remove "approx env bounds" if we already know from trait

[nikomatsakis]

Alternative to #55988 that fixes #55756 -- smaller fix that I cannot see having (correctness) repercussions beyond the test at hand, and hence better for backporting. (Famous last words, I know.)

r? @eddyb

[rust-highfive]

The job x86_64-gnu-llvm-5.0 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:03fcf380:start=1542554063353629564,finish=1542554118398878364,duration=55045248800
$ 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-5.0
---
[00:50:11] .................................................................................................... 3000/5023
[00:50:14] ..........................................i......................................................... 3100/5023
[00:50:18] .................................................................................................... 3200/5023
[00:50:21] .....i.i..ii........................................................................................ 3300/5023
[00:50:25] ...............................................................................................F.... 3400/5023
[00:50:28] ...................................................................................i.ii............. 3500/5023
[00:50:32] ...i................................................................................................ 3700/5023
[00:50:33] ...........................................................i........................................ 3800/5023
[00:50:35] .................................................................................................... 3900/5023
[00:50:39] .................................................................................................... 4000/5023
---
[00:51:13] ---- [ui] ui/nll/ty-outlives/issue-55756.rs stdout ----
[00:51:13] 
[00:51:13] error: ui test compiled successfully!
[00:51:13] status: exit code: 0
[00:51:13] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/nll/ty-outlives/issue-55756.rs" "--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/nll/ty-outlives/issue-55756/a" "-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/nll/ty-outlives/issue-55756/auxiliary" "-A" "unused"
[00:51:13] ------------------------------------------
[00:51:13] 
[00:51:13] ------------------------------------------
[00:51:13] stderr:
---
[00:51:13] failures:
[00:51:13]     [ui] ui/nll/ty-outlives/issue-55756.rs
[00:51:13] 
[
[00:51:13] Makefile:58: recipe for target 'check' failed
[00:51:13] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:25e9e218
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Sun Nov 18 16:06:41 UTC 2018
---
travis_time:end:0a797714:start=1542557202309422864,finish=1542557202315345732,duration=5922868
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0768f479
$ 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_fai

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)

[eddyb]

@bors r+

[bors]

📌 Commit a5b4cb2 has been approved by eddyb