Add a lint to catch clashing extern fn declarations.

[jumbatm]

Closes #69390.

Adds lint clashing_extern_decl to detect when, within a single crate, an extern function of the same name is declared with different types. Because two symbols of the same name cannot be resolved to two different functions at link time, and one function cannot possibly have two types, a clashing extern declaration is almost certainly a mistake.

This lint does not run between crates because a project may have dependencies which both rely on the same extern function, but declare it in a different (but valid) way. For example, they may both declare an opaque type for one or more of the arguments (which would end up distinct types), or use types that are valid conversions in the language the extern fn is defined in. In these cases, we can't say that the clashing declaration is incorrect.

r? @eddyb

[jumbatm]

Just as a reminder, per #69390 (comment), this PR should go through a crater-check run before merging (which should also let us see if changing the lint to be Deny from the get-go would break many current crates).

[eddyb]

r? @nagisa cc @hanna-kruppe (LGTM at a glance though)

[nagisa]

This is missing a test (and I think lint will fail to correctly handle) for two two not clashing declarations that use independently declared types. That is:

mod one {
    #[repr(C)] struct Banana { weight: u64 }
    extern "C" { fn weigh_banana(count: *const Banana) -> u64; }
}

mod two {
    #[repr(C)] struct Banana { weight: u64 } // note: distinct type
    // For a weirder corner case (may still be valid depending on how C code is written):
    // #[repr(C)] struct Banana { weight: u64, some_optional_field: u64 }
    extern "C" { fn weigh_banana(count: *const Banana) -> u64; }
}

[bjorn3]

Should this also check across multiple crates?

[jumbatm]

Current thinking is no, because two crates may bind to the same extern function with signatures that are different, but compatible under that language or ABI.

[bors]

☔ The latest upstream changes (presumably #71566) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #72036) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #72276) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

##[section]Starting: Linux mingw-check
##[section]Starting: Initialize job
Agent name: 'Azure Pipelines 2'
Agent machine name: 'fv-az578'
Current agent version: '2.168.2'
##[group]Operating System
16.04.6
LTS
LTS
##[endgroup]
##[group]Virtual Environment
Environment: ubuntu-16.04
Version: 20200430.2
Included Software: https://github.com/actions/virtual-environments/blob/ubuntu16/20200430.2/images/linux/Ubuntu1604-README.md
##[endgroup]
Agent running as: 'vsts'
Prepare build directory.
Set build variables.
Download all required tasks.
Download all required tasks.
Downloading task: Bash (3.163.2)
Checking job knob settings.
   Knob: AgentToolsDirectory = /opt/hostedtoolcache Source: ${AGENT_TOOLSDIRECTORY} 
   Knob: AgentPerflog = /home/vsts/perflog Source: ${VSTS_AGENT_PERFLOG} 
Start tracking orphan processes.
##[section]Finishing: Initialize job
##[section]Starting: Configure Job Name
==============================================================================
---
========================== Starting Command Output ===========================
[command]/bin/bash --noprofile --norc /home/vsts/work/_temp/f16730d5-d62e-4189-ad81-c967b265d4fd.sh

##[section]Finishing: Disable git automatic line ending conversion
##[section]Starting: Checkout rust-lang/rust@refs/pull/70946/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
---
##[command]git remote add origin https://github.com/rust-lang/rust
##[command]git config gc.auto 0
##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
##[command]git config --get-all http.proxy
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/70946/merge:refs/remotes/pull/70946/merge
---
 ---> 3adb0605cc65
Step 6/7 : ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
 ---> Using cache
 ---> 28dbc326cb7f
Step 7/7 : ENV SCRIPT python3 ../x.py test src/tools/expand-yaml-anchors &&            python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu &&            python3 ../x.py build --stage 0 src/tools/build-manifest &&            python3 ../x.py test --stage 0 src/tools/compiletest &&            python3 ../x.py test src/tools/tidy &&            python3 ../x.py doc --stage 0 src/libstd &&            /scripts/validate-toolstate.sh
 ---> 537a01811900
Successfully built 537a01811900
Successfully tagged rust-ci:latest
Built container sha256:537a018119009dc218456238dec90b5530050db1e2a1e166550c218003f6159d
---
    Checking rustc_feature v0.0.0 (/checkout/src/librustc_feature)
    Checking fmt_macros v0.0.0 (/checkout/src/libfmt_macros)
    Checking rustc_ast_pretty v0.0.0 (/checkout/src/librustc_ast_pretty)
    Checking rustc_hir v0.0.0 (/checkout/src/librustc_hir)
    Checking rustc_query_system v0.0.0 (/checkout/src/librustc_query_system)
    Checking chalk-rust-ir v0.10.0
    Checking rustc_hir_pretty v0.0.0 (/checkout/src/librustc_hir_pretty)
    Checking rustc_parse v0.0.0 (/checkout/src/librustc_parse)
    Checking rustc_ast_lowering v0.0.0 (/checkout/src/librustc_ast_lowering)
---
    Checking rustc_lint v0.0.0 (/checkout/src/librustc_lint)
error[E0531]: cannot find tuple struct or tuple variant `UnnormalizedProjection` in this scope
    --> src/librustc_lint/builtin.rs:2195:20
     |
2195 |                 | (UnnormalizedProjection(..), UnnormalizedProjection(..))

error[E0531]: cannot find tuple struct or tuple variant `UnnormalizedProjection` in this scope
    --> src/librustc_lint/builtin.rs:2195:48
     |
     |
2195 |                 | (UnnormalizedProjection(..), UnnormalizedProjection(..))

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0531`.
For more information about this error, try `rustc --explain E0531`.
error: could not compile `rustc_lint`.

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "2" "--release" "--color" "always" "--features" " llvm" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json-render-diagnostics"
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap check
Build completed unsuccessfully in 0:04:32
== clock drift check ==
  local time: Sun May 17 14:36:26 UTC 2020
  local time: Sun May 17 14:36:26 UTC 2020
  network time: Sun, 17 May 2020 14:36:26 GMT
== end clock drift check ==

##[error]Bash exited with code '1'.
##[section]Finishing: Run build
##[section]Starting: Checkout rust-lang/rust@refs/pull/70946/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
Author       : Microsoft
Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
==============================================================================
Cleaning any cached credential from repository: rust-lang/rust (GitHub)
##[section]Finishing: Checkout rust-lang/rust@refs/pull/70946/merge to s
Cleaning up task key
Start cleaning up orphan processes.
Terminate orphan process: pid (3818) (python)
##[section]Finishing: Finalize Job

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @rust-lang/infra. (Feature Requests)

[jumbatm]

Hey @nagisa, this is ready for re-review.

[nagisa]

This overall LGTM now. I think it does get the balance right.

Can you please clean up the commits/history a little? Squashing some together, especially those that refer to rustfmt, etc would be ideal.

r=me once that's done.

[nagisa]

@bors r+

[bors]

📌 Commit 337abf1 has been approved by nagisa

[bors]

🌲 The tree is currently closed for pull requests below priority 9000, this pull request will be tested once the tree is reopened

[jumbatm]

Should we do a crater-check run first? Given this fixes a soundness issue, I'd like to make this lint Deny by default, but want to see if it'd break any existing crates.

[jumbatm]

Hey @nagisa, can I get another review? Fingers crossed this can get merged soon.

[nagisa]

@bors r+

[bors]

📌 Commit 556b7ba has been approved by nagisa

[bors]

⌛ Testing commit 556b7ba with merge 4399d4f09c61ea4b3a9070ddc591ab4d72a563d9...

[bors]

💥 Test timed out

[Dylan-DPC-zz]

going to retry again

@bors retry

[bors]

⌛ Testing commit 556b7ba with merge 228a0ed...

[bors]

☀️ Test successful - checks-azure
Approved by: nagisa
Pushing 228a0ed to master...

[petrochenkov]

The lint doesn't follow the lint naming conventions - https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md#lints.

It should preferably be renamed clashing_extern_decl -> clashing_extern_declarations until it's on stable.