optimize redundant borrows and escaping paths in NLL

[nikomatsakis]

This builds on #53168 and adds a commit that addresses #53176 -- or at least I think it does. I marked this as WIP because I want to see the test results (and measure the performance). I also want to double check we're not adding in any unsoundness here.

[rust-highfive]

r? @eddyb

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[nikomatsakis]

@bors try

[bors]

⌛ Trying commit 31fe3df6543566f5671da4d78b381b9d59f2d096 with merge a7a1e7a9bda496d4f9f0fbd27d7cd126b18e8047...

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[nikomatsakis]

@rust-timer build a7a1e7a9bda496d4f9f0fbd27d7cd126b18e8047

[rust-timer]

Success: Queued a7a1e7a9bda496d4f9f0fbd27d7cd126b18e8047 with parent ccb550f, comparison URL.

[kennytm]

Perf is ready.

[BurntPizza]

*Clicks link*
*Falls out of chair*

[nikomatsakis]

So yeah, seems like a win. =)

[nikomatsakis]

So I pushed a comment that sort of convinces me that this logic is correct.

[nikomatsakis]

Removing [WIP] and I've closed #53168 in favor of this PR, since it does more.

[nikomatsakis]

r? @pnkfelix

[kennytm]

@bors retry #40474.

[bors]

⌛ Testing commit ff7f6d5 with merge 2c92308...

[bors]

💔 Test failed - status-travis

[rust-highfive]

The job dist-powerpc-linux 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.

###################################################################       94.2%
######################################################################## 100.0%
[00:07:12] extracting /checkout/obj/build/cache/2018-08-01/cargo-beta-x86_64-unknown-linux-gnu.tar.gz
[00:07:14]     Updating registry `https://github.com/rust-lang/crates.io-index`
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

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)

[kennytm]

@bors retry #40474

[bors]

⌛ Testing commit ff7f6d5 with merge 19739a77689d041e02ef7b19be5730425676a165...

[bors]

💔 Test failed - status-travis

[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.

  Downloading https://files.pythonhosted.org/packages/d1/a1/7790cc85db38daa874f6a2e6308131b9953feb1367f2ae2d1123bb93a9f5/pyasn1-0.4.4-py2.py3-none-any.whl (72kB)
Collecting futures<4.0.0,>=2.2.0; python_version == "2.6" or python_version == "2.7" (from s3transfer<0.2.0,>=0.1.12->awscli)
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
  Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'ReadTimeoutError("HTTPSConnectionPool(host='pypi.python.org', port=443): Read timed out. (read timeout=15)",)': /simple/futures/
  InsecurePlatformWarning
  Downloading https://files.pythonhosted.org/packages/2d/99/b2c4e9d5a30f6471e410a146232b4118e697fa3ffc06d6a65efde84debd0/futures-3.2.0-py2-none-any.whl
Collecting jmespath<1.0.0,>=0.7.1 (from botocore==1.10.74->awscli)
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
---
########################                                                  34.0%
######################################################################## 100.0%
[00:06:44] extracting /checkout/obj/build/cache/2018-08-01/cargo-beta-x86_64-unknown-linux-gnu.tar.gz
[00:06:44]     Updating registry `https://github.com/rust-lang/crates.io-index`
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

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)

[kennytm]

@bors retry https://www.traviscistatus.com/incidents/pxj3py3xzn14

[bors]

⌛ Testing commit ff7f6d5 with merge 0aa8d03...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing 0aa8d03 to master...