Allow to dispatch fn traits depending on number of parameters

[cjgillot]

Hello,

By following @eddyb's advise on issue #45510, I managed to have the snippets of code in #45510 and #18952 passing without breaking older diagnostics.

EDIT: the codegen tests breakage I experienced is due to the poor quality of my laptop.

If any kind reviewer has any advice, you are very welcome.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @cramertj (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[eddyb]

r? @nikomatsakis

[bors]

☔️ The latest upstream changes (presumably #56051) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Looks good! Can you add the comment and explain that one case?

[nikomatsakis]

Looks good! Can you add the comment and explain that one case?

[nikomatsakis]

can we tweak the tests a bit to show that we are actually calling the fn as expected etc?

[nikomatsakis]

can we tweak the tests a bit to show that we are actually calling the fn as expected etc?

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (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.

travis_time:end:1598c091:start=1543407844229716095,finish=1543407901626499970,duration=57396783875
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
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/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/issue-45510/auxiliary"
[00:56:55] ------------------------------------------
[00:56:55] 
[00:56:55] ------------------------------------------
[00:56:55] stderr:
[00:56:55] stderr:
[00:56:55] ------------------------------------------
[00:56:55] {"message":"can't compare `Ishmael` with `Ishmael`","code":{"code":"E0277","explanation":"\nYou tried to use a type which doesn't implement some trait in a place which\nexpected that trait. Erroneous code example:\n\n```compile_fail,E0277\n// here we declare the Foo trait with a bar method\ntrait Foo {\n    fn bar(&self);\n}\n\n// we now declare a function which takes an object implementing the Foo trait\nfn some_func<T: Foo>(foo: T) {\n    foo.bar();\n}\n\nfn main() {\n    // we now call the method with the i32 type, which doesn't implement\n    // the Foo trait\n    some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied\n}\n```\n\nIn order to fix this error, verify that the type you're using does implement\nthe trait. Example:\n\n```\ntrait Foo {\n    fn bar(&self);\n}\n\nfn some_func<T: Foo>(foo: T) {\n    foo.bar(); // we can now use this method since i32 implements the\n               // Foo trait\n}\n\n// we implement the trait on the i32 type\nimpl Foo for i32 {\n    fn bar(&self) {}\n}\n\nfn main() {\n    some_func(5i32); // ok!\n}\n```\n\nOr in a generic context, an erroneous code example would look like:\n\n```compile_fail,E0277\nfn some_func<T>(foo: T) {\n    println!(\"{:?}\", foo); // error: the trait `core::fmt::Debug` is not\n                           //        implemented for the type `T`\n}\n\nfn main() {\n    // We now call the method with the i32 type,\n    // which *does* implement the Debug trait.\n    some_func(5i32);\n}\n```\n\nNote that the error here is in the definition of the generic function: Although\nwe only call it with a parameter that does implement `Debug`, the compiler\nstill rejects the function: It must work with all possible input types. In\norder to make this example compile, we need to restrict the generic type we're\naccepting:\n\n```\nuse std::fmt;\n\n// Restrict the input type to types that implement Debug.\nfn some_func<T: fmt::Debug>(foo: T) {\n    println!(\"{:?}\", foo);\n}\n\nfn main() {\n    // Calling the method is still fine, as i32 implements Debug.\n    some_func(5i32);\n\n    // This would fail to compile now:\n    // struct WithoutDebug;\n    // some_func(WithoutDebug);\n}\n```\n\nRust only looks at the signature of the called function, as such it must\nalready specify all requirements that will be used for every type parameter.\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/run-pass/issue-45510.rs","byte_start":127,"byte_end":129,"line_start":7,"line_end":7,"column_start":17,"column_end":19,"is_primary":true,"text":[{"text":"#[derive(Debug, Eq)]","highlight_start":17,"highlight_end":19}],"label":"no implementation for `Ishmael == Ishmael`","suggested_replacement":null,"suggestion_applicability":null,"expansion":{"span":{"file_name":"/checkout/src/test/run-pass/issue-45510.rs","byte_start":127,"byte_end":129,"line_start":7,"line_end":7,"column_start":17,"column_end":19,"is_primary":falsee implement the trait on the i32 type\nimpl Foo for i32 {\n    fn bar(&self) {}\n}\n\nfn main() {\n    some_func(5i32); // ok!\n}\n```\n\nOr in a generic context, an erroneous code example would look like:\n\n```compile_fail,E0277\nfn some_func<T>(foo: T) {\n    println!(\"{:?}\", foo); // error: the trait `core::fmt::Debug` is not\n                           //        implemented for the type `T`\n}\n\nfn main() {\n    // We now call the method with the i32 type,\n    // which *does* implement the Debug trait.\n    some_func(5i32);\n}\n```\n\nNote that the error here is in the definition of the generic function: Although\nwe only call it with a parameter that does implement `Debug`, the compiler\nstill rejects the function: It must work with all possible input types. In\norder to make this example compile, we need to restrict the generic type we're\naccepting:\n\n```\nuse std::fmt;\n\n// Restrict the input type to types that implement Debug.\nfn some_func<T: fmt::Debug>(foo: T) {\n    println!(\"{:?}\", foo);\n}\n\nfn main() {\n    // Calling the method is still fine, as i32 implements Debug.\n    some_func(5i32);\n\n    // This would fail to compile now:\n    // struct WithoutDebug;\n    // some_func(WithoutDebug);\n}\n```\n\nRust only looks at the signature of the called function, as such it must\nalready specify all requirements that will be used for every type parameter.\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/run-pass/issue-45510.rs","byte_start":164,"byte_end":166,"line_start":9,"line_end":9,"column_start":17,"column_end":19,"is_primary":true,"text":[{"text":"#[derive(Debug, Eq)]","highlight_start":17,"highlight_end":19}],"label":"no implementation for `Maybe == Maybe`","suggested_replacement":null,"suggestion_applicability":null,"expansion":{"span":{"file_name":"/checkout/src/test/run-pass/issue-45510.rs","byte_start":164,"byte_end":166,"line_start":9,"line_end":9,"column_start":17,"column_end":19,"is_primary":false,"text":[{"text":"#[derive(Debug, Eq)]","highlight_start":17,"highlight_end":19}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null},"macro_decl_name":"#[derive(Eq)]","def_site_span":null}}],"children":[{"message":"the trait `std::cmp::PartialEq` is not implemented for `Maybe`","code":null,"level":"help","spans":[],"children":[],"rendered":null}],"rendered":"error[E0277]: can't compare `Maybe` with `Maybe`\n  --> /checkout/src/test/run-pass/issue-45510.rs:9:17\n   |\nLL | #[derive(Debug, Eq)]\n   |                 ^^ no implementation for `Maybe == Maybe`\n   |\n   = help: the trait `std::cmp::PartialEq` is not implemented for `Maybe`\n\n"}
[00:56:55] {"message":"aborting due to 2 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 2 previous errors\n\n"}
[00:56:55] {"message":"For more information about this error, try `rustc --explain E0277`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0277`.\n"}
[00:56:55] ------------------------------------------
[00:56:55] 
[00:56:55] thread '[run-pass] run-pass/issue-45510.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:3282:9
[00:56:55] note: Run with `RUST_BACKTRACE=1` for a backtrace.
---
[00:56:55] 
[00:56:55] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:503:22
[00:56:55] 
[00:56:55] 
[00:56:55] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/run-pass" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "run-pass" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-5.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options " "--target-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "5.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[00:56:55] 
[00:56:55] 
[00:56:55] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[00:56:55] Build completed unsuccessfully in 0:12:20
[00:56:55] Build completed unsuccessfully in 0:12:20
[00:56:55] Makefile:58: recipe for target 'check' failed
[00:56:55] make: *** [check] Error 1

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 @TimNN. (Feature Requests)

[bors]

☔️ The latest upstream changes (presumably #56502) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔️ The latest upstream changes (presumably #56875) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔️ The latest upstream changes (presumably #54252) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 91c155b has been approved by nikomatsakis

[bors]

⌛️ Testing commit 91c155b with merge cbacd34...

[bors]

💔 Test failed - status-travis

[rust-highfive]

The job x86_64-gnu-aux of your PR failed on Travis (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.

[01:47:45] test workspaces::ws_warn_path ... ok
[01:47:45] test workspaces::you_cannot_generate_lockfile_for_empty_workspaces ... ok
[01:47:45] test workspaces::workspace_with_transitive_dev_deps ... ok
[01:47:46] test workspaces::ws_warn_unused ... ok
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

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 @TimNN. (Feature Requests)

[kennytm]

@bors retry

[bors]

⌛️ Testing commit 91c155b with merge a602f13...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing a602f13 to master...