{"header":{"message":"Document the interaction of context and downcast\n","description":null,"timestamp":"2019-10-28T03:57:27Z","authors":[{"email":"dtolnay@gmail.com","name":"David Tolnay"}]},"hash":"RF4MODRCRWKMUG26O222X3ZY3MLE6X7BYZDWEQRLVUITTWYAGNUAC","authors":[],"deps":{"hashes":[["RF4MODRCRWKMUG26O222X3ZY3MLE6X7BYZDWEQRLVUITTWYAGNUAC","Document the interaction of context and downcast\n"],["AA",null],["AOQSHDBY3FLBJBFEIJQDYHGQOBECTYRN3KPWJWQMCVWDTB6L2IRQC","Work around bad diagnostic when missing Context import\n\nPreviously the compiler would say:\n\n error[E0599]: no method named `context` found for type `std::result::Result<(), anyhow::error::Error>` in the current scope\n --> src/main.rs:83:13\n |\n 83 | context(...)?;\n | ^^^^^^^ method not found in `std::result::Result<(), anyhow::error::Error>`\n |\n = help: items from traits can only be used if the trait is in scope\n help: the following trait is implemented but not in scope; perhaps add a `use` for it:\n |\n 19 | use anyhow::context::Context;\n\nwhere the `anyhow::context::Context` in the suggestion is not a path\nthat is accessible to the user.\n"],["DI4CW2M46SOKHZXATOP6HRLZB3A33FCWRFDQBIJHJY2BEDB537NQC","Add Error::root_cause accessor\n"],["CITEDKPB6MKVZUEYEDE5ZKTNVY35HCOAXKDPYG7YLLEOVFNMSRXQC","Import implementation from fehler 1.0.0-alpha.1\n"],["W655S25G7CGI4XOZ2PKITKTH26VJ4QMZFRV2RPYNI4KIUJOIGVMAC","Change all remaining exception related wording\n"],["EXRAFG37562NH775A4LDPP5FNKXLPLIZSJ3FCOBINUDKRFBJHCXAC","Move trait bounds to where-clauses\n\nI find it more readable how this appears in rustdoc.\n"],["GZZOJ7ZUSBPI3HXIV5UCFNMCDAMZPPLISY2UDV73FDN74HZ3AWJAC","Format with rustfmt 2019-09-08\n"]],"deps":["AOQSHDBY3FLBJBFEIJQDYHGQOBECTYRN3KPWJWQMCVWDTB6L2IRQC"],"known":["CITEDKPB6MKVZUEYEDE5ZKTNVY35HCOAXKDPYG7YLLEOVFNMSRXQC","GZZOJ7ZUSBPI3HXIV5UCFNMCDAMZPPLISY2UDV73FDN74HZ3AWJAC","W655S25G7CGI4XOZ2PKITKTH26VJ4QMZFRV2RPYNI4KIUJOIGVMAC","EXRAFG37562NH775A4LDPP5FNKXLPLIZSJ3FCOBINUDKRFBJHCXAC","DI4CW2M46SOKHZXATOP6HRLZB3A33FCWRFDQBIJHJY2BEDB537NQC"]},"hunks":[{"Replacement":{"inode":{"change":3,"pos":3238},"path":"src/error.rs","line":258,"ins":{"up_context":[{"change":3,"pos":6242}],"down_context":[{"change":3,"pos":818}],"flag":"BLOCK","start":0,"end":417,"inode":{"change":3,"pos":3238}},"del":{"edges":[{"previous":"BLOCK","flag":"BLOCK | DELETED","from":{"change":3,"pos":6242},"to":{"change":3,"start":391,"end":463},"introduced_by":3}],"inode":{"change":3,"pos":3238}},"ins_contents":[{"Add":" /// Returns true if `E` is the type held by this error object."},{"Add":" ///"},{"Add":" /// For errors with context, this method returns true if `E` matches the"},{"Add":" /// type of the context `C` **or** the type of the error on which the"},{"Add":" /// context has been attached. For details about the interaction between"},{"Add":" /// context and downcasting, [see here]."},{"Add":" ///"},{"Add":" /// [see here]: trait.Context.html#effect-on-downcasting"}],"del_contents":[{"Del":" /// Returns `true` if `E` is the type wrapped by this error object.\n"}]}},{"Edit":{"inode":{"change":3,"pos":11888},"path":"src/lib.rs","line":288,"atom":{"NewVertex":{"up_context":[{"change":2,"pos":222}],"down_context":[{"change":2,"pos":222}],"flag":"BLOCK","start":418,"end":431,"inode":{"change":3,"pos":11888}}},"contents":[{"Add":"///"},{"Add":"///
"}]}},{"Edit":{"inode":{"change":3,"pos":11888},"path":"src/lib.rs","line":331,"atom":{"NewVertex":{"up_context":[{"change":2,"pos":1263}],"down_context":[{"change":2,"pos":1263}],"flag":"BLOCK","start":432,"end":3492,"inode":{"change":3,"pos":11888}}},"contents":[{"Add":"///"},{"Add":"///
"},{"Add":"///"},{"Add":"/// # Effect on downcasting"},{"Add":"///"},{"Add":"/// After attaching context of type `C` onto an error of type `E`, the resulting"},{"Add":"/// `anyhow::Error` may be downcast to `C` **or** to `E`."},{"Add":"///"},{"Add":"/// That is, in codebases that rely on downcasting, Anyhow's context supports"},{"Add":"/// both of the following use cases:"},{"Add":"///"},{"Add":"/// - **Attaching context whose type is insignificant onto errors whose type"},{"Add":"/// is used in downcasts.**"},{"Add":"///"},{"Add":"/// In other error libraries whose context is not designed this way, it can"},{"Add":"/// be risky to introduce context to existing code because new context might"},{"Add":"/// break existing working downcasts. In Anyhow, any downcast that worked"},{"Add":"/// before adding context will continue to work after you add a context, so"},{"Add":"/// you should freely add human-readable context to errors wherever it would"},{"Add":"/// be helpful."},{"Add":"///"},{"Add":"/// ```"},{"Add":"/// # use anyhow::bail;"},{"Add":"/// # use thiserror::Error;"},{"Add":"/// #"},{"Add":"/// # #[derive(Error, Debug)]"},{"Add":"/// # #[error(\"???\")]"},{"Add":"/// # struct SuspiciousError;"},{"Add":"/// #"},{"Add":"/// # fn helper() -> Result<()> {"},{"Add":"/// # bail!(SuspiciousError);"},{"Add":"/// # }"},{"Add":"/// #"},{"Add":"/// use anyhow::{Context, Result};"},{"Add":"///"},{"Add":"/// fn do_it() -> Result<()> {"},{"Add":"/// helper().context(\"failed to complete the work\")?;"},{"Add":"/// # const IGNORE: &str = stringify! {"},{"Add":"/// ..."},{"Add":"/// # };"},{"Add":"/// # unreachable!()"},{"Add":"/// }"},{"Add":"///"},{"Add":"/// fn main() {"},{"Add":"/// let err = do_it().unwrap_err();"},{"Add":"/// if let Some(e) = err.downcast_ref::() {"},{"Add":"/// // If helper() returned SuspiciousError, this downcast will"},{"Add":"/// // correctly succeed even with the context in between."},{"Add":"/// # return;"},{"Add":"/// }"},{"Add":"/// # panic!(\"expected downcast to succeed\");"},{"Add":"/// }"},{"Add":"/// ```"},{"Add":"///"},{"Add":"/// - **Attaching context whose type is used in downcasts onto errors whose"},{"Add":"/// type is insignificant.**"},{"Add":"///"},{"Add":"/// Some codebases prefer to use machine-readable context to categorize"},{"Add":"/// lower level errors in a way that will be actionable to higher levels of"},{"Add":"/// the application."},{"Add":"///"},{"Add":"/// ```"},{"Add":"/// # use anyhow::bail;"},{"Add":"/// # use thiserror::Error;"},{"Add":"/// #"},{"Add":"/// # #[derive(Error, Debug)]"},{"Add":"/// # #[error(\"???\")]"},{"Add":"/// # struct HelperFailed;"},{"Add":"/// #"},{"Add":"/// # fn helper() -> Result<()> {"},{"Add":"/// # bail!(\"no such file or directory\");"},{"Add":"/// # }"},{"Add":"/// #"},{"Add":"/// use anyhow::{Context, Result};"},{"Add":"///"},{"Add":"/// fn do_it() -> Result<()> {"},{"Add":"/// helper().context(HelperFailed)?;"},{"Add":"/// # const IGNORE: &str = stringify! {"},{"Add":"/// ..."},{"Add":"/// # };"},{"Add":"/// # unreachable!()"},{"Add":"/// }"},{"Add":"///"},{"Add":"/// fn main() {"},{"Add":"/// let err = do_it().unwrap_err();"},{"Add":"/// if let Some(e) = err.downcast_ref::() {"},{"Add":"/// // If helper failed, this downcast will succeed because"},{"Add":"/// // HelperFailed is the context that has been attached to"},{"Add":"/// // that error."},{"Add":"/// # return;"},{"Add":"/// }"},{"Add":"/// # panic!(\"expected downcast to succeed\");"},{"Add":"/// }"},{"Add":"/// ```"}]}}],"owner":"yaahc","repo":"eyre","token":"cQGpHxsygqPyQq9CEfddORmnIVJE81HPbDZlG3X9WM4=","login":null}

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