Update Rust toolchain to 1.88 and MSRV to 1.86

[MichaReiser]

Summary

The most important new feature for us is that Rust now supports dyn upcasting. That allows us to remove the Upcast trait.

Test Plan

cargo build

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[AlexWaygood]

hmm, some windows path filtering stopped working?

[MeGaGiGaGon]

Looks like it's because <path>.path().canonicalize() and insta's path representation now disagree: The paths that are passed as a filter have a starting \\?\ both on main and on this PR:
On main:

---- requires_python_pyproject_toml_above_with_tool stdout ----
[crates\ruff\tests\lint.rs:19:5] path.as_ref().to_str() = Some(
    "\\\\?\\C:\\Users\\GiGaGon\\AppData\\Local\\Temp\\.tmp8VZVtR\\foo\\test.py",
)
[crates\ruff\tests\lint.rs:19:5] path.as_ref().to_str() = Some(
    "\\\\?\\C:\\Users\\GiGaGon\\AppData\\Local\\Temp\\.tmp8VZVtR",
)

On this PR

---- requires_python_pyproject_toml_above_with_tool stdout ----
[crates\ruff\tests\lint.rs:19:5] path.as_ref().to_str() = Some(
    "\\\\?\\C:\\Users\\GiGaGon\\AppData\\Local\\Temp\\.tmpUGvoNc\\foo\\test.py",
)
[crates\ruff\tests\lint.rs:19:5] path.as_ref().to_str() = Some(
    "\\\\?\\C:\\Users\\GiGaGon\\AppData\\Local\\Temp\\.tmpUGvoNc",
)

But insta's output doesn't have a starting \\?\ as of this PR:
On main with filter removed

   78       │-  "[TMP]/foo",
   79       │-  "[TMP]/foo/src",
         78 │+  "\\?\C:\Users\GiGaGon\AppData\Local\Temp\.tmpAuA7Xq\foo",
         79 │+  "\\?\C:\Users\GiGaGon\AppData\Local\Temp\.tmpAuA7Xq\foo\src",

On this PR

   78       │-  "[TMP]/foo",
   79       │-  "[TMP]/foo/src",
         78 │+  "C:\Users\GiGaGon\AppData\Local\Temp\.tmpAQDo8w\foo",
         79 │+  "C:\Users\GiGaGon\AppData\Local\Temp\.tmpAQDo8w\foo\src",

And this is supported by the fact that all of the lint tests pass if I add a .trim_start_matches(r"\\?\") to tempdir_filter

[MichaReiser]

Thanks for the analysis. The paths in your analysis look the same to me (ignoring the temp folder name) and I couldn't find any mention of changes to path handling in the Rust 1.88 changelog

[MeGaGiGaGon]

Sorry if I was unclear on the issue (I need to get better at that)
On main: path passed to tempdir_filter starts with \\?\, path insta outputs starts with \\?\, regex matches and everything works
On this pr: path passed to tempdir_filter starts with \\?\, path insta outputs does not start with \\?\, regex fails to match and path is not replaced with [TMP] causing the test failure

[ntBre]

This might have actually changed in 1.88? rust-lang/rust#139683 I didn't see it in the release notes either, though.

I found this in the blame for canonicalize but it might not be related.

[MichaReiser]

I think the culprint is rust-lang/rust#138869

canonicalize returns an UNC path on both branches. What's different is that we only see a UNC path in the spawned Ruff process on main but not with Rust 1.88.

This is now a bit annoying to fix because downstream users might run the tests with both Rust 1.86 and 1.88. I think the easiest is to use dunce to avoid UNC paths in the first place