Add Commonly Asked Questions page in guide.

src/guide.md

@@ -1,5 +1,10 @@
 # SNAFU user's guide
 
+Looking for quick help or answers? Please see the
+[troubleshooting](guide::troubleshooting) page.
+
+---
+
 Once you've got a high-level idea of what SNAFU can do by looking at
 the [quick example](crate) and some [more examples](guide::examples),
 take a peek at [our design philosophy](guide::philosophy).

src/guide/troubleshooting.md

@@ -0,0 +1,18 @@
+# Troubleshooting common issues
+
+This section covers some commonly asked and encountered questions. If
+you experience a problem and spend a significant amount of time
+answering it, please consider [submitting a pull request][pr] to add
+it here. If you cannot solve your issue after reading this, please
+[open an issue][issue] to ask your question.
+
+[pr]: https://github.com/shepmaster/snafu/blob/master/CONTRIBUTING.md
+[issue]: https://github.com/shepmaster/snafu/issues
+
+## `missing field 'source' in initializer`
+
+Do you have errors talking about the source field missing,
+`snafu::IntoError<_>` not being satisfied, and your project is
+multiple modules or files?
+
+[**answers**](troubleshooting::missing_field_source)

src/guide/troubleshooting/missing_field_source.md

@@ -0,0 +1,99 @@
+# Missing field source / IntoError is not implemented
+
+This error is encountered in multi-module / multi-file projects when
+the error type is defined in one module and constructed in another.
+
+## Failing Example
+
+**project_error.rs**
+
+```rust,ignore
+use snafu::Snafu;
+use std::io;
+
+#[derive(Debug, Snafu)]
+#[snafu(visibility(pub(crate)))]
+pub enum ProjectError {
+    #[snafu(visibility(pub(crate)))]
+    #[snafu(display("Unable to read configuration from {}: {}", path, source))]
+    IOConfigError {
+        path: &'static str,
+        source: io::Error,
+    },
+}
+```
+
+**main.rs**
+
+```rust,ignore
+mod project_error;
+
+use project_error::ProjectError;
+use snafu::ResultExt;
+use std::{fs, io};
+
+const CONFIG_PATH: &str = "/etc/example/conf.conf";
+
+pub fn read_config() -> Result<String, ProjectError> {
+    fs::read_to_string(CONFIG_PATH).context(ProjectError::IOConfigError { path: CONFIG_PATH })
+}
+
+pub fn main() {
+    println!("{}", read_config().unwrap());
+}
+```
+
+## Errors
+
+```text
+error[E0063]: missing field `source` in initializer of `project_error::ProjectError`
+  --> src/lib.rs:200:9
+   |
+27 |         ProjectError::IOConfigError { path: CONFIG_PATH }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `source`
+```
+
+and
+
+```text
+error[E0277]: the trait bound `project_error::ProjectError: snafu::IntoError<_>` is not satisfied
+  --> src/lib.rs:200:9
+   |
+27 |         ProjectError::IOConfigError { path: CONFIG_PATH }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `snafu::IntoError<_>` is not implemented for `project_error::ProjectError`
+the trait bound 'project_error::ProjectError: snafu::IntoError<_>' is not satisfied
+```
+
+## Solution
+
+Replace the `ProjectError::IOConfigError` in the `read_config()`
+function with `project_error::IOConfigError`.
+
+## Explanation
+
+This works because the `#[derive(Snafu)]` macro creates the *context
+selector* type `IoConfigError`:
+
+```rust,ignore
+#[derive(Debug, Snafu)]
+pub enum ProjectError {
+    IOConfigError {
+        source: io::Error,
+        path: &'static str,
+    },
+}
+
+// some details removed
+struct IOConfigError<P> {
+    path: P,
+}
+
+// Some impls for the IOConfigError struct
+```
+
+See [the macro section](guide::the_macro) of the guide for more details.
+
+When you use `ProjectError::IOConfigError`, you're referencing the
+enum variant, not the struct that you need. Replacing
+`ProjectError::IOConfigError` with `project_error::IOConfigError`
+fixes this problem.

src/lib.rs

@@ -159,6 +159,9 @@ generate_guide! {
         pub mod philosophy;
         pub mod structs;
         pub mod the_macro;
+        pub mod troubleshooting {
+            pub mod missing_field_source;
+        }
         pub mod upgrading;
 
         @code pub mod examples;