Add lint for pub fns returning a Result without documenting errors

The Rust Book recommends that functions that return a `Result` type have
a doc comment with an `# Errors` section describing the kind of errors
that can be returned
(https://doc.rust-lang.org/book/ch14-02-publishing-to-crates-io.html#commonly-used-sections).
This change adds a lint to enforce this. The lint is allow by default;
it can be enabled with `#![warn(clippy::missing_errors_doc)]`.

Closes #4854.

CHANGELOG.md

@@ -1092,6 +1092,7 @@ Released 2018-09-13
 [`misrefactored_assign_op`]: https://rust-lang.github.io/rust-clippy/master/index.html#misrefactored_assign_op
 [`missing_const_for_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn
 [`missing_docs_in_private_items`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_docs_in_private_items
+[`missing_errors_doc`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_errors_doc
 [`missing_inline_in_public_items`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_inline_in_public_items
 [`missing_safety_doc`]: https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
 [`mistyped_literal_suffixes`]: https://rust-lang.github.io/rust-clippy/master/index.html#mistyped_literal_suffixes

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 338 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 339 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/doc.rs

@@ -1,4 +1,4 @@
-use crate::utils::span_lint;
+use crate::utils::{match_type, paths, return_ty, span_lint};
 use itertools::Itertools;
 use pulldown_cmark;
 use rustc::hir;
@@ -8,7 +8,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_session::declare_tool_lint;
 use std::ops::Range;
 use syntax::ast::{AttrKind, Attribute};
-use syntax::source_map::{BytePos, Span};
+use syntax::source_map::{BytePos, MultiSpan, Span};
 use syntax_pos::Pos;
 use url::Url;
 
@@ -45,7 +45,7 @@ declare_clippy_lint! {
     ///
     /// **Known problems:** None.
     ///
-    /// **Examples**:
+    /// **Examples:**
     /// ```rust
     ///# type Universe = ();
     /// /// This function should really be documented
@@ -70,6 +70,35 @@ declare_clippy_lint! {
     "`pub unsafe fn` without `# Safety` docs"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks the doc comments of publicly visible functions that
+    /// return a `Result` type and warns if there is no `# Errors` section.
+    ///
+    /// **Why is this bad?** Documenting the type of errors that can be returned from a
+    /// function can help callers write code to handle the errors appropriately.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Examples:**
+    ///
+    /// Since the following function returns a `Result` it has an `# Errors` section in
+    /// its doc comment:
+    ///
+    /// ```rust
+    ///# use std::io;
+    /// /// # Errors
+    /// ///
+    /// /// Will return `Err` if `filename` does not exist or the user does not have
+    /// /// permission to read it.
+    /// pub fn read(filename: String) -> io::Result<String> {
+    ///     unimplemented!();
+    /// }
+    /// ```
+    pub MISSING_ERRORS_DOC,
+    pedantic,
+    "`pub fn` returns `Result` without `# Errors` in doc comment"
+}
+
 declare_clippy_lint! {
     /// **What it does:** Checks for `fn main() { .. }` in doctests
     ///
@@ -114,7 +143,7 @@ impl DocMarkdown {
     }
 }
 
-impl_lint_pass!(DocMarkdown => [DOC_MARKDOWN, MISSING_SAFETY_DOC, NEEDLESS_DOCTEST_MAIN]);
+impl_lint_pass!(DocMarkdown => [DOC_MARKDOWN, MISSING_SAFETY_DOC, MISSING_ERRORS_DOC, NEEDLESS_DOCTEST_MAIN]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DocMarkdown {
     fn check_crate(&mut self, cx: &LateContext<'a, 'tcx>, krate: &'tcx hir::Crate) {
@@ -122,20 +151,10 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DocMarkdown {
     }
 
     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
-        if check_attrs(cx, &self.valid_idents, &item.attrs) {
-            return;
-        }
-        // no safety header
+        let headers = check_attrs(cx, &self.valid_idents, &item.attrs);
         match item.kind {
             hir::ItemKind::Fn(ref sig, ..) => {
-                if cx.access_levels.is_exported(item.hir_id) && sig.header.unsafety == hir::Unsafety::Unsafe {
-                    span_lint(
-                        cx,
-                        MISSING_SAFETY_DOC,
-                        item.span,
-                        "unsafe function's docs miss `# Safety` section",
-                    );
-                }
+                lint_for_missing_headers(cx, item.hir_id, item.span, sig, headers);
             },
             hir::ItemKind::Impl(_, _, _, _, ref trait_ref, ..) => {
                 self.in_trait_impl = trait_ref.is_some();
@@ -151,40 +170,51 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DocMarkdown {
     }
 
     fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::TraitItem) {
-        if check_attrs(cx, &self.valid_idents, &item.attrs) {
-            return;
-        }
-        // no safety header
+        let headers = check_attrs(cx, &self.valid_idents, &item.attrs);
         if let hir::TraitItemKind::Method(ref sig, ..) = item.kind {
-            if cx.access_levels.is_exported(item.hir_id) && sig.header.unsafety == hir::Unsafety::Unsafe {
-                span_lint(
-                    cx,
-                    MISSING_SAFETY_DOC,
-                    item.span,
-                    "unsafe function's docs miss `# Safety` section",
-                );
-            }
+            lint_for_missing_headers(cx, item.hir_id, item.span, sig, headers);
         }
     }
 
     fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::ImplItem) {
-        if check_attrs(cx, &self.valid_idents, &item.attrs) || self.in_trait_impl {
+        let headers = check_attrs(cx, &self.valid_idents, &item.attrs);
+        if self.in_trait_impl {
             return;
         }
-        // no safety header
         if let hir::ImplItemKind::Method(ref sig, ..) = item.kind {
-            if cx.access_levels.is_exported(item.hir_id) && sig.header.unsafety == hir::Unsafety::Unsafe {
-                span_lint(
-                    cx,
-                    MISSING_SAFETY_DOC,
-                    item.span,
-                    "unsafe function's docs miss `# Safety` section",
-                );
-            }
+            lint_for_missing_headers(cx, item.hir_id, item.span, sig, headers);
         }
     }
 }
 
+fn lint_for_missing_headers<'a, 'tcx>(
+    cx: &LateContext<'a, 'tcx>,
+    hir_id: hir::HirId,
+    span: impl Into<MultiSpan> + Copy,
+    sig: &hir::FnSig,
+    headers: DocHeaders,
+) {
+    if !cx.access_levels.is_exported(hir_id) {
+        return; // Private functions do not require doc comments
+    }
+    if !headers.safety && sig.header.unsafety == hir::Unsafety::Unsafe {
+        span_lint(
+            cx,
+            MISSING_SAFETY_DOC,
+            span,
+            "unsafe function's docs miss `# Safety` section",
+        );
+    }
+    if !headers.errors && match_type(cx, return_ty(cx, hir_id), &paths::RESULT) {
+        span_lint(
+            cx,
+            MISSING_ERRORS_DOC,
+            span,
+            "docs for function returning `Result` missing `# Errors` section",
+        );
+    }
+}
+
 /// Cleanup documentation decoration (`///` and such).
 ///
 /// We can't use `syntax::attr::AttributeMethods::with_desugared_doc` or
@@ -243,7 +273,13 @@ pub fn strip_doc_comment_decoration(comment: &str, span: Span) -> (String, Vec<(
     panic!("not a doc-comment: {}", comment);
 }
 
-pub fn check_attrs<'a>(cx: &LateContext<'_, '_>, valid_idents: &FxHashSet<String>, attrs: &'a [Attribute]) -> bool {
+#[derive(Copy, Clone)]
+struct DocHeaders {
+    safety: bool,
+    errors: bool,
+}
+
+fn check_attrs<'a>(cx: &LateContext<'_, '_>, valid_idents: &FxHashSet<String>, attrs: &'a [Attribute]) -> DocHeaders {
     let mut doc = String::new();
     let mut spans = vec![];
 
@@ -255,7 +291,11 @@ pub fn check_attrs<'a>(cx: &LateContext<'_, '_>, valid_idents: &FxHashSet<String
             doc.push_str(&comment);
         } else if attr.check_name(sym!(doc)) {
             // ignore mix of sugared and non-sugared doc
-            return true; // don't trigger the safety check
+            // don't trigger the safety or errors check
+            return DocHeaders {
+                safety: true,
+                errors: true,
+            };
         }
     }
 
@@ -267,7 +307,10 @@ pub fn check_attrs<'a>(cx: &LateContext<'_, '_>, valid_idents: &FxHashSet<String
     }
 
     if doc.is_empty() {
-        return false;
+        return DocHeaders {
+            safety: false,
+            errors: false,
+        };
     }
 
     let parser = pulldown_cmark::Parser::new(&doc).into_offset_iter();
@@ -295,12 +338,15 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
     valid_idents: &FxHashSet<String>,
     events: Events,
     spans: &[(usize, Span)],
-) -> bool {
+) -> DocHeaders {
     // true if a safety header was found
     use pulldown_cmark::Event::*;
     use pulldown_cmark::Tag::*;
 
-    let mut safety_header = false;
+    let mut headers = DocHeaders {
+        safety: false,
+        errors: false,
+    };
     let mut in_code = false;
     let mut in_link = None;
     let mut in_heading = false;
@@ -323,7 +369,8 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
                     // text "http://example.com" by pulldown-cmark
                     continue;
                 }
-                safety_header |= in_heading && text.trim() == "Safety";
+                headers.safety |= in_heading && text.trim() == "Safety";
+                headers.errors |= in_heading && text.trim() == "Errors";
                 let index = match spans.binary_search_by(|c| c.0.cmp(&range.start)) {
                     Ok(o) => o,
                     Err(e) => e - 1,
@@ -340,7 +387,7 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
             },
         }
     }
-    safety_header
+    headers
 }
 
 fn check_code(cx: &LateContext<'_, '_>, text: &str, span: Span) {

clippy_lints/src/lib.rs

@@ -488,6 +488,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &derive::DERIVE_HASH_XOR_EQ,
         &derive::EXPL_IMPL_CLONE_ON_COPY,
         &doc::DOC_MARKDOWN,
+        &doc::MISSING_ERRORS_DOC,
         &doc::MISSING_SAFETY_DOC,
         &doc::NEEDLESS_DOCTEST_MAIN,
         &double_comparison::DOUBLE_COMPARISONS,
@@ -1013,6 +1014,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&default_trait_access::DEFAULT_TRAIT_ACCESS),
         LintId::of(&derive::EXPL_IMPL_CLONE_ON_COPY),
         LintId::of(&doc::DOC_MARKDOWN),
+        LintId::of(&doc::MISSING_ERRORS_DOC),
         LintId::of(&empty_enum::EMPTY_ENUM),
         LintId::of(&enum_glob_use::ENUM_GLOB_USE),
         LintId::of(&enum_variants::MODULE_NAME_REPETITIONS),

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 338] = [
+pub const ALL_LINTS: [Lint; 339] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -1127,6 +1127,13 @@ pub const ALL_LINTS: [Lint; 338] = [
         deprecation: None,
         module: "missing_doc",
     },
+    Lint {
+        name: "missing_errors_doc",
+        group: "pedantic",
+        desc: "`pub fn` returns `Result` without `# Errors` in doc comment",
+        deprecation: None,
+        module: "doc",
+    },
     Lint {
         name: "missing_inline_in_public_items",
         group: "restriction",

tests/ui/doc_errors.rs

@@ -0,0 +1,64 @@
+#![warn(clippy::missing_errors_doc)]
+
+use std::io;
+
+pub fn pub_fn_missing_errors_header() -> Result<(), ()> {
+    unimplemented!();
+}
+
+/// This is not sufficiently documented.
+pub fn pub_fn_returning_io_result() -> io::Result<()> {
+    unimplemented!();
+}
+
+/// # Errors
+/// A description of the errors goes here.
+pub fn pub_fn_with_errors_header() -> Result<(), ()> {
+    unimplemented!();
+}
+
+/// This function doesn't require the documentation because it is private
+fn priv_fn_missing_errors_header() -> Result<(), ()> {
+    unimplemented!();
+}
+
+pub struct Struct1;
+
+impl Struct1 {
+    /// This is not sufficiently documented.
+    pub fn pub_method_missing_errors_header() -> Result<(), ()> {
+        unimplemented!();
+    }
+
+    /// # Errors
+    /// A description of the errors goes here.
+    pub fn pub_method_with_errors_header() -> Result<(), ()> {
+        unimplemented!();
+    }
+
+    /// This function doesn't require the documentation because it is private.
+    fn priv_method_missing_errors_header() -> Result<(), ()> {
+        unimplemented!();
+    }
+}
+
+pub trait Trait1 {
+    /// This is not sufficiently documented.
+    fn trait_method_missing_errors_header() -> Result<(), ()>;
+
+    /// # Errors
+    /// A description of the errors goes here.
+    fn trait_method_with_errors_header() -> Result<(), ()>;
+}
+
+impl Trait1 for Struct1 {
+    fn trait_method_missing_errors_header() -> Result<(), ()> {
+        unimplemented!();
+    }
+
+    fn trait_method_with_errors_header() -> Result<(), ()> {
+        unimplemented!();
+    }
+}
+
+fn main() {}

tests/ui/doc_errors.stderr

@@ -0,0 +1,34 @@
+error: docs for function returning `Result` missing `# Errors` section
+  --> $DIR/doc_errors.rs:5:1
+   |
+LL | / pub fn pub_fn_missing_errors_header() -> Result<(), ()> {
+LL | |     unimplemented!();
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::missing-errors-doc` implied by `-D warnings`
+
+error: docs for function returning `Result` missing `# Errors` section
+  --> $DIR/doc_errors.rs:10:1
+   |
+LL | / pub fn pub_fn_returning_io_result() -> io::Result<()> {
+LL | |     unimplemented!();
+LL | | }
+   | |_^
+
+error: docs for function returning `Result` missing `# Errors` section
+  --> $DIR/doc_errors.rs:29:5
+   |
+LL | /     pub fn pub_method_missing_errors_header() -> Result<(), ()> {
+LL | |         unimplemented!();
+LL | |     }
+   | |_____^
+
+error: docs for function returning `Result` missing `# Errors` section
+  --> $DIR/doc_errors.rs:47:5
+   |
+LL |     fn trait_method_missing_errors_header() -> Result<(), ()>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+