Lint unsupported, target-specifc codegen flags

[wesleywiser]

Proposal

Some rustc codegen flags are target specific (or specific to families of targets) and have no effect when used on unsupported targets. For example:

• -C control-flow-guard - Only supported on Windows targets
• -C dlltool - Only used on *-windows-gnu targets
• -C dwarf-version - Has no effect on *-windows-msvc targets
• -C relro-level - Has no effect on *-windows-msvc targets

The question of "what should a codegen flag do on unsupported targets" has previously come up during stabilization of some codegen flags.

I propose that we add a lint which fires when a codegen flag is passed to the compiler which is ignored for the active target:

$ rustc -C dlltool=something.exe -C control-flow-guard=on --target x86_64-unknown-linux-gnu hello_world.rs
warning: `-C dlltool` has no effect on the target platform
warning: `-C control-flow-guard` has no effect on the target platform

The lint should default to the warn level but users can choose to deny or allow it via -D or -A flags respectively. Ideally the lint would only flag attempts to activate unsupported options as to avoid unnecessary diagnostic noise (eg rustc -C control-flow-guard=off --target x86_64-unknown-linux-gnu ... would not trigger the lint).

Mentors or Reviewers

I'm willing to mentor or implement the proposal.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
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• Once an MCP is seconded, the Final Comment Period begins.
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  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

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cc @rust-lang/compiler

[Kivooeo]

@rustbot second

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#150984.