Move trait selection error reporting to its own top-level module

[compiler-errors]

This effectively moves rustc_trait_selection::traits::error_reporting to rustc_trait_selection::error_reporting::traits. There are only a couple of actual changes to the code, like moving the pretty_impl_header fn out of the specialization module for privacy reasons.

This is quite pointless on its own, but having error_reporting as a top-level module in rustc_trait_selection is very important to make sure we have a meaningful file structure for when we move type error reporting (and region error reporting, with which it's incredibly entangled currently) into rustc_trait_selection. I've opened a tracking issue here: #127492

r? lcnr

[rustbot]

HIR ty lowering was modified

cc @fmease

Some changes occurred in engine.rs, potentially modifying the public API of ObligationCtxt.

cc @lcnr, @compiler-errors

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[lcnr]

@bors r+ rollup=iffy ❤️

[bors]

📌 Commit fe4c995 has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit fe4c995 with merge a2d5819...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing a2d5819 to master...

[rust-timer]

Finished benchmarking commit (a2d5819): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results (primary 3.9%, secondary -2.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.9%	[3.9%, 3.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.4%, -2.2%]	2
All ❌✅ (primary)	3.9%	[3.9%, 3.9%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 703.117s -> 699.685s (-0.49%)
Artifact size: 328.74 MiB -> 328.81 MiB (0.02%)