Remove ast::Guard

[Centril]

With the introduction of ast::ExprKind::Let in #60861, the ast::Guard structure is now redundant in terms of representing if let guards in AST since it can be represented by ExprKind::Let syntactically. Therefore, we remove ast::Guard here.

However, we keep hir::Guard because the semantic representation is a different matter and this story is more unclear right now (might involve goto 'arm in HIR or something...).

r? @petrochenkov

[petrochenkov]

@bors r+ rollup

[bors]

📌 Commit 4d53714 has been approved by petrochenkov

[bors]

⌛ Testing commit 4d53714 with merge b5383b8...

[bors]

💔 Test failed - checks-travis

[rust-highfive]

The job arm-android 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.

[01:44:33] test string::test_str_truncate ... ok
[01:44:33] test string::test_str_truncate_invalid_len ... ok
[01:44:33] test string::test_retain ... ok
[01:44:33] test string::test_str_truncate_split_codepoint ... ok
[01:44:33] error: test failed, to rerun pass '-p alloc --test collectionstests'
[01:44:33] died due to signal 11
[01:44:33] 
[01:44:33] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "arm-linux-androideabi" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--"
[01:44:33] expected success, got: exit code: 3
[01:44:33] 
---
travis_time:end:01eb9dbb:start=1561292788302978447,finish=1561292788312777061,duration=9798614
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:1e5bbdfb
$ 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 --batch -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:1c2e7180
travis_time:start:1c2e7180
$ 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:0a19634c
$ 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)

[Centril]

@bors retry collectionstests

[bors]

⌛ Testing commit 4d53714 with merge 3cc3486...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: petrochenkov
Pushing 3cc3486 to master...