chore: bump MSRV to Rust 1.95 and adopt new stable features #67

Summary

Bump rust-version from "1.94" to "1.95" after Rust 1.95.0 is released (April 16, 2026) and adopt newly stabilized features that benefit this crate.

Rust 1.95.0 features relevant to la-stack

core::hint::cold_path (high value)

Stabilized in rust-lang/rust#151576. This is the single most useful addition for this crate.

Where to use it:

1. det_sign_exact exact-fallback path (src/exact.rs): The Bareiss fallback in the adaptive two-stage predicate is the cold path — the fast f64 filter resolves the vast majority of calls. Adding cold_path() at the start of the exact fallback block hints the compiler to optimize the hot f64 filter path at the expense of the allocation-heavy exact path.

   // Stage 2: exact Bareiss fallback.
   core::hint::cold_path();
   let det = bareiss_det(self);

2. Error paths in all exact methods: The NonFinite / Singular / Overflow error returns are cold:

   Err(LaError::NonFinite { col: c }) => { core::hint::cold_path(); ... }

3. Lu::factor and Ldlt::factor singular/error branches: The singularity detection paths are cold in normal usage.

4. det_direct fallback arm (_ => None for D ≥ 5): Cold path in the common D ≤ 4 case.

cold_path() can also be used to build likely/unlikely helpers if needed for future hot-loop optimizations.

if let guards (moderate value)

Stabilized in rust-lang/rust#141295. Allows if let in match arm guards.

Where to use:

• Potential cleanup in det_sign_exact fast filter:

  // Before:
  if let (Some(det_f64), Some(err)) = (self.det_direct(), self.det_errbound()) {
      if det_f64.is_finite() { ... }
  }
  
  // After (within a match):
  match self.det_direct() {
      Some(det_f64) if let Some(err) = self.det_errbound() && det_f64.is_finite() => { ... }
      ...
  }

• Not a large codebase win, but worth adopting where it improves readability.

cfg_select! macro (low value for now)

Stabilized in rust-lang/rust#149783. Replaces chains of #[cfg] / #[cfg(not(...))] with a single cfg_select! block.

Where to use:

• Currently la-stack uses #[cfg(feature = "exact")] for the module include and re-export. If feature-gated code grows more complex, cfg_select! can replace nested cfg blocks. Low priority for now since the current feature gating is minimal.

Compiler improvements (indirect benefit)

• MIR SSA range propagation pass may improve optimization of the arithmetic chains in det_direct and the Bareiss inner loop
• Better const-generic ergonomics (incremental improvements)

Proposed Changes

1. Update Cargo.toml: rust-version = "1.95"
2. Update CI workflows if they pin a specific Rust version
3. Add core::hint::cold_path() to:
   • det_sign_exact Bareiss fallback
   • Error return paths in validate_finite, det_exact, solve_exact, etc.
   • Lu::factor / Ldlt::factor singular detection
   • det_direct D ≥ 5 fallback arm
4. Audit match patterns for if let guard opportunities
5. Run benchmarks before/after to measure cold_path impact on the hot path

Implementation Notes

• Wait for Rust 1.95.0 stable release (April 16, 2026)
• cold_path() is a pure hint with no semantic impact — safe to add liberally
• Benchmark the exact arithmetic paths (especially det_sign_exact on well-conditioned matrices where the fast filter dominates) to verify the hint helps
• Related: perf: stack-backed storage for exact arithmetic kernels #62, perf: custom f64 → BigRational conversion via mantissa/exponent decomposition #63, perf: integer-only Bareiss determinant (BigInt path) #64, refactor: unify determinant computation pipeline #65, perf: minor optimizations — clone reduction + det_sign_exact short-circuit #66 (all v0.4.0 milestone work)