use rustup override set to select toolchain

[fprasx]

Using rustup override set will override a local rust-toolchain.toml file while rustup default will not.

Resolves #31

[robjtede]

I'd like to contrive a test (that would fail on current main) before merging this. I can do it or you can have a crack if you want.

[fprasx]

Hmm, I don't want to spend too much time on this if you have time, because you'll definitely be more efficient than I am (I'm not great with yaml or bash). My initial idea is to do something like:

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 521e3f6..0e959d9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,10 +12,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust:
-          # Test with toolchain file override
-          - null
+        use-toolchain-file:
+          - true
+          - false

+        rust:
           # Test that the sparse registry check works.
           # 1.66 and 1.67 don't support stable sparse registry.
           - "1.66"
@@ -32,7 +33,7 @@ jobs:

       # Test toolchain file support
       - name: Write rust-toolchain.toml
-        if: matrix.rust == null
+        if: matrix.use-toolchain-file == true
         shell: bash
         run: |
           cat <<EOF >>rust-toolchain.toml
@@ -51,7 +52,11 @@ jobs:
           components: clippy

       - name: Check ${{'${{steps.toolchain.outputs.rustc-version}}'}}
-        run: echo '${{steps.toolchain.outputs.rustc-version}}'
+        run: |
+          realversion=$(rustc --version)
+          [ "$realversion" != {{steps.toolchain.outputs.rustc-version}} ] || \
+            { echo "toolchain file not overridden: found $realversion"; exit 1}

       - name: Check ${{'${{steps.toolchain.outputs.cargo-version}}'}}
         run: echo '${{steps.toolchain.outputs.cargo-version}}'

Basically you do every test case with and without a toolchain file, but check that the resulting active toolchain is always the desired one. This is a little bad though because it would immediately double CI usage.

[robjtede]

That's a good start; thanks! I'll get it sorted today then.

[obi1kenobi]

Thank you both! 🚀

[jonasbb]

Thank you @fprasx for submitting the PR and thanks @robjtede, for adding the tests.