Merge pull request #25616 from dvdksn/codex/fix-25597-cargo-cache

docs: fix concurrent Rust cache mounts

Author: dvdksn

content/guides/rust.md

@@ -107,9 +107,8 @@ RUN --mount=type=bind,source=src,target=src \
     --mount=type=bind,source=Cargo.toml,target=Cargo.toml \
     --mount=type=bind,source=Cargo.lock,target=Cargo.lock \
     --mount=type=cache,target=/app/target/ \
-    --mount=type=cache,target=/usr/local/cargo/git/db \
-    --mount=type=cache,target=/usr/local/cargo/registry/ \
-    cargo build --locked --release && \
+    --mount=type=cache,target=/var/cache/cargo \
+    CARGO_HOME=/var/cache/cargo cargo build --locked --release && \
     cp ./target/release/$APP_NAME /bin/server
 
 ################################################################################
@@ -164,9 +163,8 @@ RUN --mount=type=bind,source=src,target=src \
     --mount=type=bind,source=Cargo.toml,target=Cargo.toml \
     --mount=type=bind,source=Cargo.lock,target=Cargo.lock \
     --mount=type=cache,target=/app/target/ \
-    --mount=type=cache,target=/usr/local/cargo/git/db \
-    --mount=type=cache,target=/usr/local/cargo/registry/ \
-    cargo build --locked --release && \
+    --mount=type=cache,target=/var/cache/cargo \
+    CARGO_HOME=/var/cache/cargo cargo build --locked --release && \
     cp ./target/release/$APP_NAME /bin/server
 
 ################################################################################
@@ -206,6 +204,11 @@ CMD ["/bin/server"]
 {{< /tab >}}
 {{< /tabs >}}
 
+The cache mount uses a single `CARGO_HOME` so concurrent builds coordinate
+access through Cargo's cache lock. Setting `CARGO_HOME` also changes where
+Cargo looks for global configuration and credentials. Project-level
+`.cargo/config.toml` files are unaffected.
+
 For building an image, only the Dockerfile is necessary. Open the Dockerfile
 in your favorite IDE or text editor and see what it contains. To learn more
 about Dockerfiles, see the [Dockerfile reference](/reference/dockerfile.md).
@@ -591,21 +594,21 @@ For the sample application, you'll use a variation of the backend from the react
    WORKDIR /app
 
    # Build the application.
-   # Leverage a cache mount to /usr/local/cargo/registry/
-   # for downloaded dependencies and a cache mount to /app/target/ for
-   # compiled dependencies which will speed up subsequent builds.
+   # Leverage a cache mount to /var/cache/cargo for downloaded dependencies
+   # and a cache mount to /app/target/ for compiled dependencies which will
+   # speed up subsequent builds.
    # Leverage a bind mount to the src directory to avoid having to copy the
    # source code into the container. Once built, copy the executable to an
    # output directory before the cache mounted /app/target is unmounted.
    RUN --mount=type=bind,source=src,target=src \
        --mount=type=bind,source=Cargo.toml,target=Cargo.toml \
        --mount=type=bind,source=Cargo.lock,target=Cargo.lock \
        --mount=type=cache,target=/app/target/ \
-       --mount=type=cache,target=/usr/local/cargo/registry/ \
+       --mount=type=cache,target=/var/cache/cargo \
        --mount=type=bind,source=migrations,target=migrations \
        <<EOF
    set -e
-   cargo build --locked --release
+   CARGO_HOME=/var/cache/cargo cargo build --locked --release
    cp ./target/release/$APP_NAME /bin/server
    EOF
 

content/manuals/build/cache/optimize.md

@@ -262,11 +262,15 @@ RUN --mount=type=cache,target=/root/.gem \
 
 ```dockerfile
 RUN --mount=type=cache,target=/app/target/ \
-    --mount=type=cache,target=/usr/local/cargo/git/db \
-    --mount=type=cache,target=/usr/local/cargo/registry/ \
-    cargo build
+    --mount=type=cache,target=/var/cache/cargo \
+    CARGO_HOME=/var/cache/cargo cargo build
 ```
 
+The cache mount uses a single `CARGO_HOME` so concurrent builds coordinate
+access through Cargo's cache lock. Setting `CARGO_HOME` also changes where
+Cargo looks for global configuration and credentials. Project-level
+`.cargo/config.toml` files are unaffected.
+
 {{< /tab >}}
 {{< tab name=".NET" >}}