update README.md on how to install toolchain and components

[goetz-markgraf]

Hello community. This is my first PR, so please bear with me if I do something wrong. I will happily adjust the PR if needed.

I have tried to build rustfmt. Since I did not have the required toolchain and components installed, the build crashed. Thanks to help on discord, I was able to get running.

In this PR, I have added the needed commands to install the toolchain and the components to README.md. This will help newcomers like me, I think.

[ytmimi]

Thanks for working on the docs! I'm guessing cargo didn't automatically install the toolchain and components listed in the rust-toolchain file when you tried to build? Whenever we bump the toolchain, cargo automatically installs everything the next time I run cargo build. I wonder why it didn't do that for you 🤔

[goetz-markgraf]

I don't know. I have not found any setting. But for those, this addition to the readme could help.
If someone knows the settings or config, I'll be very happy to learn.

[ytmimi]

I've left a few suggestions on the docs you wrote. I think linking out to the docs on toolchains and components could be useful for users who might not be too familiar with those terms. Also, I'd prefer not to list a specific nightly version in the docs and would much rather link out to the rust-toolchain file.

[ytmimi]

I'd prefer not to list the content of the rust-toolchain file inline. Instead, I'd rather linked out to the file [`rust-toolchain`](https://github.com/rust-lang/rustfmt/blob/master/rust-toolchain), and let users know that they need to install the exact nightly version of the compiler that rustfmt has pinned in its rust-toolchain file.

[ytmimi]

I don't think this is correct. You should be able to install the nightly toolchain and components from anywhere.

Suggested change

All command should be run in the project's root directory.

[goetz-markgraf]

I have completly re-written the section.

[ytmimi]

Suggested change

	Make sure to have installed the correct toolchain and all components.
	The needed parts can be found in the file `rust-toolchain`. For example:
	rustfmt depends on compiler internals and therefore pins a specific version of the nightly compiler.
	Make sure you've installed the correct nightly [toolchain](https://rust-lang.github.io/rustup/concepts/toolchains.html) and all [components](https://rust-lang.github.io/rustup/concepts/components.html) listed in rustfmt's [`rust-toolchain` file](https://github.com/rust-lang/rustfmt/blob/master/rust-toolchain).

It's probably worth mentioning this information before telling users to build rustfmt with cargo build or mention that they should follow these steps if they're having issues building.

[ytmimi]

Lets point users to the rust-toolchain file to get the exact date:

Suggested change

	```
	rustup install nightly-2024-08-17
	```
	```
	rustup install nightly-YYYY-MM-DD
	```

[goetz-markgraf]

Great Idea, I will update my PR, probably tomorrow. THX for your ideas

[ytmimi]

@goetz-markgraf Thanks for making some changes. I actually preferred your first draft better than what you changed it to. Would you be able to take another go at this and apply some of my suggestions to your first draft?

Also, we strongly discourage merge commits. You should be able to rebase your branch to bring it up to date.

[goetz-markgraf]

The first draft show a way to install toolchain and components, but for rustfmt this way is wrong, as I have learnd.
Since there is a rust-toolchain-file, one should use it and not fiddle with the installation.

My problem was, that I had started nvim directly after cloning the repo. nvim in turn started the LSP rust-analyzer that tried to install the toolchain. That was only party successful. And then I was stuck with a corrupt toolchain.

I have documented what you should do, when you get a corrupt toolchain. And that, to me, is better than trying to patch up the broken installation.

As for you comments, I tried my best to apply then. I have not used any literal name of toolchain but rather your suggestion. And I have added a link to the documentation of rustup that was key to learn about the correct way.

So, I'd rather stick with the second approach.

[ytmimi]

The first draft show a way to install toolchain and components, but for rustfmt this way is wrong, as I have learnd.
Since there is a rust-toolchain-file, one should use it and not fiddle with the installation.

What's wrong with installing components by hand? Also, where did you learn this was wrong?

[goetz-markgraf]

Well. All my previous efforts did not resolve the root cause, the corrupt toolchain. In an attempt to solve the compile error I had overridden the toolchain with rustup override set nightly and so the rust-toolchain file was completely ignored.

The new approach leads to the toolchain correctly recognizing the rust-toolchain file as intended by the authors of the repo. So I think, setting up the configuration such that rustup and cargo can do their thing instead of fiddeling manually with the toolchain is the right approach.