constified str::from_utf8_unchecked_mut

[fee1-dead]

This change is small so I think it is fine to just have a fcp for this.

[rust-highfive]

r? @scottmcm

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[rust-log-analyzer]

The job x86_64-gnu-llvm-10 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling libc v0.2.93
   Compiling std v0.0.0 (/checkout/library/std)
   Compiling compiler_builtins v0.1.46
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error: const-stable function cannot use `#[feature(const_mut_refs)]`
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188 | pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
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help: if it is not part of the public API, make this function unstably const
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188 | #[rustc_const_unstable(feature = "...", issue = "...")]
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help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
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188 | #[rustc_allow_const_fn_unstable(const_mut_refs)]


error: const-stable function cannot use `#[feature(const_mut_refs)]`
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191 |     unsafe { mem::transmute(v) }
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help: if it is not part of the public API, make this function unstably const
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188 | #[rustc_const_unstable(feature = "...", issue = "...")]
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help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
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188 | #[rustc_allow_const_fn_unstable(const_mut_refs)]


error: const-stable function cannot use `#[feature(const_mut_refs)]`
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191 |     unsafe { mem::transmute(v) }
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help: if it is not part of the public API, make this function unstably const
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188 | #[rustc_const_unstable(feature = "...", issue = "...")]
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help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
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188 | #[rustc_allow_const_fn_unstable(const_mut_refs)]


error: const-stable function cannot use `#[feature(const_mut_refs)]`
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191 |     unsafe { mem::transmute(v) }
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help: if it is not part of the public API, make this function unstably const
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188 | #[rustc_const_unstable(feature = "...", issue = "...")]
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help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
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188 | #[rustc_allow_const_fn_unstable(const_mut_refs)]


error: const-stable function cannot use `#[feature(const_mut_refs)]`
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188 | pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
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help: if it is not part of the public API, make this function unstably const
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188 | #[rustc_const_unstable(feature = "...", issue = "...")]
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help: otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks
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188 | #[rustc_allow_const_fn_unstable(const_mut_refs)]

error: aborting due to 5 previous errors

error: could not compile `core`