Auto merge of #13341 - rimutaka:git-path-doc-9624, r=weihanglo

doc: Edits for git/path dependency sections

Fixes #9624

## Summary

Made changes to [[Specifying dependencies from git repositories](https://doc.rust-lang.org/nightly/cargo/reference/specifying-dependencies.html#specifying-dependencies-from-git-repositories)] and [Specifying path dependencies](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#specifying-path-dependencies) sections of the Reference.

* minor rephrasing for readability
* added more examples
* added sub-headings
* rearranged a few paragraphs for readability

### Additional info

`@weihanglo` proposed a small change in this comment #9624 (comment).
It had been a while since I read that reference page, so I had a novice's point view and expanded the scope of the change to address the questions that I had while reading it.

The combination of _git_, _path_ and _version_ keys was specially confusing until I read the Multiple locations section.

If you think this is too much change for no gain I am happy to reverse the edits and only add the new info as per `@weihanglo` 's comment.

src/doc/src/reference/specifying-dependencies.md

@@ -51,7 +51,9 @@ versions before 1.0.0. While SemVer says there is no compatibility before
 and `x > 0`.
 
 It is possible to further tweak the logic for selecting compatible versions
-using special operators, though it shouldn't be necessary most of the time.
+using special operators as described in the [Version requirement syntax](#version-requirement-syntax) section.
+
+Use the default version requirement strategy, e.g. `log = "1.2.3"` where possible to maximize compatibility.
 
 ## Version requirement syntax
 
@@ -158,16 +160,17 @@ separated with a comma, e.g., `>= 1.2, < 1.5`.
 
 ## Specifying dependencies from other registries
 
-To specify a dependency from a registry other than [crates.io], first the
-registry must be configured in a `.cargo/config.toml` file. See the [registries
-documentation] for more information. In the dependency, set the `registry` key
-to the name of the registry to use.
+To specify a dependency from a registry other than [crates.io] set the `registry` key
+to the name of the registry to use:
 
 ```toml
 [dependencies]
 some-crate = { version = "1.0", registry = "my-registry" }
 ```
 
+where `my-registry` is the registry name configured in `.cargo/config.toml` file.
+See the [registries documentation] for more information.
+
 > **Note**: [crates.io] does not allow packages to be published with
 > dependencies on code published outside of [crates.io].
 
@@ -183,69 +186,113 @@ you need to specify is the location of the repository with the `git` key:
 regex = { git = "https://github.com/rust-lang/regex.git" }
 ```
 
-Cargo will fetch the `git` repository at this location then look for a
-`Cargo.toml` for the requested crate anywhere inside the `git` repository
-(not necessarily at the root --- for example, specifying a member crate name
-of a workspace and setting `git` to the repository containing the workspace).
+Cargo fetches the `git` repository at that location and traverses the file tree to find
+`Cargo.toml` file for the requested crate anywhere inside the `git` repository. 
+For example, `regex-lite` and `regex-syntax` are members of `rust-lang/regex` repo
+and can be referred to by the repo's root URL (`https://github.com/rust-lang/regex.git`)
+regardless of where in the file tree they reside.
+
+```toml
+regex-lite   = { git = "https://github.com/rust-lang/regex.git" }
+regex-syntax = { git = "https://github.com/rust-lang/regex.git" }
+```
+
+The above rule does not apply to [`path` dependencies](#specifying-path-dependencies).
 
-Since we haven’t specified any other information, Cargo assumes that
-we intend to use the latest commit on the default branch to build
-our package, which may not necessarily be the main branch.
-You can combine the `git` key with the `rev`, `tag`, or `branch` keys to
-specify something else. Here's an example of specifying that you want to use
-the latest commit on a branch named `next`:
+### Choice of commit
+
+Cargo assumes that we intend to use the latest commit on the default branch to build
+our package if we only specify the repo URL, as in the examples above.
+
+You can combine the `git` key with the `rev`, `tag`, or `branch` keys to be more specific about
+which commit to use. Here's an example of using the latest commit on a branch named `next`:
 
 ```toml
 [dependencies]
 regex = { git = "https://github.com/rust-lang/regex.git", branch = "next" }
 ```
 
-Anything that is not a branch or tag falls under `rev`. This can be a commit
+Anything that is not a branch or a tag falls under `rev` key. This can be a commit
 hash like `rev = "4c59b707"`, or a named reference exposed by the remote
-repository such as `rev = "refs/pull/493/head"`. What references are available
-varies by where the repo is hosted; GitHub in particular exposes a reference to
-the most recent commit of every pull request as shown, but other git hosts often
-provide something equivalent, possibly under a different naming scheme.
-
-Once a `git` dependency has been added, Cargo will lock that dependency to the
-latest commit at the time. New commits will not be pulled down automatically
-once the lock is in place. However, they can be pulled down manually with
-`cargo update`.
-
-See [Git Authentication] for help with git authentication for private repos.
-
-> **Note**: Neither the `git` key nor the `path` key changes the meaning of the
-> `version` key: the `version` key always implies that the package is available
-> in a registry. `version`, `git`, and `path` keys are considered [separate
-> locations](#multiple-locations) for resolving the dependency.
->
-> When the dependency is retrieved from `git`, the `version` key will _not_
-> affect which commit is used, but the version information in the dependency's
-> `Cargo.toml` file will still be validated against the `version` requirement.
+repository such as `rev = "refs/pull/493/head"`. 
+
+What references are available for the `rev` key varies by where the repo is hosted.  
+GitHub exposes a reference to the most recent commit of every pull request as in the example above.
+Other git hosts may provide something equivalent under a different naming scheme.
+
+**More `git` dependency examples:**
+
+```toml
+# .git suffix can be omitted if the host accepts such URLs - both examples work the same
+regex = { git = "https://github.com/rust-lang/regex" }
+regex = { git = "https://github.com/rust-lang/regex.git" }
+
+# a commit with a particular tag
+regex = { git = "https://github.com/rust-lang/regex.git", tag = "1.10.3" }
+
+# a commit by its SHA1 hash
+regex = { git = "https://github.com/rust-lang/regex.git", rev = "0c0990399270277832fbb5b91a1fa118e6f63dba" }
+
+# HEAD commit of PR 493
+regex = { git = "https://github.com/rust-lang/regex.git", rev = "refs/pull/493/head" }
+
+# INVALID EXAMPLES
+
+# specifying the commit after # ignores the commit ID and generates a warning
+regex = { git = "https://github.com/rust-lang/regex.git#4c59b70" }
+
+# git and path cannot be used at the same time
+regex = { git = "https://github.com/rust-lang/regex.git#4c59b70", path = "../regex" }
+```
+
+Cargo locks the commits of `git` dependencies in `Cargo.lock` file at the time of their addition
+and checks for updates only when you run `cargo update` command.
+
+### The role of the `version` key
+
+The `version` key always implies that the package is available in a registry,
+regardless of the presence of `git` or `path` keys.
+
+The `version` key does _not_ affect which commit is used when Cargo retrieves the `git` dependency,
+but Cargo checks the version information in the dependency's `Cargo.toml` file 
+against the `version` key and raises an error if the check fails.
+
+In this example, Cargo retrieves the HEAD commit of the branch called `next` from Git and checks if the crate's version
+is compatible with `version = "1.10.3"`:
+
+```toml
+[dependencies]
+regex = { version = "1.10.3", git = "https://github.com/rust-lang/regex.git", branch = "next" }
+```
+
+`version`, `git`, and `path` keys are considered separate locations for resolving the dependency. 
+See [Multiple locations](#multiple-locations) section below for detailed explanations.
 
 > **Note**: [crates.io] does not allow packages to be published with
 > dependencies on code published outside of [crates.io] itself
 > ([dev-dependencies] are ignored). See the [Multiple
 > locations](#multiple-locations) section for a fallback alternative for `git`
 > and `path` dependencies.
 
-[Git Authentication]: ../appendix/git-authentication.md
+### Accessing private Git repositories
+
+See [Git Authentication](../appendix/git-authentication.md) for help with Git authentication for private repos.
 
 ## Specifying path dependencies
 
 Over time, our `hello_world` package from [the guide](../guide/index.md) has
 grown significantly in size! It’s gotten to the point that we probably want to
 split out a separate crate for others to use. To do this Cargo supports **path
 dependencies** which are typically sub-crates that live within one repository.
-Let’s start off by making a new crate inside of our `hello_world` package:
+Let’s start by making a new crate inside of our `hello_world` package:
 
 ```console
 # inside of hello_world/
 $ cargo new hello_utils
 ```
 
 This will create a new folder `hello_utils` inside of which a `Cargo.toml` and
-`src` folder are ready to be configured. In order to tell Cargo about this, open
+`src` folder are ready to be configured. To tell Cargo about this, open
 up `hello_world/Cargo.toml` and add `hello_utils` to your dependencies:
 
 ```toml
@@ -254,30 +301,50 @@ hello_utils = { path = "hello_utils" }
 ```
 
 This tells Cargo that we depend on a crate called `hello_utils` which is found
-in the `hello_utils` folder (relative to the `Cargo.toml` it’s written in).
+in the `hello_utils` folder, relative to the `Cargo.toml` file it’s written in.
+
+The next `cargo build` will automatically build `hello_utils` and
+all of its dependencies.
 
-And that’s it! The next `cargo build` will automatically build `hello_utils` and
-all of its own dependencies, and others can also start using the crate as well.
-However, crates that use dependencies specified with only a path are not
-permitted on [crates.io]. If we wanted to publish our `hello_world` crate, we
-would need to publish a version of `hello_utils` to [crates.io]
-and specify its version in the dependencies line as well:
+### No local path traversal
+
+The local paths must point to the exact folder with the dependency's `Cargo.toml`.
+Unlike with `git` dependencies, Cargo does not traverse local paths.
+For example, if `regex-lite` and `regex-syntax` are members of a
+locally cloned `rust-lang/regex` repo, they have to be referred to by the full path:
+
+```toml
+# git key accepts the repo root URL and Cargo traverses the tree to find the crate
+[dependencies]
+regex-lite   = { git = "https://github.com/rust-lang/regex.git" }
+regex-syntax = { git = "https://github.com/rust-lang/regex.git" }
+
+# path key requires the member name to be included in the local path
+[dependencies]
+regex-lite   = { path = "../regex/regex-lite" }
+regex-syntax = { path = "../regex/regex-syntax" }
+```
+
+### Local paths in published crates
+
+Crates that use dependencies specified with only a path are not
+permitted on [crates.io].
+
+If we wanted to publish our `hello_world` crate,
+we would need to publish a version of `hello_utils` to [crates.io] as a separate crate
+and specify its version in the dependencies line of `hello_world`:
 
 ```toml
 [dependencies]
 hello_utils = { path = "hello_utils", version = "0.1.0" }
 ```
 
-> **Note**: Neither the `git` key nor the `path` key changes the meaning of the
-> `version` key: the `version` key always implies that the package is available
-> in a registry. `version`, `git`, and `path` keys are considered [separate
-> locations](#multiple-locations) for resolving the dependency.
+The use of `path` and `version` keys together is explained in the [Multiple locations](#multiple-locations) section.
 
 > **Note**: [crates.io] does not allow packages to be published with
-> dependencies on code published outside of [crates.io] itself
-> ([dev-dependencies] are ignored). See the [Multiple
-> locations](#multiple-locations) section for a fallback alternative for `git`
-> and `path` dependencies.
+> dependencies on code outside of [crates.io], except for [dev-dependencies].
+> See the [Multiple locations](#multiple-locations) section
+> for a fallback alternative for `git` and `path` dependencies.
 
 ## Multiple locations