Warn when installing with a non-default toolchain #16131

smoelius · 2025-10-18T09:36:35Z

This PR implements the cargo changes for #11036. The rustup changes were implemented in rust-lang/rustup#4518.

The PR is currently organized as four commits:

The first expands the rustup tests to set RUSTUP_TOOLCHAIN_SOURCE. This change is not strictly necessary, but it improves the rustup tests' fidelity.
The second moves the pkg function to a location where the next commit can use it.
The third implements a test to check the fourth commit's fix.
The fourth warns when cargo install is invoked with a non-default toolchain, as indicated by RUSTUP_TOOLCHAIN_SOURCE.

In the third commit, two significant changes were made to simulated_rustup_environment:

Previously, the constructed proxy would call an always-panicking executable whenever it was asked to run cargo. Now, the proxy can be made to run the cargo under test.
The proxy can now be made to set additional environment variables (e.g., RUSTUP_TOOLCHAIN_SOURCE) before calling the proxied tool.

The PR is currently marked as draft because rust-lang/rustup#4518 has not yet been released. Technically, this PR could be merged before then. But testing it with a fixed rustup would seem to make sense.

Nits are welcome.

cc: @epage

rustbot · 2025-10-18T09:36:40Z

r? @weihanglo

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weihanglo

EDIT: The force push was to make Clippy happy.

Feel free to force-push and re-organize your commits during the iterations.

src/cargo/ops/cargo_install.rs

tests/testsuite/rustup.rs

tests/testsuite/install.rs

tests/testsuite/rustup.rs

src/cargo/ops/cargo_install.rs

smoelius · 2025-11-06T23:41:13Z

Just FYI, I know it looks like only some of the comments have been addressed. I am still hoping to get feedback on #16131 (comment) and #16131 (comment).

smoelius · 2025-12-28T21:33:31Z

I resolved comments that I thought I implemented uncontroversially. I hope that was okay.

EDIT: I also hid a few of my own comments that were old.

epage · 2025-12-30T16:43:50Z

Please clean up your commits for how they should be reviewed and merged, rather than for how it was developed.

crates/cargo-test-support/src/lib.rs

smoelius · 2026-01-01T13:36:48Z

I think the PR is ready to be looked at again.

tests/testsuite/utils/mod.rs

Thought of this when reviewing new cases for `std::env` in rust-lang#16131.

src/cargo/ops/cargo_install.rs

Thought of this when reviewing new cases for `std::env` in rust-lang#16131.

### What does this PR try to resolve? Thought of this when reviewing new cases for `std::env` in #16131. ### How to test and review this PR?

This is the first of several commits to implement a fix for rust-lang#11036. The changes fetch the `RUSTUP_TOOLCHAIN_SOURCE` environment variable set by Rustup, and warn if it is anything other than "default" or "cli".

smoelius · 2026-01-10T13:15:40Z

I need to fix the commits. I will do that within the next few hours.

smoelius · 2026-01-10T14:17:10Z

The commits should be fixed now.

src/cargo/ops/cargo_install.rs

tests/testsuite/rustup.rs

src/cargo/ops/cargo_install.rs

epage · 2026-01-13T20:44:32Z

src/cargo/ops/cargo_install.rs

+    fn is_default_or_cli_override(&self) -> Option<bool> {
+        match self {
+            Self::Default => Some(true),
+            Self::Environment => Some(false),
+            Self::CommandLine => Some(true),
+            Self::OverrideDB => Some(false),
+            Self::ToolchainFile => Some(false),
+            Self::Other(_) => None,
+        }
+    }


Instead of enumerating the cases in the function name, can we describe the policy this functions is meant to convey?

is_override_requiring_warning

That doesn't convey any meaning to the caller, requiring them to read the function to understand what is happening

One option is to flip the logic and name it is_implicit_override. I know this variant read better in the caller code but this is the most clear name I can think of to clarify the intent.

epage · 2026-01-14T16:35:58Z

src/cargo/ops/cargo_install.rs

+            let report = &[Level::WARNING
+                .secondary_title(format!(
+                    "default toolchain{maybe_toolchain} implicitly overridden by {source}"
+                ))
+                .element(Level::HELP.message(format!(
+                    "use `cargo +stable install` if you meant to use the stable toolchain."
+                )))];


in thinking over the warnings message in #16131 (comment), I'm wondering novice users will realize why this is a concern or why this is happening.

One thought is adding:

note: rustup selects the toolchain based on the parent environment and not the environment of the package being installed

I implemented this. Also, I removed the period from the help message for consistency.

epage · 2026-01-14T16:54:01Z

tests/testsuite/rustup.rs

+[WARNING] default toolchain `test-toolchain` implicitly overridden by unrecognized source
+  |
+  = [HELP] use `cargo +stable install` if you meant to use the stable toolchain.


Isn't RUSTUP_TOOLCHAIN the active toolchain, not the default, making this

[WARNING] default toolchain implicitly overridden to test-toolchain by unrecognized source

Oof. You're right.

"overridden to test-toolchain" not "overridden with test-toolchain"?

I went with "with". But I would be happy to change it if you like.

epage · 2026-01-14T17:02:45Z

Thanks for your patience through the rounds of feedback! This is looking great!

rustbot assigned weihanglo Oct 18, 2025

rustbot added Command-install S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 18, 2025

smoelius marked this pull request as draft October 18, 2025 09:36

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 18, 2025

smoelius force-pushed the second-half-of-11036 branch from 2157847 to 9006944 Compare October 18, 2025 09:52

weihanglo reviewed Oct 18, 2025

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smoelius force-pushed the second-half-of-11036 branch from 5f2fbb4 to c64e01a Compare October 19, 2025 09:49

smoelius mentioned this pull request Oct 19, 2025

Set RUSTUP_TOOLCHAIN_SOURCE rust-lang/rustup#4518

Merged

weihanglo mentioned this pull request Oct 19, 2025

Expand RUSTUP_TOOLCHAIN_SOURCE's documentation rust-lang/rustup#4549

Merged