embuild fails to compile with Rust 1.57 or older

If embuild is compiled with:

Rust toolchain version 1.57 or older
the espidf feature enabled

then compilation fails with:

% cargo build --features espidf
# .. snip
   Compiling embuild v0.29.1 (/Users/kevin/src/embuild)
error: there is no argument named `IDF_PATH_VAR`
   --> src/espidf.rs:140:65
    |
140 |             FromEnvError::NoRepo(anyhow!("environment variable `{IDF_PATH_VAR}` not found"))
    |                                                                 ^^^^^^^^^^^^^^

error: there is no argument named `IDF_PATH_VAR`
   --> src/espidf.rs:177:36
    |
177 |                          given by ${IDF_PATH_VAR} ('{}')",
    |                                    ^^^^^^^^^^^^^^

error: there is no argument named `v`
   --> src/espidf.rs:262:32
    |
262 |             Ok(v) => format!("v{v}"),
    |                                ^^^

error: there is no argument named `MANAGED_ESP_IDF_REPOS_DIR_BASE`
   --> src/espidf.rs:321:22
    |
321 |             format!("{MANAGED_ESP_IDF_REPOS_DIR_BASE}-{hash}")
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: there is no argument named `hash`
   --> src/espidf.rs:321:55
    |
321 |             format!("{MANAGED_ESP_IDF_REPOS_DIR_BASE}-{hash}")
    |                                                       ^^^^^^

error: there is no argument named `IDF_PYTHON_ENV_PATH_VAR`
   --> src/espidf.rs:482:58
    |
482 |                     let python_env_var_prefix = format!("{IDF_PYTHON_ENV_PATH_VAR}=");
    |                                                          ^^^^^^^^^^^^^^^^^^^^^^^^^

error: there is no argument named `IDF_PYTHON_ENV_PATH_VAR`
   --> src/espidf.rs:490:72
    |
490 | ...                   "`idf_tools.py export` result contains no `{IDF_PYTHON_ENV_PATH_VAR}` item \
    |                                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: expected a `Fn<(char,)>` closure, found `[char; 2]`
   --> src/espidf.rs:375:45
    |
375 |         let mut ref_name = ref_name.replace(&['/', '\\'], "-");
    |                                     ------- ^^^^^^^^^^^^ expected an `Fn<(char,)>` closure, found `[char; 2]`
    |                                     |
    |                                     required by a bound introduced by this call
    |
    = help: the trait `Fn<(char,)>` is not implemented for `[char; 2]`
    = note: required because of the requirements on the impl of `FnOnce<(char,)>` for `&[char; 2]`
    = note: required because of the requirements on the impl of `Pattern<'_>` for `&[char; 2]`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `embuild` due to 8 previous errors

This is because embuild uses captured identifiers in format strings, and support for this was added in Rust 1.58.

Ideally compilation would require the correct toolchain or fail early and with a clear error message.

I am aware of two approaches to improve this:

Cargo.toml's rust-version field
The channel field of a rust-toolchain file

I've added a PR for the rust-version field here: #45.

esp-rs / embuild

embuild fails to compile with Rust 1.57 or older #46