Access individual fields of tuples, closures and generators on drop.

[matthewjasper]

Fixes #48623, by extending the change in #47917 to closures. Also does this for tuples and generators for consistency.

Enums are unchanged because there is no way to borrow *enum.field without borrowing enum.field at the moment, so any error would be reported when the enum goes out of scope. Unions aren't changed because unions they don't automatically drop their fields.

r? @nikomatsakis

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[nikomatsakis]

Generators are a bit funny -- I'm not sure if upvar_tys is correct for that case?

Maybe @Zoxc can comment how to get the list of fields for a generator type, else I'll dig in later.

[Zoxc]

I think this is correct here. upvar_tys gives you just the upvars for a generator. Pre-transform generators have an additional u32 state field, but it's only used when constructing a generator and it is never accessed.

[nikomatsakis]

This looks good (modulo generators), though I keep having this nagging feeling we can refactor this another way. But this seems like it is extending the existing fix, which is fine.

[nikomatsakis]

Hmm, maybe we can extract a helper function here? If feels a bit silly to have basically the same code repeated so many times.

[matthewjasper]

Done. I preferred a closure to avoid having to pass all of the captured variables as arguments.

[pietroalbini]

Ping from triage! Could @nikomatsakis (or someone else from @rust-lang/compiler) review this?

[nikomatsakis]

@bors r+

[bors]

📌 Commit dab4884 has been approved by nikomatsakis

[bors]

⌛ Testing commit dab488406f5f948e6f61e793f0f498233a781cfc with merge 6e9d3f14539a96ebcfcb7ee4312592b490318750...

[bors]

💔 Test failed - status-appveyor

[rust-highfive]

The job x86_64-gnu-llvm-3.9 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.

[00:44:39] 
[00:44:39] ---- [ui (nll)] ui/generator/yield-while-iterating.rs stdout ----
[00:44:39]  diff of stderr:
[00:44:39] 
[00:44:39] 6 LL |             yield();
[00:44:39] 7    |             ------- possible yield occurs here
[00:44:39] 8 
[00:44:39] - error[E0597]: borrowed value does not live long enough
[00:44:39] -   --> $DIR/yield-while-iterating.rs:50:17
[00:44:39] -    |
[00:44:39] - LL |       let mut b = || {
[00:44:39] -    |  _________________^
[00:44:39] - LL | |         for p in &mut x {
[00:44:39] - LL | |             yield p;
[00:44:39] - LL | |         }
[00:44:39] - LL | |     };
[00:44:39] -    | |     |
[00:44:39] -    | |     |
[00:44:39] -    | |_____temporary value only lives until here
[00:44:39] -    |       temporary value does not live long enough
[00:44:39] - 
[00:44:39] 23 error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable
[00:44:39] 24   --> $DIR/yield-while-iterating.rs:67:20
[00:44:39] 
[00:44:39] 
[00:44:39] 35 LL |       b.resume();
[00:44:39] 36    |       - borrow later used here
[00:44:39] 37 
[00:44:39] - error[E0597]: borrowed value does not live long enough
[00:44:39] -   --> $DIR/yield-while-iterating.rs:62:17
[00:44:39] -    |
[00:44:39] - LL |       let mut b = || {
[00:44:39] -    |  _________________^
[00:44:39] - LL | |         for p in &mut x {
[00:44:39] - LL | |             yield p;
[00:44:39] - LL | |         }
[00:44:39] - LL | |     };
[00:44:39] -    | |     |
[00:44:39] -    | |     |
[00:44:39] -    | |_____temporary value only lives until here
[00:44:39] -    |       temporary value does not live long enough
[00:44:39] + error: aborting due to 2 previous errors
[00:44:39] - error: aborting due to 4 previous errors
[00:44:39] - 
[00:44:39] - Some errors occurred: E0502, E0597, E0626.
[00:44:39] + Some errors occurred: E0502, E0626.
[00:44:39] + Some errors occurred: E0502, E0626.
[00:44:39] 55 For more information about an error, try `rustc --explain E0502`.
[00:44:39] 56 
[00:44:39] 
[00:44:39] 
[00:44:39] The actual stderr differed from the expected stderr.
[00:44:39] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/generator/yield-while-iterating.nll.stderr
[00:44:39] /checkout/src/test/ui/update-references.sh '/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui' 'generator/yield-while-iterating.rs'
[00:44:39] 
[00:44:39] error: 1 errors occurred comparing output.
[00:44:39] status: exit code: 101
[00:44:39] status: exit code: 101
[00:44:39] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/generator/yield-while-iterating.rs" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/generator/yield-while-iterating.stage2-x86_64-unknown-linux-gnu" "-Zborrowck=mir" "-Ztwo-phase-borrows" "-Crpath" "-O" "-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/generator/yield-while-iterating.stage2-x86_64-unknown-linux-gnu.aux" "-A" "unused"
[00:44:39] ------------------------------------------
[00:44:39] 
[00:44:39] ------------------------------------------
[00:44:39] stderr:
[00:44:39] stderr:
[00:44:39] ------------------------------------------
[00:44:39] {"message":"borrow may still be in use when generator yields","code":{"code":"E0626","explanation":"\nThis error occurs because a borrow in a generator persists across a\nyield point.\n\n```compile_fail,E0626\n# #![feature(generators, generator_trait)]\n# use std::ops::Generator;\nlet mut b = || {\n    let a = &String::new(); // <-- This borrow...\n    yield (); // ...is still in scope here, when the yield occurs.\n    println!(\"{}\", a);\n};\nunsafe { b.resume() };\n```\n\nAt present, it is not permitted to have a yield that occurs while a\nborrow is still in scope. To resolve this error, the borrow must\neither be \"contained\" to a smaller scope that does not overlap the\nyield or else eliminated in another way. So, for example, we might\nresolve the previous example by removing the borrow and just storing\nthe integer by value:\n\n```\n# #![feature(generators, generator_trait)]\n# use std::ops::Generator;\nlet mut b = || {\n    let a = 3;\n    yield ();\n    println!(\"{}\", a);\n};\nunsafe { b.resume() };\n```\n\nThis is a very simple case, of course. In more complex cases, we may\nwish to have more than one reference to the value that was borrowed --\nin those cases, something like the `Rc` or `Arc` types may be useful.\n\nThis error also frequently arises with iteration:\n\n```compile_fail,E0626\n# #![feature(generators, generator_trait)]\n# use std::ops::Generator;\nlet mut b = || {\n  let v = vec![1,2,3];\n  for &x in &v { // <-- borrow of `v` is still in scope...\n    yield x; // ...when this yield occurs.\n  }\n};\nunsafe { b.resume() };\n```\n\nSuch cases can sometimes be resolved by iterating \"by value\" (or using\n`into_iter()`) to avoid borrowing:\n\n```\n# #![feature(generators, generator_trait)]\n# use std::ops::Generator;\nlet mut b = || {\n  let v = vec![1,2,3];\n  for x in v { // <-- Take ownership of the values instead!\n    yield x; // <-- Now yield is OK.\n  }\n};\nunsafe { b.resume() };\n```\n\nIf taking ownership is not an option, using indices can work too:\n\n```\n# #![feature(generators, generator_trait)]\n# use std::ops::Generator;\nlet mut b = || {\n  let v = vec![1,2,3];\n  let len = v.len(); // (*)\n  for i in 0..len {\n    let x = v[i]; // (*)\n    yield x; // <-- Now yield is OK.\n  }\n};\nunsafe { b.resume() };\n\n// (*) -- Unfortunately, these temporaries are currently required.\n// See <https://github.com/rust-lang/rust/issues/43122>.\n```\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/ui/generator/yield-while-iterating.rs","byte_start":948,"byte_end":955,"line_start":23,"line_end":23,"column_start":13,"column_end":20,"is_primary":false,"text":[{"text":"            yield();","highlight_start":13,"highlight_end":20}],"label":"possible yield occurs here","suggested_replacement":null,"expansion":null},{"file_name":"/checkout/src/test/ui/generator/yield-while-iterating.rs","byte_start":921,"byte_end":923,"line_start":22,"line_end":22,"column_start":18,"column_end":20,"is_primary":true,"text":[{"text":"        for p in &x { //~ ERROR","highlight_start":18,"highlight_end":20}],"label":null,"suggested_replacement":null,"expansion":null}],"children":[],"rendered":"error[E0626]: borrow may still be in use when generator yields\n  --> /checkout/src/test/ui/generator/yield-while-iterating.rs:22:18\n   |\nLL |         for p in &x { //~ ERROR\n   |                  ^^\nLL |             yield();\n   |             ------- possible yield occurs here\n\n"}
[00:44:39] {"message":"cannot borrow `x` as immutable because it is also borrowed as mutable","code":{"code":"E0502","explanation":"\nThis error indicates that you are trying to borrow a variable as mutable when it\nhas already been borrowed as immutable.\n\nExample of erroneous code:\n\n```compile_fail,E0502\nfn bar(x: &mut i32) {}\nfn foo(a: &mut i32) {\n    let ref y = a; // a is borrowed as immutable.\n    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed\n            //        as immutable\n}\n```\n\nTo fix this error, ensure that you don't have any other references to the\nvariable before trying to access it mutably:\n\n```\nfn bar(x: &mut i32) {}\nfn foo(a: &mut i32) {\n    bar(a);\n    let ref y = a; // ok!\n}\n```\n\nFor more information on the rust ownership system, take a look at\nhttps://doc.rust-lang.org/stable/book/references-and-borrowing.html.\n"},"level":"error","spans":[{"file_name":"/checkout/src/test/ui/generator/yield-while-iterating.rs","byte_start":1798,"byte_end":1799,"line_start":67,"line_end":67,"column_start":20,"column_end":21,"is_primary":true,"text":[{"text":"    println!(\"{}\", x[0]); //~ ERROR","highlight_start":20,"highlight_end":21}],"label":"immutable borrow occurs here","suggested_replacement":null,"expansion":null},{"file_name":"/checkout/src/test/ui/generator/yield-while-iterating.rs","byte_start":1710,"byte_end":1777,"line_start":62,"line_end":66,"column_start":17,"column_end":6,"is_primary":false,"text":[{"text":"    let mut b = || {","highlight_start":17,"highlight_end":21},{"text":"        for p in &mut x {","highlight_start":1,"highlight_end":26},{"text":"            yield p;","highlight_start":1,"highlight_end":21},{"text":"        }","highlight_start":1,"highlight_end":10},{"text":"    };","highlight_start":1,"highlight_end":6}],"label":"mutable borrow occurs here","suggested_replacement":null,"expansion":null},{"file_name":"/checkout/src/test/ui/generator/yield-while-iterating.rs","byte_start":1819,"byte_end":1820,"line_start":68,"line_end":68,"column_start":5,"column_end":6,"is_primary":false,"text":[{"text":"    b.resume();","highlight_start":5,"highlight_end":6}],"label":"borrow later used here","suggested_replacement":null,"expansion":null}],"children":[],"rendered":"error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable\n  --> /checkout/src/test/ui/generator/yield-while-iterating.rs:67:20\n   |\nLL |       let mut b = || {\n   |  _________________-\nLL | |         for p in &mut x {\nLL | |             yield p;\nLL | |         }\nLL | |     };\n   | |_____- mutable borrow occurs here\nLL |       println!(\"{}\", x[0]); //~ ERROR\n   |                      ^ immutable borrow occurs here\nLL |       b.resume();\n   |       - borrow later used here\n\n"}
[00:44:39] {"message":"aborting due to 2 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 2 previous errors\n\n"}
[00:44:39] {"message":"Some errors occurred: E0502, E0626.","code":null,"level":"","spans":[],"children":[],"rendered":"Some errors occurred: E0502, E0626.\n"}
[00:44:39] {"message":"For more information about an error, try `rustc --explain E0502`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about an error, try `rustc --explain E0502`.\n"}
[00:44:39] ------------------------------------------
[00:44:39] 
[00:44:39] thread '[ui (nll)] ui/generator/yield-while-iterating.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:2963:9
[00:44:39] note: Run with `RUST_BACKTRACE=1` for a backtrace.
---
[00:44:39] 
[00:44:39] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:488:22
[00:44:39] 
[00:44:39] 
[00:44:39] 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/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-3.9/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" "--llvm-version" "3.9.1\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always" "--compare-mode" "nll"
[00:44:39] 
[00:44:39] 
[00:44:39] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[00:44:39] Build completed unsuccessfully in 0:03:42
[00:44:39] Build completed unsuccessfully in 0:03:42
[00:44:39] Makefile:58: recipe for target 'check' failed
[00:44:39] make: *** [check] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0bb5397b
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[nikomatsakis]

@bors r+

[bors]

📌 Commit 902bc0f has been approved by nikomatsakis

[kennytm]

@bors p=6

[bors]

⌛ Testing commit 902bc0f with merge ede7f94...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing ede7f94 to master...