Make the unused_mut lint smarter

[arielb1]

This makes it somewhat more aggressive, so this is kind-of a [breaking-change] for these compiling with #[deny(unused_mut)].

r? @pnkfelix

[bors]

☔ The latest upstream changes (presumably #24527) made this pull request unmergeable. Please resolve the merge conflicts.

[arielb1]

bump

[pnkfelix]

(I don't know if I agree that #26332 is actually a bug ... that is, I don't know if I like the idea of flagging that sort of code with a warning... but I'm willing to assume that it is for the purposes of completing this review)

[pnkfelix]

cc @rust-lang/lang

[pnkfelix]

(The cleanup commit looks fine to me; I am just hesistant to r-plus this PR without double-checking that others agree that this change to the lint in question is desirable.)

[nrc]

f+ on the principle here - it sounds like it is making the lint better, and I don't think lints count as a breaking change

[pnkfelix]

@nrc okay, my main concern was that someone who writes:

letmut x;
...x = 3;

may intend, in the long term, to add additional statements that assign to x, and this lint will be annoying for that case.

But I am willing to believe that the change here will help more than it hurts. Especially if it helps new-comers understand the idea that the "delayed single-assignment style" like above is not considered mutation by rustc.

[pnkfelix]

@bors r+ a18d984

[bors]

⌛ Testing commit a18d984 with merge 73165be...

[bors]

💔 Test failed - auto-mac-64-nopt-t

[alexcrichton]

@bors: retry

On Fri, Jul 3, 2015 at 6:59 AM, bors notifications@github.com wrote:

[image: 💔] Test failed - auto-mac-64-nopt-t
http://buildbot.rust-lang.org/builders/auto-mac-64-nopt-t/builds/5606

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#26378 (comment).

[bors]

⌛ Testing commit a18d984 with merge 728af15...

[bors]

💔 Test failed - auto-mac-64-nopt-t

[alexcrichton]

@bors: retry

On Fri, Jul 3, 2015 at 12:59 PM, bors notifications@github.com wrote:

[image: 💔] Test failed - auto-mac-64-nopt-t
http://buildbot.rust-lang.org/builders/auto-mac-64-nopt-t/builds/5609

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#26378 (comment).

[bors]

⌛ Testing commit a18d984 with merge f027bdc...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt