Replace nbsp in all rustdoc code blocks

[dtolnay]

Fixes #106098. This makes code copyable from rustdoc rendered documentation into a Rust source file.

Tested by building the docs of syn and scanning for any missing indentation.

white-space: pre makes consecutive spaces not collapse into one, and makes linebreaks not get inserted, exactly what   was doing.

[rustbot]

r? @notriddle

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[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @Folyd, @jsha

[notriddle]

Please run ./x test --bless src/test/rustdoc to update the rustdoc test HTML samples, and also fix the indentation in rustdoc.css to use tabs instead of spaces.

[est31]

cc @hsivonen

[notriddle]

Is this what we want?

With the old setup, code headers were allowed to wrap, and there are GUI tests for it. in the notable traits popup.

[GuillaumeGomez]

They are supposed to indeed.

[rust-log-analyzer]

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

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

  IMAGE: x86_64-gnu-tools
##[endgroup]
From https://github.com/rust-lang/rust
 * branch              master     -> FETCH_HEAD
Searching for toolstate changes between f5c3dfdbbf06d5997079ac7339de5002f7ced2a3 and 52d1dfac2e7b9eb15d8adf7b0a2ddfeb1fff2501
Rustdoc was updated
##[group]Run src/ci/scripts/verify-channel.sh
src/ci/scripts/verify-channel.sh
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
---
.......... (90/95)
.....      (95/95)


/checkout/src/test/rustdoc-gui/notable-trait.goml notable-trait... FAILED
[ERROR] (line 48) Error: Evaluation failed: assert didn't fail: for command `compare-elements-position-false: (
    "//*[@id='method.create_an_iterator_from_read']//a[text()='NotableStructWithLongName']",
    "//*[@id='method.create_an_iterator_from_read']//*[@class='notable-traits']",
    ("y", "x"),
)`
[ERROR] (line 69) Error: Evaluation failed: The following errors happened: [different X values: 677.03125 (or 677) != 245]: for command `assert-position: (
    "//*[@id='method.create_an_iterator_from_read']//a[text()='NotableStructWithLongName']",
    {"x": 245},
)`
[ERROR] (line 73) Error: Evaluation failed: The following errors happened: [different X values: 955.453125 (or 955) != 523]: for command `assert-position: (
    "//*[@id='method.create_an_iterator_from_read']//*[@class='notable-traits']",
    {"x": 523},
)`
[ERROR] (line 93) Error: Evaluation failed: The following errors happened: [different X values: 447.03125 (or 447) != 15]: for command `assert-position: (
    "//*[@id='method.create_an_iterator_from_read']//a[text()='NotableStructWithLongName']",
    {"x": 15},
)`
[ERROR] (line 97) Error: Evaluation failed: The following errors happened: [different X values: 725.453125 (or 725) != 293]: for command `assert-position: (
    "//*[@id='method.create_an_iterator_from_read']//*[@class='notable-traits']",
    {"x": 293},
)`
[ERROR] (line 102) Error: Node is either not clickable or not an HTMLElement: for command `click: "//*[@id='method.create_an_iterator_from_read']//*[@class='notable-traits']"`
Build completed unsuccessfully in 0:02:31

[dtolnay]

I'll leave it to someone else to work out what to do about notable traits popup.

Copying code from those is a less common thing so maybe it's not so bad for notable traits popups to continue using nbsp. I still think the non-popup rendered APIs should never use nbsp.