Make SortedMap::new const, also faster is_empty

[llogiq]

(The latter is because [T]::is_empty compiles to a single pointer comparison, whereas len() == 0 first subtracts the pointers and only then compares with zero)

[rust-highfive]

r? @davidtwco

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The job x86_64-gnu-llvm-6.0 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:260093f8:start=1545184131617532413,finish=1545184132753039487,duration=1135507074
$ 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-6.0
---
[00:05:33]    Compiling backtrace v0.3.11
[00:05:34]    Compiling parking_lot v0.6.4
[00:05:36]    Compiling rustc-rayon v0.1.1
travis_time:start:0247b6fc
$ 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:177ba5db
travis_time:start:177ba5db
$ 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:33988aee
$ 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)

[sinkuu]

Vec (self.data) has len field, and a slice is (ptr, len), right? It seems strange if len() == 0 involves pointer subtraction. len() == 0 -> is_empty() is a nice cleanup though.

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The job x86_64-gnu-llvm-6.0 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:0212eb17:start=1545219886964955679,finish=1545219889233250839,duration=2268295160
$ 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-6.0

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)

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

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travis_time:end:002583c8:start=1545220725012526478,finish=1545220726029633601,duration=1017107123
$ 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-6.0
---
[00:05:23]    |                   ^^^^^^ allocation not allowed in constant functions
[00:05:23]    |
[00:05:23]    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
[00:05:23] 
[00:05:23] error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
[00:05:23]   --> src/librustc_data_structures/sorted_map.rs:35:19
[00:05:23] 35 |             data: vec![]
[00:05:23]    |                   ^^^^^^
[00:05:23]    |
[00:05:23]    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
---
[00:05:24] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:05:24] expected success, got: exit code: 101
[00:05:24] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:05:24] Build completed unsuccessfully in 0:01:59
[00:05:24] make: *** [all] Error 1
[00:05:24] Makefile:28: recipe for target 'all' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0c39a6b0
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Dec 19 12:04:18 UTC 2018
---
travis_time:end:0d502902:start=1545221059383467347,finish=1545221059388869534,duration=5402187
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0bb124fc
$ 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:0070f886
travis_time:start:0070f886
$ 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:01c3157c
$ dmesg | grep -i kill

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[oli-obk]

While these can be const fn, we've previously opted not to make internal functions const fn without concrete use cases

[oli-obk]

You need to use Vec::new() here

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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:01f02d68:start=1545235788330528876,finish=1545235843265671827,duration=54935142951
$ 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-6.0
---
[00:05:16]    |                   ^^^^^^ allocation not allowed in constant functions
[00:05:16]    |
[00:05:16]    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
[00:05:16] 
[00:05:16] error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
[00:05:16]   --> src/librustc_data_structures/sorted_map.rs:35:19
[00:05:16] 35 |             data: vec![]
[00:05:16]    |                   ^^^^^^
[00:05:16]    |
[00:05:16]    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
---
[00:05:16] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:05:16] expected success, got: exit code: 101
[00:05:16] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:05:16] Build completed unsuccessfully in 0:02:00
[00:05:16] Makefile:28: recipe for target 'all' failed
[00:05:16] make: *** [all] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:01ab4770
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Dec 19 16:16:10 UTC 2018
---
travis_time:end:04163b64:start=1545236170886325549,finish=1545236170892816231,duration=6490682
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:035cd859
$ 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:1d614368
travis_time:start:1d614368
$ 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:00ecb710
$ 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)

[bors]

☔ The latest upstream changes (presumably #56613) made this pull request unmergeable. Please resolve the merge conflicts.

[scottmcm]

I'm all in favour of replacing .len() == 0 with .is_empty(), but it's not a perf fix. LLVM is quite capable of changing a-b==0 into a == b. (And data is a Vec anyway, which implements is_empty() as len() == 0: https://doc.rust-lang.org/src/alloc/vec.rs.html#1184-1186)

[Dylan-DPC-zz]

ping from triage @davidtwco can you review this?
@llogiq you have a few conflicts to attend to.

[davidtwco]

ping from triage @davidtwco can you review this?

I've been checking regularly to see if the conflicts have been resolved before reviewing.

[Dylan-DPC-zz]

Okay. Updated the label to reflect the state.

[llogiq]

This is more trouble than it's worth. Closing.