rust-toolchain should allow setting the equivalent of `--profile minimal`

[Arnavion]

Describe the problem you are trying to solve

For things like CI that can make do with the minimal profile but also need additional components, I usually do something like this to install the desired toolchain:

rust_toolchain="$(cat rust-toolchain)"
rustup toolchain install "$rust_toolchain" --profile minimal --component clippy,rustfmt
rustup default "$rust_toolchain"

The new TOML format allows this to be skipped since the first invocation of rustc / cargo will auto-install the toolchain with the required components, except that it does so using the default profile and thus also installs rust-docs.

Describe the solution you'd like

It would be nice if, when rustup installs a new toolchain as a result of a discovered TOML rust-toolchain file, it either defaults to using the minimal profile if components is specified (too late now perhaps due to backward compatibility), or the TOML has a profile field I can set to "minimal", or some other way.

[DJMcNab]

Does running rustup set profile minimal before running any cargo commands (etc) work?

[Arnavion]

Good idea. Yes, that does work.

$ rustup set profile minimal

info: profile set to 'minimal'

$ cd foo/
$ cat rust-toolchain

[toolchain]
channel = "stable"
components = ["clippy", "rustfmt"]

$ rustc -vV

info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: latest update on 2020-11-19, rust version 1.48.0 (7eac88abb 2020-11-16)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: using up to 500.0 MiB of RAM to unpack components
info: installing component 'clippy'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'

rustc 1.48.0 (7eac88abb 2020-11-16)
binary: rustc
commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
commit-date: 2020-11-16
host: x86_64-unknown-linux-gnu
release: 1.48.0
LLVM version: 11.0

@kinnison Do you still want this enhancement?

[Arnavion]

Actually, as part of making a PR to fix it, I've learned that the concept of "profile" is at the global rustup level, not per toolchain.

$ cat ~/.rustup/settings.toml

default_host_triple = "x86_64-unknown-linux-gnu"
default_toolchain = "nightly-x86_64-unknown-linux-gnu"
profile = "minimal"
version = "12"

[overrides]

So this really doesn't make sense in a rust-toolchain file after all. Running rustup set profile or pre-provisioning the ~/.rustup/settings.toml in builders is the right approach.

[DJMcNab]

It probably would make sense to default to minimal profile in a CI setting anyway, although that might be a breaking change :(

[kinnison]

It'd involve modifying DistributableToolchain::install_from_dist() to support having an Option<Profile> passed in to override the config profile (not entirely hard, just laborious to touch all the relevant callsites) and then adding the profile capability to the toml file and into the OverrideCfg - if people are still interested, I can reopen this and do the PR.

[jyn514]

I would still like this change - rustdoc is looking into building out of tree, and it needs neither clippy nor rustfmt, since clippy isn't used at all and rustfmt will be the wrong version (rust-lang/rust#76419 (comment)). It would be nice not to download them if they'll never be used.