add `explain` subcommand and explanations to all lints

19 files changed, 578 insertions, 206 deletions

diff --git a/bin/src/config.rs b/bin/src/config.rs
index ef3a29d..1649017 100644
--- a/bin/src/config.rs
+++ b/bin/src/config.rs
@@ -26,6 +26,8 @@ pub enum SubCommand {
     Fix(Fix),
     /// Fix exactly one issue at provided position
     Single(Single),
+    /// Print detailed explanation for a lint warning
+    Explain(Explain),
 }
 
 #[derive(Clap, Debug)]
@@ -88,6 +90,13 @@ pub struct Single {
     pub diff_only: bool,
 }
 
+#[derive(Clap, Debug)]
+pub struct Explain {
+    /// Warning code to explain
+    #[clap(parse(try_from_str = parse_warning_code))]
+    pub target: u32,
+}
+
 mod dirs {
     use std::{
         fs,
@@ -160,6 +169,21 @@ fn parse_line_col(src: &str) -> Result<(usize, usize), ConfigErr> {
     }
 }
 
+fn parse_warning_code(src: &str) -> Result<u32, ConfigErr> {
+    let mut char_stream = src.chars();
+    let severity = char_stream
+        .next()
+        .ok_or(ConfigErr::InvalidWarningCode(src.to_owned()))?
+        .to_ascii_lowercase();
+    match severity {
+        'w' => char_stream
+            .collect::<String>()
+            .parse::<u32>()
+            .map_err(|_| ConfigErr::InvalidWarningCode(src.to_owned())),
+        _ => Ok(0),
+    }
+}
+
 fn build_ignore_set(ignores: &[String]) -> Result<GlobSet, GlobError> {
     let mut set = GlobSetBuilder::new();
     for pattern in ignores {
diff --git a/bin/src/err.rs b/bin/src/err.rs
index 4c16d69..1e52c2b 100644
--- a/bin/src/err.rs
+++ b/bin/src/err.rs
@@ -12,6 +12,8 @@ pub enum ConfigErr {
     InvalidPath(#[from] io::Error),
     #[error("unable to parse `{0}` as line and column")]
     InvalidPosition(String),
+    #[error("unable to parse `{0}` as warning code")]
+    InvalidWarningCode(String),
 }
 
 // #[derive(Error, Debug)]
@@ -41,6 +43,12 @@ pub enum SingleFixErr {
 }
 
 #[derive(Error, Debug)]
+pub enum ExplainErr {
+    #[error("lint with code `{0}` not found")]
+    LintNotFound(u32),
+}
+
+#[derive(Error, Debug)]
 pub enum StatixErr {
     // #[error("linter error: {0}")]
     // Lint(#[from] LintErr),
@@ -50,4 +58,6 @@ pub enum StatixErr {
     Single(#[from] SingleFixErr),
     #[error("config error: {0}")]
     Config(#[from] ConfigErr),
+    #[error("explain error: {0}")]
+    Explain(#[from] ExplainErr),
 }
diff --git a/bin/src/explain.rs b/bin/src/explain.rs
new file mode 100644
index 0000000..6aefa7e
--- /dev/null
+++ b/bin/src/explain.rs
@@ -0,0 +1,15 @@
+use crate::err::ExplainErr;
+
+use lib::LINTS;
+
+pub fn explain(code: u32) -> Result<&'static str, ExplainErr> {
+    match code {
+        0 => Ok("syntax error"),
+        _ => LINTS
+            .values()
+            .flatten()
+            .find(|l| l.code() == code)
+            .map(|l| l.explanation())
+            .ok_or(ExplainErr::LintNotFound(code)),
+    }
+}
diff --git a/bin/src/main.rs b/bin/src/main.rs
index 90b79ce..31f6823 100644
--- a/bin/src/main.rs
+++ b/bin/src/main.rs
@@ -1,5 +1,6 @@
 mod config;
 mod err;
+mod explain;
 mod fix;
 mod lint;
 mod traits;
@@ -86,6 +87,10 @@ fn _main() -> Result<(), StatixErr> {
                 print!("{}", &*single_fix_result.src)
             }
         }
+        SubCommand::Explain(explain_config) => {
+            let explanation = explain::explain(explain_config.target)?;
+            println!("{}", explanation)
+        }
     }
     Ok(())
 }
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 196cbf8..5347666 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -226,25 +226,29 @@ pub trait Rule {
 /// Contains information about the lint itself. Do not implement manually,
 /// look at the `lint` attribute macro instead for implementing rules
 pub trait Metadata {
-    fn name() -> &'static str
-    where
-        Self: Sized;
-    fn note() -> &'static str
-    where
-        Self: Sized;
-    fn code() -> u32
-    where
-        Self: Sized;
-    fn report() -> Report
-    where
-        Self: Sized;
+    fn name(&self) -> &'static str;
+    fn note(&self) -> &'static str;
+    fn code(&self) -> u32;
+    fn report(&self) -> Report;
     fn match_with(&self, with: &SyntaxKind) -> bool;
     fn match_kind(&self) -> Vec<SyntaxKind>;
 }
 
+/// Contains offline explanation for each lint
+/// The `lint` macro scans nearby doc comments for
+/// explanations and derives this trait.
+///
+/// FIXME: the lint macro does way too much, maybe
+/// split it into smaller macros.
+pub trait Explain {
+    fn explanation(&self) -> &'static str {
+        "no explanation found"
+    }
+}
+
 /// Combines Rule and Metadata, do not implement manually, this is derived by
 /// the `lint` macro.
-pub trait Lint: Metadata + Rule + Send + Sync {}
+pub trait Lint: Metadata + Explain + Rule + Send + Sync {}
 
 /// Helper utility to take lints from modules and insert them into a map for efficient
 /// access. Mapping is from a SyntaxKind to a list of lints that apply on that Kind.
diff --git a/lib/src/lints/bool_comparison.rs b/lib/src/lints/bool_comparison.rs
index 0b5733b..5c9bee8 100644
--- a/lib/src/lints/bool_comparison.rs
+++ b/lib/src/lints/bool_comparison.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,25 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind, SyntaxNode,
 };
 
+/// ## What it does
+/// Checks for expressions of the form `x == true`, `x != true` and
+/// suggests using the variable directly.
+///
+/// ## Why is this bad?
+/// Unnecessary code.
+///
+/// ## Example
+/// Instead of checking the value of `x`:
+///
+/// ```
+/// if x == true then 0 else 1
+/// ```
+///
+/// Use `x` directly:
+///
+/// ```
+/// if x then 0 else 1
+/// ```
 #[lint(
     name = "bool_comparison",
     note = "Unnecessary comparison with boolean",
@@ -71,7 +90,7 @@ impl Rule for BoolComparison {
                     non_bool_side,
                     bool_side
                 );
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/collapsible_let_in.rs b/lib/src/lints/collapsible_let_in.rs
index 878d218..21199a8 100644
--- a/lib/src/lints/collapsible_let_in.rs
+++ b/lib/src/lints/collapsible_let_in.rs
@@ -1,13 +1,41 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
-use rowan::Direction;
 use rnix::{
     types::{LetIn, TypedNode},
-    NodeOrToken, SyntaxElement, SyntaxKind, TextRange
+    NodeOrToken, SyntaxElement, SyntaxKind, TextRange,
 };
+use rowan::Direction;
 
+/// ## What it does
+/// Checks for `let-in` expressions whose body is another `let-in`
+/// expression.
+///
+/// ## Why is this bad?
+/// Unnecessary code, the `let-in` expressions can be merged.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   a = 2;
+/// in
+/// let
+///   b = 3;
+/// in
+///   a + b
+/// ```
+///
+/// Merge both `let-in` expressions:
+///
+/// ```
+/// let
+///   a = 2;
+///   b = 3;
+/// in
+///   a + b
+/// ```
 #[lint(
     name = "collapsible let in",
     note = "These let-in expressions are collapsible",
@@ -47,7 +75,7 @@ impl Rule for CollapsibleLetIn {
                 let replacement = make::empty().node().clone();
 
                 Some(
-                    Self::report()
+                    self.report()
                         .diagnostic(first_annotation, first_message)
                         .suggest(second_annotation, second_message, Suggestion::new(replacement_at, replacement))
                 )
@@ -57,4 +85,3 @@ impl Rule for CollapsibleLetIn {
         }
     }
 }
-
diff --git a/lib/src/lints/empty_let_in.rs b/lib/src/lints/empty_let_in.rs
index aae1377..b255c23 100644
--- a/lib/src/lints/empty_let_in.rs
+++ b/lib/src/lints/empty_let_in.rs
@@ -1,12 +1,30 @@
-use crate::{Lint, Metadata, Report, Rule, Suggestion};
+use crate::{Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
 use rnix::{
-    types::{LetIn, TypedNode, EntryHolder},
+    types::{EntryHolder, LetIn, TypedNode},
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for `let-in` expressions which create no new bindings.
+///
+/// ## Why is this bad?
+/// `let-in` expressions that create no new bindings are useless.
+/// These are probably remnants from debugging or editing expressions.
+///
+/// ## Example
+///
+/// ```
+/// let in pkgs.statix
+/// ```
+///
+/// Preserve only the body of the `let-in` expression:
+///
+/// ```
+/// pkgs.statix
+/// ```
 #[lint(
     name = "empty let-in",
     note = "Useless let-in expression",
@@ -31,11 +49,10 @@ impl Rule for EmptyLetIn {
                 let at = node.text_range();
                 let replacement = body;
                 let message = "This let-in expression has no entries";
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
         }
     }
 }
-
diff --git a/lib/src/lints/empty_pattern.rs b/lib/src/lints/empty_pattern.rs
index 6fb7558..5312548 100644
--- a/lib/src/lints/empty_pattern.rs
+++ b/lib/src/lints/empty_pattern.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,33 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for an empty variadic pattern: `{...}`, in a function
+/// argument.
+///
+/// ## Why is this bad?
+/// The intention with empty patterns is not instantly obvious. Prefer
+/// an underscore identifier instead, to indicate that the argument
+/// is being ignored.
+///
+/// ## Example
+///
+/// ```
+/// client = { ... }: {
+///   imports = [ self.nixosModules.irmaseal-pkg ];
+///   services.irmaseal-pkg.enable = true;
+/// };
+/// ```
+///
+/// Replace the empty variadic pattern with `_` to indicate that you
+/// intend to ignore the argument:
+///
+/// ```
+/// client = _: {
+///   imports = [ self.nixosModules.irmaseal-pkg ];
+///   services.irmaseal-pkg.enable = true;
+/// };
+/// ```
 #[lint(
     name = "empty pattern",
     note = "Found empty pattern in function argument",
@@ -28,7 +55,7 @@ impl Rule for EmptyPattern {
                 let at = node.text_range();
                 let message = "This pattern is empty, use `_` instead";
                 let replacement = make::ident("_").node().clone();
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/eta_reduction.rs b/lib/src/lints/eta_reduction.rs
index 79a5101..3a483d0 100644
--- a/lib/src/lints/eta_reduction.rs
+++ b/lib/src/lints/eta_reduction.rs
@@ -1,4 +1,4 @@
-use crate::{Lint, Metadata, Report, Rule, Suggestion};
+use crate::{Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,32 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind, SyntaxNode,
 };
 
+/// ## What it does
+/// Checks for eta-reducible functions, i.e.: converts lambda
+/// expressions into free standing functions where applicable.
+///
+/// ## Why is this bad?
+/// Oftentimes, eta-reduction results in code that is more natural
+/// to read.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   double = i: 2 * i;
+/// in
+/// map (x: double x) [ 1 2 3 ]
+/// ```
+///
+/// The lambda passed to the `map` function is eta-reducible, and the
+/// result reads more naturally:
+///
+/// ```
+/// let
+///   double = i: 2 * i;
+/// in
+/// map double [ 1 2 3 ]
+/// ```
 #[lint(
     name = "eta reduction",
     note = "This function expression is eta reducible",
@@ -43,7 +69,7 @@ impl Rule for EtaReduction {
                         "Found eta-reduction: `{}`",
                         replacement.text().to_string()
                     );
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/legacy_let_syntax.rs b/lib/src/lints/legacy_let_syntax.rs
index 2087e27..139f633 100644
--- a/lib/src/lints/legacy_let_syntax.rs
+++ b/lib/src/lints/legacy_let_syntax.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,34 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for legacy-let syntax that was never formalized.
+///
+/// ## Why is this bad?
+/// This syntax construct is undocumented, refrain from using it.
+///
+/// ## Example
+///
+/// Legacy let syntax makes use of an attribute set annotated with
+/// `let` and expects a `body` attribute.
+/// ```
+/// let {
+///   body = x + y;
+///   x = 2;
+///   y = 3;
+/// }
+/// ```
+///
+/// This is trivially representible via `rec`, which is documented
+/// and more widely known:
+///
+/// ```
+/// rec {
+///   body = x + y;
+///   x = 2;
+///   y = 3;
+/// }.body
+/// ```
 #[lint(
     name = "legacy let syntax",
     note = "Using undocumented `let` syntax",
@@ -36,7 +64,7 @@ impl Rule for ManualInherit {
                 let message = "Prefer `rec` over undocumented `let` syntax";
                 let replacement = selected.node().clone();
 
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/manual_inherit.rs b/lib/src/lints/manual_inherit.rs
index 0a6933c..2d119c3 100644
--- a/lib/src/lints/manual_inherit.rs
+++ b/lib/src/lints/manual_inherit.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,30 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for bindings of the form `a = a`.
+///
+/// ## Why is this bad?
+/// If the aim is to bring attributes from a larger scope into
+/// the current scope, prefer an inherit statement.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   a = 2;
+/// in
+///   { a = a; b = 3; }
+/// ```
+///
+/// Try `inherit` instead:
+///
+/// ```
+/// let
+///   a = 2;
+/// in
+///   { inherit a; b = 3; }
+/// ```
 #[lint(
     name = "manual inherit",
     note = "Assignment instead of inherit",
@@ -35,7 +59,7 @@ impl Rule for ManualInherit {
                 let at = node.text_range();
                 let replacement = make::inherit_stmt(&[key]).node().clone();
                 let message = "This assignment is better written with `inherit`";
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/manual_inherit_from.rs b/lib/src/lints/manual_inherit_from.rs
index 794aaf9..8d0f539 100644
--- a/lib/src/lints/manual_inherit_from.rs
+++ b/lib/src/lints/manual_inherit_from.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,30 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for bindings of the form `a = someAttr.a`.
+///
+/// ## Why is this bad?
+/// If the aim is to extract or bring attributes of an attrset into
+/// scope, prefer an inherit statement.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   mtl = pkgs.haskellPackages.mtl;
+/// in
+///   null
+/// ```
+///
+/// Try `inherit` instead:
+///
+/// ```
+/// let
+///   inherit (pkgs.haskellPackages) mtl;
+/// in
+///   null
+/// ```
 #[lint(
     name = "manual inherit from",
     note = "Assignment instead of inherit from",
@@ -40,7 +64,7 @@ impl Rule for ManualInherit {
                     make::inherit_from_stmt(set, &[key]).node().clone()
                 };
                 let message = "This assignment is better written with `inherit`";
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/redundant_pattern_bind.rs b/lib/src/lints/redundant_pattern_bind.rs
index aebc549..5b0711f 100644
--- a/lib/src/lints/redundant_pattern_bind.rs
+++ b/lib/src/lints/redundant_pattern_bind.rs
@@ -1,4 +1,4 @@
-use crate::{Lint, Metadata, Report, Rule, Suggestion};
+use crate::{Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,10 +7,29 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for binds of the form `inputs @ { ... }` in function
+/// arguments.
+///
+/// ## Why is this bad?
+/// The variadic pattern here is redundant, as it does not capture
+/// anything.
+///
+/// ## Example
+///
+/// ```
+/// inputs @ { ... }: inputs.nixpkgs
+/// ```
+///
+/// Remove the pattern altogether:
+///
+/// ```
+/// inputs: inputs.nixpkgs
+/// ```
 #[lint(
     name = "redundant pattern bind",
     note = "Found redundant pattern bind in function argument",
-    code = 10,
+    code = 11,
     match_with = SyntaxKind::NODE_PATTERN
 )]
 struct RedundantPatternBind;
@@ -32,7 +51,7 @@ impl Rule for RedundantPatternBind {
                 let at = node.text_range();
                 let message = format!("This pattern bind is redundant, use `{}` instead", ident.as_str());
                 let replacement = ident.node().clone();
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/unquoted_splice.rs b/lib/src/lints/unquoted_splice.rs
index 4d1ed69..c2fd6e4 100644
--- a/lib/src/lints/unquoted_splice.rs
+++ b/lib/src/lints/unquoted_splice.rs
@@ -1,4 +1,4 @@
-use crate::{make, Lint, Metadata, Report, Rule, Suggestion};
+use crate::{make, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
@@ -7,6 +7,30 @@ use rnix::{
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for antiquote/splice expressions that are not quoted.
+///
+/// ## Why is this bad?
+/// An *anti*quoted expression should always occur within a *quoted*
+/// expression.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   pkgs = nixpkgs.legacyPackages.${system};
+/// in
+///   pkgs
+/// ```
+///
+/// Quote the splice expression:
+///
+/// ```
+/// let
+///   pkgs = nixpkgs.legacyPackages."${system}";
+/// in
+///   pkgs
+/// ```
 #[lint(
     name = "unquoted splice",
     note = "Found unquoted splice expression",
@@ -24,7 +48,7 @@ impl Rule for UnquotedSplice {
                 let at = node.text_range();
                 let replacement = make::quote(&node).node().clone();
                 let message = "Consider quoting this splice expression";
-                Some(Self::report().suggest(at, message, Suggestion::new(at, replacement)))
+                Some(self.report().suggest(at, message, Suggestion::new(at, replacement)))
             } else {
                 None
             }
diff --git a/lib/src/lints/useless_parens.rs b/lib/src/lints/useless_parens.rs
index 2d6ba8f..36ad1b7 100644
--- a/lib/src/lints/useless_parens.rs
+++ b/lib/src/lints/useless_parens.rs
@@ -1,12 +1,37 @@
-use crate::{Lint, Metadata, Report, Rule, Suggestion, Diagnostic};
+use crate::{Diagnostic, Metadata, Report, Rule, Suggestion};
 
 use if_chain::if_chain;
 use macros::lint;
 use rnix::{
-    types::{ParsedType, KeyValue, Paren, TypedNode, Wrapper},
+    types::{KeyValue, Paren, ParsedType, TypedNode, Wrapper},
     NodeOrToken, SyntaxElement, SyntaxKind,
 };
 
+/// ## What it does
+/// Checks for unnecessary parentheses.
+///
+/// ## Why is this bad?
+/// Unnecessarily parenthesized code is hard to read.
+///
+/// ## Example
+///
+/// ```
+/// let
+///   double = (x: 2 * x);
+///   ls = map (double) [ 1 2 3 ];
+/// in
+///   (2 + 3)
+/// ```
+///
+/// Remove unnecessary parentheses:
+///
+/// ```
+/// let
+///   double = x: 2 * x;
+///   ls = map double [ 1 2 3 ];
+/// in
+///   2 + 3
+/// ```
 #[lint(
     name = "useless parens",
     note = "These parentheses can be omitted",
@@ -27,7 +52,7 @@ impl Rule for UselessParens {
 
             if let Some(diagnostic) = do_thing(parsed_type_node);
             then {
-                let mut report = Self::report();
+                let mut report = self.report();
                 report.diagnostics.push(diagnostic);
                 Some(report)
             } else {
@@ -79,7 +104,7 @@ fn do_thing(parsed_type_node: ParsedType) -> Option<Diagnostic> {
             if let Some(parsed_inner) = ParsedType::cast(inner_node);
             if matches!(
                 parsed_inner,
-                ParsedType::List(_) 
+                ParsedType::List(_)
                 | ParsedType::Paren(_)
                 | ParsedType::Str(_)
                 | ParsedType::AttrSet(_)
@@ -95,6 +120,6 @@ fn do_thing(parsed_type_node: ParsedType) -> Option<Diagnostic> {
                 None
             }
         },
-        _ => None
+        _ => None,
     }
 }
diff --git a/macros/src/explain.rs b/macros/src/explain.rs
new file mode 100644
index 0000000..41dc5d4
--- /dev/null
+++ b/macros/src/explain.rs
@@ -0,0 +1,28 @@
+use proc_macro2::TokenStream as TokenStream2;
+use quote::quote;
+use syn::{ItemStruct, Lit, Meta, MetaNameValue};
+
+pub fn generate_explain_impl(struct_item: &ItemStruct) -> TokenStream2 {
+    let struct_name = &struct_item.ident;
+    let explain = struct_item
+        .attrs
+        .iter()
+        .filter_map(|attr| match attr.parse_meta().ok() {
+            Some(Meta::NameValue(MetaNameValue {
+                path,
+                lit: Lit::Str(str_lit),
+                ..
+            })) if path.is_ident("doc") => Some(str_lit.value()),
+            _ => None,
+        })
+        .map(|s| s.strip_prefix(' ').unwrap_or(&s).to_owned())
+        .collect::<Vec<_>>()
+        .join("\n");
+    quote! {
+        impl crate::Explain for #struct_name {
+            fn explanation(&self) -> &'static str {
+                #explain
+            }
+        }
+    }
+}
diff --git a/macros/src/lib.rs b/macros/src/lib.rs
index 127b4cb..86fa509 100644
--- a/macros/src/lib.rs
+++ b/macros/src/lib.rs
@@ -1,44 +1,12 @@
-use std::collections::HashMap;
+mod explain;
+mod metadata;
 
+use explain::generate_explain_impl;
+use metadata::{generate_meta_impl, RawLintMeta};
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
-
-use quote::{format_ident, quote};
-use syn::{
-    parse::{Parse, ParseStream, Result as ParseResult},
-    parse_macro_input,
-    punctuated::Punctuated,
-    Expr, Ident, ItemStruct, Lit, Token,
-};
-
-struct KeyValue {
-    key: Ident,
-    _eq: Token![=],
-    value: Expr,
-}
-
-impl Parse for KeyValue {
-    fn parse(input: ParseStream) -> ParseResult<Self> {
-        Ok(Self {
-            key: input.parse()?,
-            _eq: input.parse()?,
-            value: input.parse()?,
-        })
-    }
-}
-
-struct LintMeta(HashMap<Ident, Expr>);
-
-impl Parse for LintMeta {
-    fn parse(input: ParseStream) -> ParseResult<Self> {
-        Ok(Self(
-            Punctuated::<KeyValue, Token![,]>::parse_terminated(input)?
-                .into_iter()
-                .map(|item| (item.key, item.value))
-                .collect(),
-        ))
-    }
-}
+use quote::quote;
+use syn::{parse_macro_input, Ident, ItemStruct};
 
 fn generate_self_impl(struct_name: &Ident) -> TokenStream2 {
     quote! {
@@ -50,136 +18,16 @@ fn generate_self_impl(struct_name: &Ident) -> TokenStream2 {
     }
 }
 
-fn generate_meta_impl(struct_name: &Ident, meta: &LintMeta) -> TokenStream2 {
-    let name_fn = generate_name_fn(meta);
-    let note_fn = generate_note_fn(meta);
-    let code_fn = generate_code_fn(meta);
-    let report_fn = generate_report_fn();
-    let match_with_fn = generate_match_with_fn(meta);
-    let match_kind = generate_match_kind(meta);
-    quote! {
-        impl Metadata for #struct_name {
-            #name_fn
-            #note_fn
-            #code_fn
-            #report_fn
-            #match_with_fn
-            #match_kind
-        }
-    }
-}
-
-fn generate_name_fn(meta: &LintMeta) -> TokenStream2 {
-    let name = meta
-        .0
-        .get(&format_ident!("name"))
-        .unwrap_or_else(|| panic!("`name` not present"));
-    if let syn::Expr::Lit(name_lit) = name {
-        if let Lit::Str(name_str) = &name_lit.lit {
-            return quote! {
-                fn name() -> &'static str {
-                    #name_str
-                }
-            };
-        }
-    }
-    panic!("Invalid value for `name`");
-}
-
-fn generate_note_fn(meta: &LintMeta) -> TokenStream2 {
-    let note = meta
-        .0
-        .get(&format_ident!("note"))
-        .unwrap_or_else(|| panic!("`note` not present"));
-    if let syn::Expr::Lit(note_lit) = note {
-        if let Lit::Str(note_str) = &note_lit.lit {
-            return quote! {
-                fn note() -> &'static str {
-                    #note_str
-                }
-            };
-        }
-    }
-    panic!("Invalid value for `note`");
-}
-
-fn generate_code_fn(meta: &LintMeta) -> TokenStream2 {
-    let code = meta
-        .0
-        .get(&format_ident!("code"))
-        .unwrap_or_else(|| panic!("`code` not present"));
-    if let syn::Expr::Lit(code_lit) = code {
-        if let Lit::Int(code_int) = &code_lit.lit {
-            return quote! {
-                fn code() -> u32 {
-                    #code_int
-                }
-            };
-        }
-    }
-    panic!("Invalid value for `note`");
-}
-
-fn generate_report_fn() -> TokenStream2 {
-    quote! {
-        fn report() -> Report {
-            Report::new(Self::note(), Self::code())
-        }
-    }
-}
-
-fn generate_match_with_fn(meta: &LintMeta) -> TokenStream2 {
-    let match_with_lit = meta
-        .0
-        .get(&format_ident!("match_with"))
-        .unwrap_or_else(|| panic!("`match_with` not present"));
-    if let syn::Expr::Path(match_path) = match_with_lit {
-        quote! {
-            fn match_with(&self, with: &SyntaxKind) -> bool {
-                #match_path == *with
-            }
-        }
-    } else if let syn::Expr::Array(array_expr) = match_with_lit {
-        quote! {
-            fn match_with(&self, with: &SyntaxKind) -> bool {
-                #array_expr.contains(with)
-            }
-        }
-    } else {
-        panic!("`match_with` has non-path value")
-    }
-}
-
-fn generate_match_kind(meta: &LintMeta) -> TokenStream2 {
-    let match_with_lit = meta
-        .0
-        .get(&format_ident!("match_with"))
-        .unwrap_or_else(|| panic!("`match_with` not present"));
-    if let syn::Expr::Path(match_path) = match_with_lit {
-        quote! {
-            fn match_kind(&self) -> Vec<SyntaxKind> {
-                vec![#match_path]
-            }
-        }
-    } else if let syn::Expr::Array(array_expr) = match_with_lit {
-        quote! {
-            fn match_kind(&self) -> Vec<SyntaxKind> {
-                #array_expr.to_vec()
-            }
-        }
-    } else {
-        panic!("`match_with` has non-path value")
-    }
-}
-
 #[proc_macro_attribute]
 pub fn lint(attr: TokenStream, item: TokenStream) -> TokenStream {
     let struct_item = parse_macro_input!(item as ItemStruct);
-    let meta = parse_macro_input!(attr as LintMeta);
+    let meta = parse_macro_input!(attr as RawLintMeta);
 
     let struct_name = &struct_item.ident;
     let self_impl = generate_self_impl(struct_name);
     let meta_impl = generate_meta_impl(struct_name, &meta);
+    let explain_impl = generate_explain_impl(&struct_item);
+
     (quote! {
         #struct_item
 
@@ -189,8 +37,9 @@ pub fn lint(attr: TokenStream, item: TokenStream) -> TokenStream {
 
         #self_impl
         #meta_impl
+        #explain_impl
 
-        impl Lint for #struct_name {}
+        impl crate::Lint for #struct_name {}
     })
     .into()
 }
diff --git a/macros/src/metadata.rs b/macros/src/metadata.rs
new file mode 100644
index 0000000..b41eb9c
--- /dev/null
+++ b/macros/src/metadata.rs
@@ -0,0 +1,177 @@
+use std::collections::HashMap;
+
+use proc_macro2::TokenStream as TokenStream2;
+use quote::{format_ident, quote};
+use syn::{
+    parse::{Parse, ParseStream, Result},
+    punctuated::Punctuated,
+    Expr, ExprArray, Ident, Lit, Path, Token,
+};
+
+struct KeyValue {
+    key: Ident,
+    _eq: Token![=],
+    value: Expr,
+}
+
+impl Parse for KeyValue {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Ok(Self {
+            key: input.parse()?,
+            _eq: input.parse()?,
+            value: input.parse()?,
+        })
+    }
+}
+
+pub struct RawLintMeta(HashMap<Ident, Expr>);
+
+impl Parse for RawLintMeta {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Ok(Self(
+            Punctuated::<KeyValue, Token![,]>::parse_terminated(input)?
+                .into_iter()
+                .map(|item| (item.key, item.value))
+                .collect(),
+        ))
+    }
+}
+
+pub struct LintMeta<'μ> {
+    name: &'μ Lit,
+    note: &'μ Lit,
+    code: &'μ Lit,
+    match_with: MatchWith<'μ>,
+}
+
+enum MatchWith<'π> {
+    Path(&'π Path),
+    Array(&'π ExprArray),
+}
+
+fn extract<'λ>(id: &str, raw: &'λ RawLintMeta) -> &'λ Expr {
+    raw.0
+        .get(&format_ident!("{}", id))
+        .unwrap_or_else(|| panic!("`{}` not present", id))
+}
+
+fn as_lit(e: &Expr) -> &Lit {
+    match e {
+        Expr::Lit(l) => &l.lit,
+        _ => panic!("expected a literal"),
+    }
+}
+
+impl<'μ> LintMeta<'μ> {
+    fn from_raw(raw: &'μ RawLintMeta) -> Self {
+        let name = as_lit(extract("name", raw));
+        let note = as_lit(extract("note", raw));
+        let code = as_lit(extract("code", raw));
+        let match_with_expr = extract("match_with", raw);
+        let match_with = match match_with_expr {
+            Expr::Path(p) => MatchWith::Path(&p.path),
+            Expr::Array(a) => MatchWith::Array(a),
+            _ => panic!("`match_with` is neither a path nor an array"),
+        };
+        Self {
+            name,
+            note,
+            code,
+            match_with,
+        }
+    }
+
+    fn generate_name_fn(&self) -> TokenStream2 {
+        let name_str = self.name;
+        return quote! {
+            fn name(&self) -> &'static str {
+                #name_str
+            }
+        };
+    }
+
+    fn generate_note_fn(&self) -> TokenStream2 {
+        let note_str = self.note;
+        return quote! {
+            fn note(&self) -> &'static str {
+                #note_str
+            }
+        };
+    }
+
+    fn generate_code_fn(&self) -> TokenStream2 {
+        let code_int = self.code;
+        return quote! {
+            fn code(&self) -> u32 {
+                #code_int
+            }
+        };
+    }
+
+    fn generate_match_with_fn(&self) -> TokenStream2 {
+        match self.match_with {
+            MatchWith::Path(p) => {
+                quote! {
+                    fn match_with(&self, with: &SyntaxKind) -> bool {
+                        #p == *with
+                    }
+                }
+            }
+            MatchWith::Array(a) => {
+                quote! {
+                    fn match_with(&self, with: &SyntaxKind) -> bool {
+                        #a.contains(with)
+                    }
+                }
+            }
+        }
+    }
+
+    fn generate_match_kind_fn(&self) -> TokenStream2 {
+        match self.match_with {
+            MatchWith::Path(p) => {
+                quote! {
+                    fn match_kind(&self) -> Vec<SyntaxKind> {
+                        vec![#p]
+                    }
+                }
+            }
+            MatchWith::Array(a) => {
+                quote! {
+                    fn match_kind(&self) -> Vec<SyntaxKind> {
+                        #a.to_vec()
+                    }
+                }
+            }
+        }
+    }
+
+    fn generate_report_fn(&self) -> TokenStream2 {
+        quote! {
+            fn report(&self) -> crate::Report {
+                crate::Report::new(self.note(), self.code())
+            }
+        }
+    }
+}
+
+pub fn generate_meta_impl(struct_name: &Ident, meta: &RawLintMeta) -> TokenStream2 {
+    let not_raw = LintMeta::from_raw(&meta);
+    let name_fn = not_raw.generate_name_fn();
+    let note_fn = not_raw.generate_note_fn();
+    let code_fn = not_raw.generate_code_fn();
+    let match_with_fn = not_raw.generate_match_with_fn();
+    let match_kind = not_raw.generate_match_kind_fn();
+    let report_fn = not_raw.generate_report_fn();
+
+    quote! {
+        impl crate::Metadata for #struct_name {
+            #name_fn
+            #note_fn
+            #code_fn
+            #match_with_fn
+            #match_kind
+            #report_fn
+        }
+    }
+}