Suggest removing `&mut` when encountering mutability error #68723

LeSeulArtichaut · 2020-02-01T01:14:37Z

Resolves #68697

I haven't checked that this change doesn't break any of the already existing UI tests, I'll use the CI to figure out for me.

r? @Centril

Centril · 2020-02-01T01:31:14Z

src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs

-                    Applicability::MachineApplicable,
-                );
+
+                if let ty::Ref(_, _, Mutability::Mut) = local_decl.ty.kind {


This condition seems insufficient. You'd need to check that the expected type requires &mut T and not &mut &mut T, otherwise it seems like it would trigger on this (which is a good test to add, to ensure that this behaves correctly):

struct S; fn bar(x: &mut &mut S) {} fn foo() { let x = &mut S; bar(&mut x); }

...but this makes me wonder whether this is doable in borrowck if you don't know what type is expected (seems you only know that you want to take a mutable borrow and cannot due to lack of mut on the binding. On the other hand, if you do check it in typeck, then we need to check that:

struct S; fn bar(x: &mut S) {} fn foo() { let mut x = &mut S; bar(&mut x); }

still compiles... which is probably not a good idea in typeck.

Centril · 2020-02-01T01:31:25Z

r? @matthewjasper

rust-highfive · 2020-02-01T02:21:44Z

The job x86_64-gnu-llvm-7 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.

2020-02-01T01:15:11.3948654Z ========================== Starting Command Output ===========================
2020-02-01T01:15:11.3950454Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/af415ec5-73f0-4d42-ad43-8579b1f114cc.sh
2020-02-01T01:15:11.3950505Z 
2020-02-01T01:15:11.3953027Z ##[section]Finishing: Disable git automatic line ending conversion
2020-02-01T01:15:11.3961573Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68723/merge to s
2020-02-01T01:15:11.3964008Z Task         : Get sources
2020-02-01T01:15:11.3964050Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-02-01T01:15:11.3964080Z Version      : 1.0.0
2020-02-01T01:15:11.3964110Z Author       : Microsoft
---
2020-02-01T01:15:12.3762662Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-02-01T01:15:12.3774238Z ##[command]git config gc.auto 0
2020-02-01T01:15:12.3776865Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-02-01T01:15:12.3779079Z ##[command]git config --get-all http.proxy
2020-02-01T01:15:12.3785714Z ##[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/68723/merge:refs/remotes/pull/68723/merge
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---
2020-02-01T02:21:38.5337548Z 5    |         ^^^^^^
2020-02-01T02:21:38.5337577Z 
2020-02-01T02:21:38.5337603Z 
2020-02-01T02:21:38.5337648Z The actual stderr differed from the expected stderr.
2020-02-01T02:21:38.5338265Z Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/issue-68697-remove-mut-ref/issue-68697-remove-mut-ref.stderr
2020-02-01T02:21:38.5338524Z To update references, rerun the tests and pass the `--bless` flag
2020-02-01T02:21:38.5338846Z To only update this specific test, also pass `--test-args borrowck/issue-68697-remove-mut-ref.rs`
2020-02-01T02:21:38.5338952Z error: 1 errors occurred comparing output.
2020-02-01T02:21:38.5339123Z status: exit code: 1
2020-02-01T02:21:38.5339123Z status: exit code: 1
2020-02-01T02:21:38.5340050Z command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/borrowck/issue-68697-remove-mut-ref.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/issue-68697-remove-mut-ref" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/issue-68697-remove-mut-ref/auxiliary" "-A" "unused"
2020-02-01T02:21:38.5340425Z ------------------------------------------
2020-02-01T02:21:38.5340462Z 
2020-02-01T02:21:38.5340701Z ------------------------------------------
2020-02-01T02:21:38.5340766Z stderr:
2020-02-01T02:21:38.5340766Z stderr:
2020-02-01T02:21:38.5341090Z ------------------------------------------
2020-02-01T02:21:38.5341152Z error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
2020-02-01T02:21:38.5341419Z   --> /checkout/src/test/ui/borrowck/issue-68697-remove-mut-ref.rs:9:9
2020-02-01T02:21:38.5341469Z    |
2020-02-01T02:21:38.5341509Z LL |     bar(&mut x);
2020-02-01T02:21:38.5341550Z    |         ^^^^^^
2020-02-01T02:21:38.5341728Z    |         |
2020-02-01T02:21:38.5341774Z    |         cannot borrow as mutable
2020-02-01T02:21:38.5341831Z    |         help: remove the unnecessary `&mut` here: `x`
2020-02-01T02:21:38.5341922Z error: aborting due to previous error
2020-02-01T02:21:38.5341953Z 
2020-02-01T02:21:38.5342203Z For more information about this error, try `rustc --explain E0596`.
2020-02-01T02:21:38.5342255Z 
2020-02-01T02:21:38.5342255Z 
2020-02-01T02:21:38.5342469Z ------------------------------------------
2020-02-01T02:21:38.5342502Z 
2020-02-01T02:21:38.5342527Z 
2020-02-01T02:21:38.5342771Z ---- [ui] ui/borrowck/mut-borrow-of-mut-ref.rs stdout ----
2020-02-01T02:21:38.5342828Z diff of stderr:
2020-02-01T02:21:38.5342856Z 
2020-02-01T02:21:38.5342903Z 1 error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
2020-02-01T02:21:38.5343192Z 3    |
2020-02-01T02:21:38.5343192Z 3    |
2020-02-01T02:21:38.5343391Z - LL | fn f(b: &mut i32) {
2020-02-01T02:21:38.5343647Z -    |      - help: consider changing this to be mutable: `mut b`
2020-02-01T02:21:38.5343697Z 6 LL |     g(&mut b)
2020-02-01T02:21:38.5343976Z +    |       ^^^^^^
2020-02-01T02:21:38.5344037Z +    |       |
2020-02-01T02:21:38.5344080Z +    |       cannot borrow as mutable
2020-02-01T02:21:38.5344080Z +    |       cannot borrow as mutable
2020-02-01T02:21:38.5344129Z +    |       help: remove the unnecessary `&mut` here: `b`
2020-02-01T02:21:38.5344232Z 9 error: aborting due to previous error
2020-02-01T02:21:38.5344273Z 10 
2020-02-01T02:21:38.5344299Z 
2020-02-01T02:21:38.5344341Z 
2020-02-01T02:21:38.5344341Z 
2020-02-01T02:21:38.5344393Z The actual stderr differed from the expected stderr.
2020-02-01T02:21:38.5344715Z Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/mut-borrow-of-mut-ref/mut-borrow-of-mut-ref.stderr
2020-02-01T02:21:38.5345092Z To update references, rerun the tests and pass the `--bless` flag
2020-02-01T02:21:38.5345380Z To only update this specific test, also pass `--test-args borrowck/mut-borrow-of-mut-ref.rs`
2020-02-01T02:21:38.5345573Z error: 1 errors occurred comparing output.
2020-02-01T02:21:38.5345617Z status: exit code: 1
2020-02-01T02:21:38.5345617Z status: exit code: 1
2020-02-01T02:21:38.5346523Z command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/borrowck/mut-borrow-of-mut-ref.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/mut-borrow-of-mut-ref" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/borrowck/mut-borrow-of-mut-ref/auxiliary" "-A" "unused"
2020-02-01T02:21:38.5346904Z ------------------------------------------
2020-02-01T02:21:38.5346956Z 
2020-02-01T02:21:38.5347187Z ------------------------------------------
2020-02-01T02:21:38.5347244Z stderr:
2020-02-01T02:21:38.5347244Z stderr:
2020-02-01T02:21:38.5347471Z ------------------------------------------
2020-02-01T02:21:38.5347541Z error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
2020-02-01T02:21:38.5347850Z    |
2020-02-01T02:21:38.5347850Z    |
2020-02-01T02:21:38.5347911Z LL |     g(&mut b) //~ ERROR cannot borrow
2020-02-01T02:21:38.5347996Z    |       |
2020-02-01T02:21:38.5348054Z    |       cannot borrow as mutable
2020-02-01T02:21:38.5348054Z    |       cannot borrow as mutable
2020-02-01T02:21:38.5348110Z    |       help: remove the unnecessary `&mut` here: `b`
2020-02-01T02:21:38.5348185Z error: aborting due to previous error
2020-02-01T02:21:38.5348230Z 
2020-02-01T02:21:38.5348492Z For more information about this error, try `rustc --explain E0596`.
2020-02-01T02:21:38.5348526Z 
---
2020-02-01T02:21:38.5360162Z thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:348:22
2020-02-01T02:21:38.5360258Z note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2020-02-01T02:21:38.5373315Z 
2020-02-01T02:21:38.5373393Z 
2020-02-01T02:21:38.5375150Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "/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/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-7/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0 -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" "7.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
2020-02-01T02:21:38.5375590Z 
2020-02-01T02:21:38.5375620Z 
2020-02-01T02:21:38.5383227Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
2020-02-01T02:21:38.5383382Z Build completed unsuccessfully in 1:00:45
2020-02-01T02:21:38.5383382Z Build completed unsuccessfully in 1:00:45
2020-02-01T02:21:38.5434266Z == clock drift check ==
2020-02-01T02:21:38.5473304Z   local time: Sat Feb  1 02:21:38 UTC 2020
2020-02-01T02:21:39.0936713Z   network time: Sat, 01 Feb 2020 02:21:39 GMT
2020-02-01T02:21:39.0941178Z == end clock drift check ==
2020-02-01T02:21:39.4547913Z 
2020-02-01T02:21:39.4664710Z ##[error]Bash exited with code '1'.
2020-02-01T02:21:39.4677268Z ##[section]Finishing: Run build
2020-02-01T02:21:39.4699965Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68723/merge to s
2020-02-01T02:21:39.4701888Z Task         : Get sources
2020-02-01T02:21:39.4701954Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-02-01T02:21:39.4702017Z Version      : 1.0.0
2020-02-01T02:21:39.4702059Z Author       : Microsoft
2020-02-01T02:21:39.4702059Z Author       : Microsoft
2020-02-01T02:21:39.4702105Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
2020-02-01T02:21:39.4702171Z ==============================================================================
2020-02-01T02:21:39.8920523Z Cleaning any cached credential from repository: rust-lang/rust (GitHub)
2020-02-01T02:21:39.8962631Z ##[section]Finishing: Checkout rust-lang/rust@refs/pull/68723/merge to s
2020-02-01T02:21:39.9083076Z Cleaning up task key
2020-02-01T02:21:39.9083918Z Start cleaning up orphan processes.
2020-02-01T02:21:39.9194662Z Terminate orphan process: pid (5152) (python)
2020-02-01T02:21:39.9428038Z ##[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 @TimNN. (Feature Requests)

JohnCSimon · 2020-02-10T01:26:25Z

Ping from triage: @LeSeulArtichaut Can you please address the test failures?

LeSeulArtichaut · 2020-02-11T21:26:45Z

@JohnCSimon As Centril pointed out, my fix was very naive and will not work. I'll close this now

Suggest removing &mut when encountering mutability error

afd38e9

rust-highfive assigned Centril Feb 1, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 1, 2020

Centril reviewed Feb 1, 2020

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rust-highfive assigned matthewjasper and unassigned Centril Feb 1, 2020

JohnCSimon added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 10, 2020

LeSeulArtichaut closed this Feb 11, 2020

LeSeulArtichaut deleted the diagnose-mut-borrow branch February 18, 2021 20:28

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Suggest removing `&mut` when encountering mutability error #68723

Suggest removing `&mut` when encountering mutability error #68723

LeSeulArtichaut commented Feb 1, 2020

Centril Feb 1, 2020

Centril commented Feb 1, 2020

rust-highfive commented Feb 1, 2020

JohnCSimon commented Feb 10, 2020

LeSeulArtichaut commented Feb 11, 2020

Suggest removing &mut when encountering mutability error #68723

Suggest removing &mut when encountering mutability error #68723

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LeSeulArtichaut commented Feb 1, 2020

Centril Feb 1, 2020

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Centril commented Feb 1, 2020

rust-highfive commented Feb 1, 2020

JohnCSimon commented Feb 10, 2020

LeSeulArtichaut commented Feb 11, 2020

Suggest removing `&mut` when encountering mutability error #68723

Suggest removing `&mut` when encountering mutability error #68723