Optimize string handling in lit_token().

[nnethercote]

In the common case, the string value in a string literal Token is the
same as the string value in a string literal LitKind. (The exception is
when escapes or \r are involved.) This patch takes advantage of that to
avoid calling str_lit() and re-interning the string in that case. This
speeds up incremental builds for a few of the rustc-benchmarks, the best
by 3%.

Benchmarks that got a speedup of 1% or more:

coercions
        avg: -1.1%      min: -3.5%      max: 0.4%
regex-check
        avg: -1.2%      min: -1.5%      max: -0.6%
futures-check
        avg: -0.9%      min: -1.4%      max: -0.3%
futures
        avg: -0.8%      min: -1.3%      max: -0.3%
futures-opt
        avg: -0.7%      min: -1.2%      max: -0.1%
regex
        avg: -0.5%      min: -1.2%      max: -0.1%
regex-opt
        avg: -0.5%      min: -1.1%      max: -0.1%
hyper-check
        avg: -0.7%      min: -1.0%      max: -0.3%

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[michaelwoerister]

Since we are already micro-optimizing: This will iterate the string twice. Something like the following might be faster:

if s.as_bytes().iter().any(|&c| c == b'\\' || c == b'\r') {

[michaelwoerister]

Looks good to me.

[nnethercote]

I updated to include @michaelwoerister's suggestion.

[michaelwoerister]

@bors r+

Thanks, @nnethercote!

[bors]

📌 Commit 65ea0ff has been approved by michaelwoerister