Add map_err_ignore lint

CHANGELOG.md

@@ -1675,6 +1675,7 @@ Released 2018-09-13
 [`many_single_char_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#many_single_char_names
 [`map_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_clone
 [`map_entry`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_entry
+[`map_err_ignore`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_err_ignore
 [`map_flatten`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten
 [`map_identity`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_identity
 [`map_unwrap_or`]: https://rust-lang.github.io/rust-clippy/master/index.html#map_unwrap_or

clippy_lints/src/lib.rs

@@ -230,6 +230,7 @@ mod main_recursion;
 mod manual_async_fn;
 mod manual_non_exhaustive;
 mod map_clone;
+mod map_err_ignore;
 mod map_identity;
 mod map_unit_fn;
 mod match_on_vec_items;
@@ -624,6 +625,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &manual_async_fn::MANUAL_ASYNC_FN,
         &manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE,
         &map_clone::MAP_CLONE,
+        &map_err_ignore::MAP_ERR_IGNORE,
         &map_identity::MAP_IDENTITY,
         &map_unit_fn::OPTION_MAP_UNIT_FN,
         &map_unit_fn::RESULT_MAP_UNIT_FN,
@@ -916,6 +918,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box implicit_saturating_sub::ImplicitSaturatingSub);
     store.register_late_pass(|| box methods::Methods);
     store.register_late_pass(|| box map_clone::MapClone);
+    store.register_late_pass(|| box map_err_ignore::MapErrIgnore);
     store.register_late_pass(|| box shadow::Shadow);
     store.register_late_pass(|| box types::LetUnitValue);
     store.register_late_pass(|| box types::UnitCmp);
@@ -1327,6 +1330,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&manual_async_fn::MANUAL_ASYNC_FN),
         LintId::of(&manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE),
         LintId::of(&map_clone::MAP_CLONE),
+        LintId::of(&map_err_ignore::MAP_ERR_IGNORE),
         LintId::of(&map_identity::MAP_IDENTITY),
         LintId::of(&map_unit_fn::OPTION_MAP_UNIT_FN),
         LintId::of(&map_unit_fn::RESULT_MAP_UNIT_FN),
@@ -1534,6 +1538,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&manual_async_fn::MANUAL_ASYNC_FN),
         LintId::of(&manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE),
         LintId::of(&map_clone::MAP_CLONE),
+        LintId::of(&map_err_ignore::MAP_ERR_IGNORE),
         LintId::of(&matches::INFALLIBLE_DESTRUCTURING_MATCH),
         LintId::of(&matches::MATCH_LIKE_MATCHES_MACRO),
         LintId::of(&matches::MATCH_OVERLAPPING_ARM),

clippy_lints/src/map_err_ignore.rs

@@ -0,0 +1,117 @@
+use crate::utils::span_lint_and_sugg;
+use rustc_errors::Applicability;
+use rustc_hir::{CaptureBy, Expr, ExprKind, PatKind, QPath};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for instances of `map_err(|_| Some::Enum)`
+    ///
+    /// **Why is this bad?** This map_err throws away the original error rather than allowing the enum to bubble the original error
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// enum Errors {
+    ///    Ignore
+    ///}
+    ///fn main() -> Result<(), Errors> {
+    ///
+    ///    let x = u32::try_from(-123_i32);
+    ///
+    ///    println!("{:?}", x.map_err(|_| Errors::Ignore));
+    ///
+    ///    Ok(())
+    ///}
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// enum Errors {
+    ///    WithContext(TryFromIntError)
+    ///}
+    ///fn main() -> Result<(), Errors> {
+    ///
+    ///    let x = u32::try_from(-123_i32);
+    ///
+    ///    println!("{:?}", x.map_err(|e| Errors::WithContext(e)));
+    ///
+    ///    Ok(())
+    ///}
+    /// ```
+    pub MAP_ERR_IGNORE,
+    style,
+    "`map_err` should not ignore the original error"
+}
+
+declare_lint_pass!(MapErrIgnore => [MAP_ERR_IGNORE]);
+
+impl<'tcx> LateLintPass<'tcx> for MapErrIgnore {
+    // do not try to lint if this is from a macro or desugaring
+    fn check_expr(&mut self, cx: &LateContext<'_>, e: &Expr<'_>) {
+        if e.span.from_expansion() {
+            return;
+        }
+
+        // check if this is a method call (e.g. x.foo())
+        if let ExprKind::MethodCall(ref method, _t_span, ref args, _) = e.kind {
+            // only work if the method name is `map_err` and there are only 2 arguments (e.g. x.map_err(|_|[1]
+            // Enum::Variant[2]))
+            if method.ident.as_str() == "map_err" && args.len() == 2 {
+                // make sure the first argument is a closure, and grab the CaptureRef, body_id, and body_span fields
+                if let ExprKind::Closure(capture, _, body_id, body_span, _) = args[1].kind {
+                    // check if this is by Reference (meaning there's no move statement)
+                    if capture == CaptureBy::Ref {
+                        // Get the closure body to check the parameters and values
+                        let closure_body = cx.tcx.hir().body(body_id);
+                        // make sure there's only one parameter (`|_|`)
+                        if closure_body.params.len() == 1 {
+                            // make sure that parameter is the wild token (`_`)
+                            if let PatKind::Wild = closure_body.params[0].pat.kind {
+                                // Check the value of the closure to see if we can build the enum we are throwing away
+                                // the error for make sure this is a Path
+                                if let ExprKind::Path(q_path) = &closure_body.value.kind {
+                                    // this should be a resolved path, only keep the path field
+                                    if let QPath::Resolved(_, path) = q_path {
+                                        // finally get the idents for each path segment collect them as a string and
+                                        // join them with the path separator ("::"")
+                                        let closure_fold: String = path
+                                            .segments
+                                            .iter()
+                                            .map(|x| x.ident.as_str().to_string())
+                                            .collect::<Vec<String>>()
+                                            .join("::");
+                                        //Span the body of the closure (the |...| bit) and suggest the fix by taking
+                                        // the error and encapsulating it in the enum
+                                        span_lint_and_sugg(
+                                            cx,
+                                            MAP_ERR_IGNORE,
+                                            body_span,
+                                            "`map_err` has thrown away the original error",
+                                            "Allow the error enum to encapsulate the original error",
+                                            format!("|e| {}(e)", closure_fold),
+                                            Applicability::HasPlaceholders,
+                                        );
+                                    }
+                                } else {
+                                    //If we cannot build the enum in a human readable way just suggest not throwing way
+                                    // the error
+                                    span_lint_and_sugg(
+                                        cx,
+                                        MAP_ERR_IGNORE,
+                                        body_span,
+                                        "`map_err` has thrown away the original error",
+                                        "Allow the error enum to encapsulate the original error",
+                                        "|e|".to_string(),
+                                        Applicability::HasPlaceholders,
+                                    );
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

src/lintlist/mod.rs

@@ -1165,6 +1165,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "entry",
     },
+    Lint {
+        name: "map_err_ignore",
+        group: "style",
+        desc: "`map_err` should not ignore the original error",
+        deprecation: None,
+        module: "map_err_ignore",
+    },
     Lint {
         name: "map_flatten",
         group: "pedantic",

tests/ui/drop_ref.rs

@@ -1,5 +1,6 @@
 #![warn(clippy::drop_ref)]
 #![allow(clippy::toplevel_ref_arg)]
+#![allow(clippy::map_err_ignore)]
 
 use std::mem::drop;
 

tests/ui/drop_ref.stderr

@@ -1,108 +1,108 @@
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:9:5
+  --> $DIR/drop_ref.rs:10:5
    |
 LL |     drop(&SomeStruct);
    |     ^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::drop-ref` implied by `-D warnings`
 note: argument has type `&SomeStruct`
-  --> $DIR/drop_ref.rs:9:10
+  --> $DIR/drop_ref.rs:10:10
    |
 LL |     drop(&SomeStruct);
    |          ^^^^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:12:5
+  --> $DIR/drop_ref.rs:13:5
    |
 LL |     drop(&owned1);
    |     ^^^^^^^^^^^^^
    |
 note: argument has type `&SomeStruct`
-  --> $DIR/drop_ref.rs:12:10
+  --> $DIR/drop_ref.rs:13:10
    |
 LL |     drop(&owned1);
    |          ^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:13:5
+  --> $DIR/drop_ref.rs:14:5
    |
 LL |     drop(&&owned1);
    |     ^^^^^^^^^^^^^^
    |
 note: argument has type `&&SomeStruct`
-  --> $DIR/drop_ref.rs:13:10
+  --> $DIR/drop_ref.rs:14:10
    |
 LL |     drop(&&owned1);
    |          ^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:14:5
+  --> $DIR/drop_ref.rs:15:5
    |
 LL |     drop(&mut owned1);
    |     ^^^^^^^^^^^^^^^^^
    |
 note: argument has type `&mut SomeStruct`
-  --> $DIR/drop_ref.rs:14:10
+  --> $DIR/drop_ref.rs:15:10
    |
 LL |     drop(&mut owned1);
    |          ^^^^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:18:5
+  --> $DIR/drop_ref.rs:19:5
    |
 LL |     drop(reference1);
    |     ^^^^^^^^^^^^^^^^
    |
 note: argument has type `&SomeStruct`
-  --> $DIR/drop_ref.rs:18:10
+  --> $DIR/drop_ref.rs:19:10
    |
 LL |     drop(reference1);
    |          ^^^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:21:5
+  --> $DIR/drop_ref.rs:22:5
    |
 LL |     drop(reference2);
    |     ^^^^^^^^^^^^^^^^
    |
 note: argument has type `&mut SomeStruct`
-  --> $DIR/drop_ref.rs:21:10
+  --> $DIR/drop_ref.rs:22:10
    |
 LL |     drop(reference2);
    |          ^^^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:24:5
+  --> $DIR/drop_ref.rs:25:5
    |
 LL |     drop(reference3);
    |     ^^^^^^^^^^^^^^^^
    |
 note: argument has type `&SomeStruct`
-  --> $DIR/drop_ref.rs:24:10
+  --> $DIR/drop_ref.rs:25:10
    |
 LL |     drop(reference3);
    |          ^^^^^^^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:29:5
+  --> $DIR/drop_ref.rs:30:5
    |
 LL |     drop(&val);
    |     ^^^^^^^^^^
    |
 note: argument has type `&T`
-  --> $DIR/drop_ref.rs:29:10
+  --> $DIR/drop_ref.rs:30:10
    |
 LL |     drop(&val);
    |          ^^^^
 
 error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
-  --> $DIR/drop_ref.rs:37:5
+  --> $DIR/drop_ref.rs:38:5
    |
 LL |     std::mem::drop(&SomeStruct);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: argument has type `&SomeStruct`
-  --> $DIR/drop_ref.rs:37:20
+  --> $DIR/drop_ref.rs:38:20
    |
 LL |     std::mem::drop(&SomeStruct);
    |                    ^^^^^^^^^^^

tests/ui/map_err.rs

@@ -0,0 +1,24 @@
+use std::convert::TryFrom;
+use std::error::Error;
+use std::fmt;
+
+#[derive(Debug)]
+enum Errors {
+    Ignored,
+}
+
+impl Error for Errors {}
+
+impl fmt::Display for Errors {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Error")
+    }
+}
+
+fn main() -> Result<(), Errors> {
+    let x = u32::try_from(-123_i32);
+
+    println!("{:?}", x.map_err(|_| Errors::Ignored));
+
+    Ok(())
+}

tests/ui/map_err.stderr

@@ -0,0 +1,10 @@
+error: `map_err` has thrown away the original error
+  --> $DIR/map_err.rs:21:32
+   |
+LL |     println!("{:?}", x.map_err(|_| Errors::Ignored));
+   |                                ^^^ help: Allow the error enum to encapsulate the original error: `|e| Errors::Ignored(e)`
+   |
+   = note: `-D clippy::map-err-ignore` implied by `-D warnings`
+
+error: aborting due to previous error
+