Implement try block expressions

[scottmcm]

I noticed that try wasn't a keyword yet in Rust 2018, so...

Fix​es #52604 That was fixed by PR #53135
cc #31436 #50412

[rust-highfive]

r? @michaelwoerister

(rust_highfive has picked a reviewer for you, use r? to override)

[scottmcm]

Ok, I think this is in good enough for a review pass. I've never touched most of these parts of the compiler before, so I suspect there are opportunities for improvement in how I did things 🙃

[petrochenkov]

Nit: Keywords are not normally moved into the "used" category until the feature is stable (e.g. macro/yield/async are all "reserved for future use").

[Centril]

Should probably rename this to try_expr?

[michaelwoerister]

r? @nikomatsakis for re-assignment. I don't think I'm the right person to review this.

[nikomatsakis]

Very nice!

[nikomatsakis]

@bors r+

[bors]

📌 Commit ec85915c5ccf9ec1ba2b4183add0fe7efbbaa00c has been approved by nikomatsakis

[nikomatsakis]

@bors r-

[nikomatsakis]

One question: do we want to provide a kind of smoother path for people using do catch today? e.g., a nice error message?

[nikomatsakis]

Or do we just not care :)

[Centril]

@nikomatsakis Here's a sourcegraph query for it, https://sourcegraph.com/search?q=repogroup:crates+case:yes++%5Cbdo%5Cs%2Bcatch%5Cb+max:400

Most of these occurrences are irrelevant and so I think we should not care.

[Centril]

Also; having the feature called catch_expr for a try { .. } block will be confusing, so let's change that bit :)

[kennytm]

@bors retry

An error occurred while generating the build script.

[bors]

⌛ Testing commit 0095471 with merge 127d02057977122d7ca0707c27d3ce2a4cbd66db...

[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.

[01:31:26] 
[01:31:26] testing https://github.com/servo/servo
[01:31:26] Initialized empty Git repository in /checkout/obj/build/ct/servo/.git/
[01:31:26] fatal: Could not parse object '17e97b9320fdb7cdb33bbc5f4d0fde0653bbf2e4'.
[01:31:46] fatal: unable to access 'https://github.com/servo/servo/': Could not resolve host: github.com
[01:31:46] thread 'main' panicked at 'assertion failed: status.success()', tools/cargotest/main.rs:128:13
[01:31:46] 
[01:31:46] 
[01:31:46] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/cargotest" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "/checkout/obj/build/ct"
[01:31:46] expected success, got: exit code: 101
[01:31:46] expected success, got: exit code: 101
[01:31:46] 
[01:31:46] 
[01:31:46] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/test/pretty src/test/run-pass/pretty src/test/run-fail/pretty src/test/run-pass-valgrind/pretty src/test/run-pass-fulldeps/pretty src/test/run-fail-fulldeps/pretty src/tools/cargo src/tools/cargotest
[01:31:46] Build completed unsuccessfully in 0:35:04
[01:31:46] make: *** [check-aux] Error 1
[01:31:46] Makefile:60: recipe for target 'check-aux' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1aac925a
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:156fb9f1:start=1535005949727505251,finish=1535005949733912231,duration=6406980
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0255de5b
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:00dd4bfc
travis_time:start:00dd4bfc
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:0252de30
$ dmesg | grep -i kill

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)

[scottmcm]

I'm cursed today:

[01:31:46] fatal: unable to access 'https://github.com/servo/servo/': Could not resolve host: github.com

https://api.travis-ci.org/v3/job/419489535/log.txt

[varkor]

@bors retry

[bors]

⌛ Testing commit 0095471 with merge 35bf1ae...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 35bf1ae to master...

[alexcrichton]

As a heads up, we've got a pretty serious issue which may be a bit of a roadblock for this. Currently the compiler doesn't actually warn about usages of the try identifier on the 2015 edition, but that's tracked by #53077 and fix for it is at #53685. Unfortunately though the primary use of the try keyword, the try! macro in the standard library, suffers from #53686.

For the edition we'll need to have at least some warning for users of the try! macro to rustfix into r#try or suggest ?, but currently no warnings are emitted which can make for a difficult edition transition experience. Some eyes and/or thoughts on #53686 would be greatly appreciated!