rust/treefile: Include filename in more error msgs #1735
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Slightly prep for next patch (was in the area so just did the whole module).
This uses the `Context` feature of the failure crate to make error messages more useful when we fail to open a file. The difference with `map_err` is that one can still obtain the underlying error from the context if need be. Though surprisingly, the normal `Display` for a `Context` doesn't include the original error, so we essentially have to do a prefix here (see [1]). Before: ``` error: Failed to load YAML treefile: No such file or directory (os error 2) ``` After: ``` error: Failed to load YAML treefile: Can't open file "treecompose-post.sh": No such file or directory (os error 2) ``` [1] rust-lang-deprecated/failure#182
cgwalters
reviewed
Jan 21, 2019
| Err(ref e) if e.kind() == io::ErrorKind::InvalidInput => {} | ||
| Err(ref e) => panic!("Expected invalid treefile, not {}", e.to_string()), | ||
| Err(ref e) => { | ||
| match e.downcast_ref::<io::Error>() { |
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Interesting. I need to learn more about the TypeId stuff, I guess it boils down to a unique id for a vtable or so?
jlebon
commented
Jan 21, 2019
| /// Open file and provide context containing filename on failures. | ||
| fn open_file<P: AsRef<Path>>(filename: P) -> Fallible<fs::File> { | ||
| return Ok(fs::File::open(filename.as_ref()).with_context( | ||
| |e| format!("Can't open file {:?}: {}", filename.as_ref().display(), e))?); |
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@lucab was asking why we're doing Ok(...?) here instead of just .... I think what's happening there is that we're relying on the .into() behaviour of ? in the case where opening the file fails. Doing just:
return fs::File::open(filename.as_ref()).with_context(
|e| format!("Can't open file {:?}: {}", filename.as_ref().display(), e));fails with:
error[E0308]: mismatched types
--> src/treefile.rs:145:12
|
145 | return fs::File::open(filename.as_ref()).with_context(
| ____________^
146 | | |e| format!("Can't open file {:?}: {}", filename.as_ref().display(), e));
| |________________________________________________________________________________^ expected struct `failure::Error`, found struct `failure::Context`
|
= note: expected type `std::result::Result<_, failure::Error>`
found type `std::result::Result<_, failure::Context<std::string::String>>`Similarly, doing .into() at the end will also fail because there's no conversion implementation from Result<T, E> to Result<T, failure::Error>. Whereas failure::Context<T> can be converted to failure::Error via the blanket Fail implementation.
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I feel like there's probably a .map_err() way to do this that would be more elegant but not worth redoing!
|
⚡ Test exempted: merge already tested. |
rh-atomic-bot
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This uses the `Context` feature of the failure crate to make error messages more useful when we fail to open a file. The difference with `map_err` is that one can still obtain the underlying error from the context if need be. Though surprisingly, the normal `Display` for a `Context` doesn't include the original error, so we essentially have to do a prefix here (see [1]). Before: ``` error: Failed to load YAML treefile: No such file or directory (os error 2) ``` After: ``` error: Failed to load YAML treefile: Can't open file "treecompose-post.sh": No such file or directory (os error 2) ``` [1] rust-lang-deprecated/failure#182 Closes: #1735 Approved by: cgwalters
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This uses the
Contextfeature of the failure crate to make errormessages more useful when we fail to open a file. The difference with
map_erris that one can still obtain the underlying error from thecontext if need be. Though surprisingly, the normal
Displayfor aContextdoesn't include the original error, so we essentially have todo a prefix here (see [1]).
Before:
After:
[1] rust-lang-deprecated/failure#182