Auto-Generate Tables in platform-support.md

[qb42]

This is a rough draft inspired by @ChrisDenton in #150501 (comment). I won't have much time to work on it until next month but I wanted to get it out there.

This is implemented as an mdbook preprocessor that pulls data from the rustc_target crate, so it will always be accurate.

Assuming this works as intended (I am not set up to test locally), it's still missing the following:

• Links to target detail pages
• Footnotes

[Noratrieb]

This is very cool! I started a similar effort a while ago in #120745 but never got to completing it. Maybe some of the code in the linked PR thee may be useful

[qb42]

@Noratrieb Thanks, I'll take a look.

[qb42]

Alright, I'm set up properly now so no more failing tidy or pushing obvious mistakes.

Unfortunately it looks like depending on rustc_target directly is a dead-end, I'm getting a variety of errors related to it being a compiler-internal crate that is supposed to be compiled as part of bootstrap. The commit I just pushed (8c1125f) is my last attempt in that direction unless someone else has a solution.

I'm going to switch to pulling data from rustc -Z unstable-options --print all-target-specs-json and see how that goes.

[Noratrieb]

Executing rustc like that seems like the best approach, yes.

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
[TIMING:start] doc::RustcBook> { target: aarch64-unknown-linux-gnu, name: "rustc", src: "/checkout/obj/build/aarch64-unknown-linux-gnu/md-doc/rustc", parent: Some(RustcBook { build_compiler: Compiler { stage: 1, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu, validate: false }), languages: [], build_compiler: None }
Rustbook (aarch64-unknown-linux-gnu) - rustc
 INFO Book building has started
ERROR The command `cargo run --manifest-path ../../../../src/tools/mdbook-rustc/Cargo.toml` wasn't found, is the `rustc-gen-target-tables` preprocessor installed? If you want to ignore this error when the `rustc-gen-target-tables` preprocessor is not installed, set `optional = true` in the `[preprocessor.rustc-gen-target-tables]` section of the book.toml configuration file.
Error: Unable to run the preprocessor `rustc-gen-target-tables`
 Caused By: No such file or directory (os error 2)
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rustbook build /checkout/obj/build/aarch64-unknown-linux-gnu/md-doc/rustc -d /checkout/obj/build/aarch64-unknown-linux-gnu/doc/rustc --rust-root /checkout` failed with exit code 101
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/doc.rs:176:18

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:36:30
  local time: Mon Jan  5 19:30:07 UTC 2026
  network time: Mon, 05 Jan 2026 19:30:07 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[rust-bors]

☔ The latest upstream changes (presumably #151409) made this pull request unmergeable. Please resolve the merge conflicts.