[docs] Add docs for `flake8-errmsg` (#2888)

README.md

@@ -1125,9 +1125,9 @@ For more, see [flake8-errmsg](https://pypi.org/project/flake8-errmsg/) on PyPI.
 
 | Code | Name | Message | Fix |
 | ---- | ---- | ------- | --- |
-| EM101 | raw-string-in-exception | Exception must not use a string literal, assign to variable first |  |
-| EM102 | f-string-in-exception | Exception must not use an f-string literal, assign to variable first |  |
-| EM103 | dot-format-in-exception | Exception must not use a `.format()` string directly, assign to variable first |  |
+| EM101 | [raw-string-in-exception](https://beta.ruff.rs/docs/rules/raw-string-in-exception/) | Exception must not use a string literal, assign to variable first |  |
+| EM102 | [f-string-in-exception](https://beta.ruff.rs/docs/rules/f-string-in-exception/) | Exception must not use an f-string literal, assign to variable first |  |
+| EM103 | [dot-format-in-exception](https://beta.ruff.rs/docs/rules/dot-format-in-exception/) | Exception must not use a `.format()` string directly, assign to variable first |  |
 
 ### flake8-executable (EXE)
 

clippy.toml

@@ -1 +1 @@
-doc-valid-idents = ["StackOverflow", "CodeQL", ".."]
+doc-valid-idents = ["StackOverflow", "CodeQL", "IPython", ".."]

crates/ruff/src/rules/flake8_errmsg/rules.rs

@@ -7,6 +7,43 @@ use crate::registry::{Diagnostic, Rule};
 use crate::violation::Violation;
 
 define_violation!(
+    /// ## What it does
+    /// Checks for the use of string literals in exception constructors.
+    ///
+    /// ## Why is this bad?
+    /// Python includes the `raise` in the default traceback (and formatters
+    /// like Rich and IPython do too).
+    ///
+    /// By using a string literal, the error message will be duplicated in the
+    /// traceback, which can make the traceback less readable.
+    ///
+    /// ## Example
+    /// Given:
+    /// ```python
+    /// raise RuntimeError("'Some value' is incorrect")
+    /// ```
+    ///
+    /// Python will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 2, in <module>
+    ///     raise RuntimeError("Some value is incorrect")
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
+    ///
+    /// Instead, assign the string to a variable:
+    /// ```python
+    /// msg = "'Some value' is incorrect"
+    /// raise RuntimeError(msg)
+    /// ```
+    ///
+    /// Which will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 3, in <module>
+    ///     raise RuntimeError(msg)
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
     pub struct RawStringInException;
 );
 impl Violation for RawStringInException {
@@ -17,6 +54,45 @@ impl Violation for RawStringInException {
 }
 
 define_violation!(
+    /// ## What it does
+    /// Checks for the use of f-strings in exception constructors.
+    ///
+    /// ## Why is this bad?
+    /// Python includes the `raise` in the default traceback (and formatters
+    /// like Rich and IPython do too).
+    ///
+    /// By using an f-string, the error message will be duplicated in the
+    /// traceback, which can make the traceback less readable.
+    ///
+    /// ## Example
+    /// Given:
+    /// ```python
+    /// sub = "Some value"
+    /// raise RuntimeError(f"{sub!r} is incorrect")
+    /// ```
+    ///
+    /// Python will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 2, in <module>
+    ///     raise RuntimeError(f"{sub!r} is incorrect")
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
+    ///
+    /// Instead, assign the string to a variable:
+    /// ```python
+    /// sub = "Some value"
+    /// msg = f"{sub!r} is incorrect"
+    /// raise RuntimeError(msg)
+    /// ```
+    ///
+    /// Which will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 3, in <module>
+    ///     raise RuntimeError(msg)
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
     pub struct FStringInException;
 );
 impl Violation for FStringInException {
@@ -27,6 +103,46 @@ impl Violation for FStringInException {
 }
 
 define_violation!(
+    /// ## What it does
+    /// Checks for the use of `.format` calls on string literals in exception
+    /// constructors.
+    ///
+    /// ## Why is this bad?
+    /// Python includes the `raise` in the default traceback (and formatters
+    /// like Rich and IPython do too).
+    ///
+    /// By using a `.format` call, the error message will be duplicated in the
+    /// traceback, which can make the traceback less readable.
+    ///
+    /// ## Example
+    /// Given:
+    /// ```python
+    /// sub = "Some value"
+    /// raise RuntimeError("'{}' is incorrect".format(sub))
+    /// ```
+    ///
+    /// Python will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 2, in <module>
+    ///     raise RuntimeError("'{}' is incorrect".format(sub))
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
+    ///
+    /// Instead, assign the string to a variable:
+    /// ```python
+    /// sub = "Some value"
+    /// msg = "'{}' is incorrect".format(sub)
+    /// raise RuntimeError(msg)
+    /// ```
+    ///
+    /// Which will produce a traceback like:
+    /// ```python
+    /// Traceback (most recent call last):
+    ///   File "tmp.py", line 3, in <module>
+    ///     raise RuntimeError(msg)
+    /// RuntimeError: 'Some value' is incorrect
+    /// ```
     pub struct DotFormatInException;
 );
 impl Violation for DotFormatInException {