Restore compatibility with Rust 1.74

PR #544 added support for `no_std` feature. The PR changed
`std::error::Error` to `core::error::Error`. The `core::error` trait was
stabilized in Rust 1.81, so the change bumped MSRV to 1.81. Before the
Python package built with Rust 1.66 and the `safetensors` create with
all features built with 1.74.

This commit restores compatibility with Rust 1.74 for `std` builds:

- `mixed_integer_ops` feature requires 1.66
- `half v2.4.1` requires 1.70
- `clap_lex v0.7.4` requires 1.74

I'm also adding `rust-version` to `Cargo.toml`, so cargo creates a
backwards compatible `Cargo.lock`. By default, Cargo >= 1.83 creates a
`v4` lock file, which is not compatible with Cargo < 1.78.

Signed-off-by: Christian Heimes <christian@python.org>

.github/workflows/rust.yml

@@ -4,7 +4,7 @@ on:
   pull_request:
 
 jobs:
-  build:
+  build-stable:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
@@ -59,3 +59,28 @@ jobs:
         #     token: ${{ secrets.CODECOV_TOKEN }} # not required for public repos
         #     working-directory: ./safetensors
         #     fail_ci_if_error: true
+
+  build-msrv:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./safetensors
+    env:
+      MSRV: "1.74"
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install Rust ${{ env.MSRV }}
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{ env.MSRV }}
+          override: true
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Build
+        run: cargo build --all-targets --verbose
+
+      - name: Run Tests
+        run: cargo test --verbose

bindings/python/Cargo.toml

@@ -2,6 +2,7 @@
 name = "safetensors-python"
 version = "0.5.3-dev.0"
 edition = "2021"
+rust-version = "1.74"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 [lib]

safetensors/Cargo.toml

@@ -2,6 +2,7 @@
 name = "safetensors"
 version = "0.5.3-dev.0"
 edition = "2021"
+rust-version = "1.74"
 homepage = "https://github.com/huggingface/safetensors"
 repository = "https://github.com/huggingface/safetensors"
 documentation = "https://docs.rs/safetensors/"

safetensors/src/tensor.rs

@@ -64,8 +64,12 @@ impl core::fmt::Display for SafeTensorError {
     }
 }
 
+#[cfg(not(feature = "std"))]
 impl core::error::Error for SafeTensorError {}
 
+#[cfg(feature = "std")]
+impl std::error::Error for SafeTensorError {}
+
 struct PreparedData {
     n: u64,
     header_bytes: Vec<u8>,