smaller PR just to fix #50002

[ExpHP]

I pulled this out of #50010 to make it easier to backport to beta if necessary, considering that inclusive range syntax is stabilizing soon (?).

It fixes a bug in <str>::index_mut with (..=end) ranges (#50002), which prior to this fix was not only unusable but also UB in the cases where it "worked" (it gave improperly truncated UTF-8).

(not that I can imagine why anybody would use <str>::index_mut... but I'm not here to judge)

[rust-highfive]

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.

[00:00:45] configure: rust.quiet-tests     := True
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[01:37:39] failures:
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[01:37:39] ---- str::test_str_slicemut_rangetoinclusive_ok stdout ----
[01:37:39]  thread 'str::test_str_slicemut_rangetoinclusive_ok' panicked at 'byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`', libcore/str/mod.rs:2268:5
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[01:37:39] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--" "--quiet"
[01:37:39] expected success, got: exit code: 101
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[01:37:39] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:37:39] Build completed unsuccessfully in 0:39:30
[01:37:39] make: *** [check] Error 1
[01:37:39] Makefile:58: recipe for target 'check' failed

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[pietroalbini]

Highfive failed to assign a reviewer for this, assigning the same one as the other PR. r? @bluss

[alexcrichton]

@bors: r+ p=1

Thanks @ExpHP and sorry for the delay!

FWIW if you'd like to give similar treatment to all the inclusive range methods here (basically delegate to the other range implementations), that'd be awesome as well!

[bors]

📌 Commit 90b361b has been approved by alexcrichton

[bors]

⌛ Testing commit 90b361b with merge 699eb95...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 699eb95 to master...