Use Rust 1.96.1 as development toolchain

Author: eldryoth

CHANGELOG.md

@@ -12,6 +12,9 @@
 - Updated README installation snippets, latest-stable Rust wording, and release
   metadata for `1.1.1`.
 - Added local compatibility evidence for Rust `1.96.1`.
+- Switched the pinned development toolchain to Rust `1.96.1` while keeping the
+  crate MSRV at Rust `1.90.0`.
+- Added CI/release checks that verify backward compatibility on Rust `1.90.0`.
 
 ## 1.1.0
 

README.md

@@ -83,6 +83,7 @@ Planned or intentionally external:
 | --- | --- |
 | License | `MIT OR Apache-2.0` |
 | MSRV | Rust `1.90.0` |
+| Development toolchain | Rust `1.96.1` |
 | Crate shape | Library only |
 | Runtime graph | `image`, `palette`, `rand`, `sanitization`, `sanitization-crypto-interop`, transitive `sha2`, `subtle`; optional `blake3`, `xxhash-rust`, `image/png`, `image/jpeg`, `image/gif` |
 | Unsafe policy | `#![forbid(unsafe_code)]` |
@@ -103,26 +104,27 @@ to split it.
 
 ## Rust Version Support
 
-The minimum supported Rust version is Rust `1.90.0`. New deployments should
-prefer the latest stable Rust; as of June 30, 2026, that is Rust `1.96.1`.
+The minimum supported Rust version is Rust `1.90.0`, as declared by
+`Cargo.toml`. Local development and release checks use the pinned
+`rust-toolchain.toml` toolchain, currently Rust `1.96.1`. CI also checks back
+to Rust `1.90.0` so the MSRV stays honest. New deployments should prefer the
+latest stable Rust; as of June 30, 2026, that is Rust `1.96.1`.
 
 Compatibility evidence for `1.1.1`:
 
 | Rust | Local Evidence |
 | --- | --- |
-| `1.90.0` | ✓ full release gate |
+| `1.90.0` | ✓ `cargo check`; ✓ `cargo check --features all-formats`; ✓ `cargo check --features "blake3 all-formats"`; ✓ `cargo check --features "xxh3 all-formats"` |
 | `1.91.0` | ✓ `cargo check --features all-formats` |
 | `1.92.0` | ✓ `cargo check --features all-formats` |
 | `1.93.0` | ✓ `cargo check --features all-formats` |
 | `1.94.0` | ✓ `cargo check --features all-formats` |
 | `1.95.0` | ✓ `cargo check --features all-formats` |
 | `1.96.0` | ✓ `cargo check --features all-formats` |
-| `1.96.1` | ✓ `cargo check --features all-formats` |
+| `1.96.1` | ✓ full release gate; ✓ `cargo check --features all-formats` |
 
 Optional hash modes are mutually exclusive, so `hashavatar` cannot use a single
-`--all-features` evidence run. The `1.90.0` MSRV was also checked with
-`cargo check --features "blake3 all-formats"` and
-`cargo check --features "xxh3 all-formats"`.
+`--all-features` evidence run.
 
 ## Install
 

docs/RELEASE.md

@@ -13,6 +13,11 @@ cargo publish --dry-run
 freshness still has to be checked before release through crates.io, docs.rs,
 upstream repositories, and RustSec advisories.
 
-The stable gate runs the normal project checks, package verification, documentation generation, fuzz harness compilation, and reproducibility checks. Optional SBOM generation runs when `cargo-sbom` is installed.
+The stable gate runs on the pinned development toolchain from
+`rust-toolchain.toml`. The normal project checks also install and run focused
+compatibility checks against the `Cargo.toml` MSRV, currently Rust `1.90.0`.
+Package verification, documentation generation, fuzz harness compilation, and
+reproducibility checks run as part of the same gate. Optional SBOM generation
+runs when `cargo-sbom` is installed.
 
 The crate package should contain the reusable library, metadata, documentation, and policy scripts. It should not contain binaries, the demo/API server, fuzz harnesses, or generated build output.

docs/release-notes/RELEASE_NOTES_1.1.1.md

@@ -28,6 +28,15 @@ the public API.
 - Updated latest-stable Rust wording to Rust `1.96.1`.
 - Added local `cargo check --features all-formats` compatibility evidence for
   Rust `1.96.1`.
+- Documented the split between the Rust `1.96.1` development toolchain and the
+  Rust `1.90.0` MSRV.
+
+## Toolchain
+
+- Switched `rust-toolchain.toml` from Rust `1.90.0` to Rust `1.96.1`.
+- Kept `Cargo.toml` `rust-version = "1.90"` as the public MSRV.
+- Added project checks that run focused compatibility coverage on Rust `1.90.0`
+  in addition to the normal development-toolchain checks.
 
 ## Compatibility
 

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "1.90.0"
+channel = "1.96.1"
 components = ["clippy", "rustfmt"]

scripts/checks.sh

@@ -28,6 +28,20 @@ cargo clippy --all-targets -- -D warnings
 echo "checks: tests"
 cargo test
 
+echo "checks: MSRV compatibility"
+if command -v rustup >/dev/null 2>&1; then
+    if ! rustup toolchain list | grep -q '^1\.90\.0'; then
+        rustup toolchain install 1.90.0 --profile minimal
+    fi
+    cargo +1.90.0 check
+    cargo +1.90.0 check --features all-formats
+    cargo +1.90.0 check --features "blake3 all-formats"
+    cargo +1.90.0 check --features "xxh3 all-formats"
+else
+    echo "checks: rustup is required for MSRV compatibility checks" >&2
+    exit 1
+fi
+
 echo "checks: docs"
 cargo doc --no-deps
 

scripts/validate-release-metadata.sh

@@ -29,8 +29,13 @@ if [ -z "$cargo_rust_version" ]; then
     exit 1
 fi
 
-if [ "$toolchain_version" != "$cargo_rust_version.0" ]; then
-    echo "release metadata: rust-toolchain.toml channel $toolchain_version does not match Cargo.toml rust-version $cargo_rust_version" >&2
+if [ "$cargo_rust_version" != "1.90" ]; then
+    echo "release metadata: Cargo.toml rust-version must remain the MSRV 1.90" >&2
+    exit 1
+fi
+
+if [ "$toolchain_version" != "1.96.1" ]; then
+    echo "release metadata: rust-toolchain.toml channel must be the development toolchain 1.96.1, got $toolchain_version" >&2
     exit 1
 fi