Cannot merge env vars in `[env]` due to type mismatches

[weihanglo]

Problem

Cargo doesn't know how to merge toml strings and tables for the [env] table.

Steps

mkdir outer
cd outer
mkdir .cargo
echo -e "[env]\nFOO = { value = '0' }" > .cargo/config.toml

cargo new inner
cd inner
mkdir .cargo
echo -e "[env]\nFOO = '1'" > .cargo/config.toml
cargo b

And you will see the error.

error: could not load Cargo configuration

Caused by:
  failed to merge configuration at `/home/user/inner/.cargo/config.toml`

Caused by:
  failed to merge key `env` between /home/user/inner/outer/.cargo/config.toml and /home/user/inner/.cargo/config.toml

Caused by:
  failed to merge key `FOO` between /home/user/inner/outer/.cargo/config.toml and /home/user/inner/.cargo/config.toml

Caused by:
  failed to merge config value from `/home/user/inner/.cargo/config.toml` into `/home/user/dirty/inner/outer/.cargo/config.toml`: expected string, but found table

Possible Solution(s)

According to The Cargo Book, there is no merge strategy for TOML table.

If a key is specified in multiple config files, the values will get merged together. Numbers, strings, and booleans will use the value in the deeper config directory taking precedence over ancestor directories, where the home directory is the lowest priority. Arrays will be joined together with higher precedence items being placed later in the merged array.

For what I can recall, it is because TOML table merge is handled case by case. I think it makes sense for the [env] table to just replace one with the other, regardless of string or table.

Notes

No response

Version

* cargo 1.83.0 (5ffbef321 2024-10-29)
* cargo 1.85.0-nightly (20a443231 2024-12-06)

[arlosi]

I think it makes sense for the [env] table to just replace one with the other, regardless of string or table.

When merging, cargo doesn't know that it's the [env] table. The merging is done early, so whatever merging strategy needs to be done consistently for all config. I'm not sure what other examples we have where a config value could be legally changed from a table / list to something else. It might be good to enumerate them all.

cargo/src/cargo/util/context/mod.rs

Lines 2262 to 2266 in 58b2d60

	// Allow switching types except for tables or arrays.
	(expected @ &mut CV::List(_, _), found)
	| (expected @ &mut CV::Table(_, _), found)
	| (expected, found @ CV::List(_, _))
	| (expected, found @ CV::Table(_, _)) => {

[weihanglo]

By reading the configuration doc, here are the config value that cannot switch types from table/array.

• [env.key] — accept either a string or a table of {value = string, force = bool}.
• build.target — accept either a string or an array of strings
• rustflags family — accept either a string or an array of strings.
  • build.rustflags/rustdocflags
  • target.<cfg>.rustflags...
• http.ssl-version — accept either a string of a {max = string, min = string} table.

It may make sense that Cargo just doesn't merge them at all, and leave for users to manually resolve conflicts. Users always have the option to use the more verbose form to fix the type conflicts.