wgpu-hal's MSRV is only 1.60. Explain MSRV rules in README.md.

README.md

@@ -35,6 +35,23 @@ Minimum Supported Rust Version is **1.64**.
 It is enforced on CI (in "/.github/workflows/ci.yml") with `RUST_VERSION` variable.
 This version can only be upgraded in breaking releases.
 
+The `wgpu-core`, `wgpu-hal`, and `wgpu-types` crates should never
+require an MSRV ahead of Firefox's MSRV for nightly builds, as
+determined by the value of `MINIMUM_RUST_VERSION` in
+[`python/mozboot/mozboot/util.py`][util]. However, Firefox uses `cargo
+vendor` to extract only those crates it actually uses, so the
+workspace's other crates can have more recent MSRVs.
+
+*Note for Rust 1.64*: The workspace itself can even use a newer MSRV
+than Firefox, as long as the vendoring step's `Cargo.toml` rewriting
+removes any features Firefox's MSRV couldn't handle. For example,
+`wgpu` can use manifest key inheritance, added in Rust 1.64, even
+before Firefox reaches that MSRV, because `cargo vendor` copies
+inherited values directly into the individual crates' `Cargo.toml`
+files, producing 1.63-compatible files.
+
+[util]: https://searchfox.org/mozilla-central/source/python/mozboot/mozboot/util.py
+
 ## Getting Started
 
 ### Rust

wgpu-hal/Cargo.toml

@@ -8,7 +8,12 @@ homepage.workspace = true
 repository.workspace = true
 keywords.workspace = true
 license.workspace = true
-rust-version.workspace = true
+
+# Override the workspace's `rust-version` key. Firefox uses `cargo vendor` to
+# copy the crates it actually uses out of the workspace, so it's meaningful for
+# them to have less restrictive MSRVs individually than the workspace as a
+# whole, if their code permits. See `../README.md` for details.
+rust-version = "1.60"
 
 [package.metadata.docs.rs]
 # Ideally we would enable all the features.