Restore behavior to not touch existing RUSTFLAGS variable

Add changelog

CHANGELOG.md

@@ -15,6 +15,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     Before only the items listed in `rust-toolchain` were installed.
     Now all the items from the toolchain file are installed and then all the `target`s and `components` that are provided as action inputs.
     This allows installing extra tools only for CI or simplify testing special targets in CI.
+* Allow skipping the creation of a `RUSTFLAGS` environment variable.
+    Cargos logic for rustflags is complicated, and setting the `RUSTFLAGS` environment variable prevents other ways of working.
+    Provide a new `rustflags` input, which controls the environment variable creation.
+    If the value is set to the empty string, then `RUSTFLAGS` is not created.
+
+    Pre-existing `RUSTFLAGS` variables are never modified by this extension.
 
 ## [1.4.4] - 2023-03-18
 

README.md

@@ -57,10 +57,12 @@ Afterward, the `components` and `target` specified via inputs are installed in a
 ### RUSTFLAGS
 
 By default, this action sets the `RUSTFLAGS` environment variable to `-D warnings`.
+However, rustflags sources are mutually exclusive, so setting this environment variable omits any configuration through `target.*.rustflags` or `build.rustflags`.
 
-However, rustflags sources are mutually exclusive, so setting this environment
-variable omits any configuration through `target.*.rustflags` or
-`build.rustflags`.
+* If `RUSTFLAGS` is already set, no modifications of the variable are made and the original value remains.
+* If `RUSTFLAGS` is unset and the `rustflags` input is empty (i.e., the empty string), then it will remain unset.
+    Use this, if you want to prevent the value from being set because you make use of `target.*.rustflags` or `build.rustflags`.
+* Otherwise, the environment variable `RUSTFLAGS` is set to the content of `rustflags`.
 
 To prevent this from happening, set the `rustflags` input to an empty string, which will
 prevent the action from setting `RUSTFLAGS` at all, keeping any existing preferences.
@@ -82,4 +84,4 @@ License].
 
 [MIT License]: LICENSE
 [Problem Matchers]: https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md
-[Cargo reference]: https://doc.rust-lang.org/cargo/reference/config.html?highlight=unknown#buildrustflags
+[Cargo reference]: https://doc.rust-lang.org/cargo/reference/config.html?highlight=unknown#buildrustflags

action.yml

@@ -80,7 +80,7 @@ runs:
         if [[ ! -v RUST_BACKTRACE ]]; then
           echo "RUST_BACKTRACE=short" >> $GITHUB_ENV
         fi
-        if [[ "$NEW_RUSTFLAGS" != "" ]]; then
+        if [[ ( ! -v RUSTFLAGS ) && $NEW_RUSTFLAGS != "" ]]; then
           echo "RUSTFLAGS=$NEW_RUSTFLAGS" >> $GITHUB_ENV
         fi
         # Enable faster sparse index on nightly