Auto merge of #50052 - nnethercote:char_lit, r=Mark-Simulacrum
Avoid allocating when parsing \u{...} literals.
`char_lit` uses an allocation in order to ignore '_' chars in \u{...}
literals. This patch changes it to not do that by processing the chars
more directly.
This improves various rustc-perf benchmark measurements by up to 6%,
particularly regex, futures, clap, coercions, hyper, and encoding.
rustc-perf results, on a stage 2 build with jemalloc disabled:
<details>
```
regex-check
avg: -5.4% min: -6.5% max: -2.7%
futures-check
avg: -3.5% min: -5.3% max: -1.7%
regex-opt
avg: -2.0% min: -5.1% max: -0.2%
regex
avg: -2.3% min: -5.0% max: -0.6%
futures-opt
avg: -3.0% min: -4.8% max: -1.1%
futures
avg: -3.1% min: -4.8% max: -1.3%
clap-rs-check
avg: -1.8% min: -3.5% max: -0.9%
coercions-check
avg: -2.0% min: -3.3% max: -1.0%
hyper-check
avg: -2.2% min: -3.1% max: -1.3%
hyper
avg: -1.3% min: -2.4% max: -0.3%
hyper-opt
avg: -0.9% min: -2.3% max: -0.1%
coercions
avg: -1.1% min: -2.2% max: -0.4%
encoding-check
avg: -1.7% min: -2.2% max: -0.9%
clap-rs-opt
avg: -0.7% min: -2.2% max: 0.0%
coercions-opt
avg: -1.2% min: -2.1% max: -0.3%
clap-rs
avg: -0.8% min: -1.9% max: -0.4%
encoding-opt
avg: -1.0% min: -1.9% max: -0.3%
encoding
avg: -1.1% min: -1.9% max: -0.4%
piston-image-check
avg: -0.7% min: -1.3% max: -0.3%
inflate-opt
avg: -0.3% min: -0.9% max: -0.0%
piston-image
avg: -0.3% min: -0.8% max: -0.1%
piston-image-opt
avg: -0.3% min: -0.7% max: -0.1%
syn-check
avg: -0.3% min: -0.6% max: -0.1%
deep-vector
avg: 0.1% min: -0.1% max: 0.5%
syn-opt
avg: -0.1% min: -0.4% max: 0.0%
html5ever
avg: -0.2% min: -0.4% max: -0.0%
deep-vector-check
avg: 0.0% min: -0.3% max: 0.3%
syn
avg: -0.2% min: -0.3% max: -0.1%
html5ever-check
avg: -0.3% min: -0.3% max: -0.2%
issue-46449-check
avg: -0.1% min: -0.2% max: 0.2%
html5ever-opt
avg: -0.0% min: -0.2% max: 0.1%
deep-vector-opt
avg: -0.0% min: -0.2% max: 0.1%
issue-46449-opt
avg: -0.0% min: -0.2% max: 0.1%
unify-linearly-check
avg: -0.0% min: -0.2% max: 0.1%
helloworld-check
avg: 0.0% min: -0.0% max: 0.2%
parser-check
avg: -0.0% min: -0.2% max: 0.0%
inflate
avg: 0.0% min: -0.0% max: 0.1%
tokio-webpush-simple-check
avg: -0.1% min: -0.1% max: -0.0%
regression-31157-check
avg: 0.0% min: -0.1% max: 0.1%
issue-46449
avg: 0.0% min: -0.1% max: 0.1%
tuple-stress-opt
avg: 0.0% min: -0.0% max: 0.1%
tuple-stress-check
avg: -0.0% min: -0.1% max: 0.1%
tuple-stress
avg: 0.0% min: -0.0% max: 0.1%
deeply-nested-check
avg: 0.0% min: -0.0% max: 0.1%
regression-31157
avg: -0.0% min: -0.1% max: 0.1%
deeply-nested-opt
avg: -0.0% min: -0.1% max: 0.1%
parser-opt
avg: -0.0% min: -0.1% max: 0.0%
parser
avg: 0.1% min: 0.0% max: 0.1%
tokio-webpush-simple
avg: -0.0% min: -0.1% max: 0.1%
regression-31157-opt
avg: -0.0% min: -0.1% max: 0.1%
helloworld-opt
avg: 0.0% min: -0.0% max: 0.1%
unify-linearly-opt
avg: 0.0% min: -0.0% max: 0.1%
unused-warnings-check
avg: 0.0% min: 0.0% max: 0.1%
tokio-webpush-simple-opt
avg: -0.0% min: -0.1% max: 0.0%
helloworld
avg: -0.0% min: -0.0% max: 0.1%
unused-warnings
avg: 0.0% min: -0.0% max: 0.0%
deeply-nested
avg: -0.0% min: -0.0% max: -0.0%
unused-warnings-opt
avg: 0.0% min: -0.0% max: 0.0%
unify-linearly
avg: 0.0% min: -0.0% max: 0.0%
inflate-check
avg: 0.0% min: -0.0% max: 0.0%
```
</details>
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