Add line number to error messages about parsing Cargo.toml

[andresovela]

Problem

When there's a problem with Cargo.toml, cargo outputs an error but it does not mention where the error is coming from. For example:

$ cargo build
error: failed to parse manifest at `path/to/Cargo.toml`

Caused by:
  error inheriting `$crate` from workspace root manifest's `workspace.dependencies.$crate`

Caused by:
  failed to find a workspace root

Proposed Solution

It'd be nice if cargo mentioned which is the offending line in the manifest. For example:

$ cargo build
error: failed to parse manifest at `path/to/Cargo.toml:24`

Caused by:
$crate = { workspace = true } 

Caused by:
  error inheriting `$crate` from workspace root manifest's `workspace.dependencies.$crate`

Caused by:
  failed to find a workspace root

Notes

No response

[epage]

So with Rust 1.74, more errors will include span information (#12556).

We also have #12810 about adding span information. Closing in favor of that. If there is a reason to keep both open, let us know!