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