints the backtrace:
[layout_of_other_places.diff.txt](https://github.com/user-attachments/files/21235523/layout_of_other_places.diff.txt)
- adding this to my bootstrap.toml to have debug symbols inside the Miri binary: `rust.debuginfo-level = "line-tables-only"` and also `build.tool.miri.features = ["tracing"]`
- obtaining a trace file with `MIRI_TRACING=1 ./x.py run miri --stage 1 --warnings warn --args src/tools/miri/tests/pass/hello.rs` (note: maybe using a file different than "src/tools/miri/tests/pass/hello.rs" would lead to more places where layout_of is called?)
-  running this query in Perfetto to select all `layout_of` spans that have as a direct parent a span named "frame" (as opposed to the parent being `InterpCx::layout_of`) and extract their backtrace: `select args.string_value from slice left join args on slice.arg_set_id = args.id where slice.name = "tcx.layout_of" and slice.parent_id in (select slice2.id from slice as slice2 where slice2.name = "frame") group by args.string_value`
- exporting the data as `.tsv` and processing that file through this Python script. It finds the first path in the backtraces where "layout" isn't mentioned, which imo is a good heuristic to not consider `layout_of` wrappers/friends as call places, but rather go down the backtrace until an actual call place is reached. [layout_of_other_places.py.txt](https://github.com/user-attachments/files/21235529/layout_of_other_places.py.txt)

</details>
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