On Ubuntu Linux, this is what I did:

~> rustc --version
rustc 1.85.1 (4eb161250 2025-03-15) (built from a source tarball)

I then installed rustup:

~> rustup --version
rustup 1.27.1 (2025-03-14)

~> sudo rustup update stable
[sudo: authenticate] Password:
info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: latest update on 2025-12-11, rust version 1.92.0 (ded5c06cf 2025-12-08)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
74.3 MiB / 74.3 MiB (100 %) 54.1 MiB/s in 1s ETA: 0s
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
info: installing component 'rust-std'
info: installing component 'rustc'
74.3 MiB / 74.3 MiB (100 %) 32.7 MiB/s in 2s ETA: 0s
info: installing component 'rustfmt'

stable-x86_64-unknown-linux-gnu installed - rustc 1.92.0 (ded5c06cf 2025-12-08)

info: default toolchain set to 'stable-x86_64-unknown-linux-gnu'
info: self-update is disabled for this build of rustup
info: any updates to rustup will need to be fetched with your system package manager
~>
~> rustup --version
rustup 1.27.1 (2025-03-14)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
~>
~> rustc --version
error: rustup could not choose a version of rustc to run, because one wasn't specified explicitly, and no default is configured.
help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain.

~> sudo rustup default stable
[sudo: authenticate] Password:
info: using existing install for 'stable-x86_64-unknown-linux-gnu'
info: default toolchain set to 'stable-x86_64-unknown-linux-gnu'

stable-x86_64-unknown-linux-gnu unchanged - rustc 1.92.0 (ded5c06cf 2025-12-08)

~> rustc --version
error: rustup could not choose a version of rustc to run, because one wasn't specified explicitly, and no default is configured.
help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain.

~> ls -a ./.cargo/
. .. registry .global-cache .package-cache .package-cache-mutate
~> which rustc
/usr/bin/rustc
~>
~> type rustc
rustc is /usr/bin/rustc
~>
~> locate -b rustc
/home/pietrom/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rustc_version-0.4.1
/home/pietrom/.cargo/registry/src/index.crates.io-6f17d22bba15001f/autocfg-1.5.0/src/rustc.rs
… /usr/bin/rustc
/usr/bin/rustc-1.81
/usr/bin/rustc-1.84
/usr/bin/rustc-1.85
…
/usr/lib/rust-1.85/bin/rustc
/usr/lib/rust-1.85/lib/librustc_driver-498d2b6d02538b82.so
/usr/lib/rust-1.85/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-9ff52a53bbba3a7a.rlib
…
/usr/lib/x86_64-linux-gnu/librustc_driver-f660ce0928bc5f8b.so
/usr/share/rustc
/usr/share/rustc-1.81
/usr/share/rustc-1.84
/usr/share/rustc-1.85
/usr/share/cargo/registry/rustc-demangle-0.1.24
…
/usr/share/doc/librust-compiler-builtins+rustc-dep-of-std-dev

What is wrong? Why the latest Rust version (rustc 1.92.0) is not installed, and why I cannot use an old version by default using rustc?

Thank you for your help.

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