Auto merge of #6980 - Jarcho:len_without_is_empty_sig, r=llogiq

`len_without_is_empty` improvements

fixes: #6958
fixes: #6972

changelog: Check the return type of `len`. Only integral types, or an `Option` or `Result` wrapping one.
changelog: Ensure the return type of `is_empty` matches. e.g. `Option<usize>` -> `Option<bool>`

Author: bors

clippy_lints/src/len_zero.rs

@@ -10,9 +10,12 @@ use rustc_hir::{
     ItemKind, Mutability, Node, TraitItemRef, TyKind,
 };
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty::{self, AssocKind, FnSig};
+use rustc_middle::ty::{self, AssocKind, FnSig, Ty, TyS};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::source_map::{Span, Spanned, Symbol};
+use rustc_span::{
+    source_map::{Span, Spanned, Symbol},
+    symbol::sym,
+};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for getting the length of something via `.len()`
@@ -137,6 +140,7 @@ impl<'tcx> LateLintPass<'tcx> for LenZero {
             if let Some(local_id) = ty_id.as_local();
             let ty_hir_id = cx.tcx.hir().local_def_id_to_hir_id(local_id);
             if !is_allowed(cx, LEN_WITHOUT_IS_EMPTY, ty_hir_id);
+            if let Some(output) = parse_len_output(cx, cx.tcx.fn_sig(item.def_id).skip_binder());
             then {
                 let (name, kind) = match cx.tcx.hir().find(ty_hir_id) {
                     Some(Node::ForeignItem(x)) => (x.ident.name, "extern type"),
@@ -148,7 +152,7 @@ impl<'tcx> LateLintPass<'tcx> for LenZero {
                     }
                     _ => return,
                 };
-                check_for_is_empty(cx, sig.span, sig.decl.implicit_self, ty_id, name, kind)
+                check_for_is_empty(cx, sig.span, sig.decl.implicit_self, output, ty_id, name, kind)
             }
         }
     }
@@ -231,10 +235,62 @@ fn check_trait_items(cx: &LateContext<'_>, visited_trait: &Item<'_>, trait_items
     }
 }
 
+#[derive(Debug, Clone, Copy)]
+enum LenOutput<'tcx> {
+    Integral,
+    Option(DefId),
+    Result(DefId, Ty<'tcx>),
+}
+fn parse_len_output(cx: &LateContext<'_>, sig: FnSig<'tcx>) -> Option<LenOutput<'tcx>> {
+    match *sig.output().kind() {
+        ty::Int(_) | ty::Uint(_) => Some(LenOutput::Integral),
+        ty::Adt(adt, subs) if cx.tcx.is_diagnostic_item(sym::option_type, adt.did) => {
+            subs.type_at(0).is_integral().then(|| LenOutput::Option(adt.did))
+        },
+        ty::Adt(adt, subs) if cx.tcx.is_diagnostic_item(sym::result_type, adt.did) => subs
+            .type_at(0)
+            .is_integral()
+            .then(|| LenOutput::Result(adt.did, subs.type_at(1))),
+        _ => None,
+    }
+}
+
+impl LenOutput<'_> {
+    fn matches_is_empty_output(self, ty: Ty<'_>) -> bool {
+        match (self, ty.kind()) {
+            (_, &ty::Bool) => true,
+            (Self::Option(id), &ty::Adt(adt, subs)) if id == adt.did => subs.type_at(0).is_bool(),
+            (Self::Result(id, err_ty), &ty::Adt(adt, subs)) if id == adt.did => {
+                subs.type_at(0).is_bool() && TyS::same_type(subs.type_at(1), err_ty)
+            },
+            _ => false,
+        }
+    }
+
+    fn expected_sig(self, self_kind: ImplicitSelfKind) -> String {
+        let self_ref = match self_kind {
+            ImplicitSelfKind::ImmRef => "&",
+            ImplicitSelfKind::MutRef => "&mut ",
+            _ => "",
+        };
+        match self {
+            Self::Integral => format!("expected signature: `({}self) -> bool`", self_ref),
+            Self::Option(_) => format!(
+                "expected signature: `({}self) -> bool` or `({}self) -> Option<bool>",
+                self_ref, self_ref
+            ),
+            Self::Result(..) => format!(
+                "expected signature: `({}self) -> bool` or `({}self) -> Result<bool>",
+                self_ref, self_ref
+            ),
+        }
+    }
+}
+
 /// Checks if the given signature matches the expectations for `is_empty`
-fn check_is_empty_sig(cx: &LateContext<'_>, sig: FnSig<'_>, self_kind: ImplicitSelfKind) -> bool {
+fn check_is_empty_sig(sig: FnSig<'_>, self_kind: ImplicitSelfKind, len_output: LenOutput<'_>) -> bool {
     match &**sig.inputs_and_output {
-        [arg, res] if *res == cx.tcx.types.bool => {
+        [arg, res] if len_output.matches_is_empty_output(res) => {
             matches!(
                 (arg.kind(), self_kind),
                 (ty::Ref(_, _, Mutability::Not), ImplicitSelfKind::ImmRef)
@@ -250,6 +306,7 @@ fn check_for_is_empty(
     cx: &LateContext<'_>,
     span: Span,
     self_kind: ImplicitSelfKind,
+    output: LenOutput<'_>,
     impl_ty: DefId,
     item_name: Symbol,
     item_kind: &str,
@@ -289,7 +346,7 @@ fn check_for_is_empty(
         },
         Some(is_empty)
             if !(is_empty.fn_has_self_parameter
-                && check_is_empty_sig(cx, cx.tcx.fn_sig(is_empty.def_id).skip_binder(), self_kind)) =>
+                && check_is_empty_sig(cx.tcx.fn_sig(is_empty.def_id).skip_binder(), self_kind, output)) =>
         {
             (
                 format!(
@@ -309,14 +366,7 @@ fn check_for_is_empty(
             db.span_note(span, "`is_empty` defined here");
         }
         if let Some(self_kind) = self_kind {
-            db.note(&format!(
-                "expected signature: `({}self) -> bool`",
-                match self_kind {
-                    ImplicitSelfKind::ImmRef => "&",
-                    ImplicitSelfKind::MutRef => "&mut ",
-                    _ => "",
-                }
-            ));
+            db.note(&output.expected_sig(self_kind));
         }
     });
 }

tests/ui/len_without_is_empty.rs

@@ -1,3 +1,5 @@
+// edition:2018
+
 #![warn(clippy::len_without_is_empty)]
 #![allow(dead_code, unused)]
 
@@ -172,9 +174,9 @@ pub trait DependsOnFoo: Foo {
     fn len(&mut self) -> usize;
 }
 
+// issue #1562
 pub struct MultipleImpls;
 
-// issue #1562
 impl MultipleImpls {
     pub fn len(&self) -> usize {
         1
@@ -187,4 +189,99 @@ impl MultipleImpls {
     }
 }
 
+// issue #6958
+pub struct OptionalLen;
+
+impl OptionalLen {
+    pub fn len(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    pub fn is_empty(&self) -> Option<bool> {
+        Some(true)
+    }
+}
+
+pub struct OptionalLen2;
+impl OptionalLen2 {
+    pub fn len(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        true
+    }
+}
+
+pub struct OptionalLen3;
+impl OptionalLen3 {
+    pub fn len(&self) -> usize {
+        0
+    }
+
+    // should lint, len is not an option
+    pub fn is_empty(&self) -> Option<bool> {
+        None
+    }
+}
+
+pub struct ResultLen;
+impl ResultLen {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    // Differing result types
+    pub fn is_empty(&self) -> Option<bool> {
+        Some(true)
+    }
+}
+
+pub struct ResultLen2;
+impl ResultLen2 {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    pub fn is_empty(&self) -> Result<bool, ()> {
+        Ok(true)
+    }
+}
+
+pub struct ResultLen3;
+impl ResultLen3 {
+    pub fn len(&self) -> Result<usize, ()> {
+        Ok(0)
+    }
+
+    // Non-fallible result is ok.
+    pub fn is_empty(&self) -> bool {
+        true
+    }
+}
+
+pub struct OddLenSig;
+impl OddLenSig {
+    // don't lint
+    pub fn len(&self) -> bool {
+        true
+    }
+}
+
+// issue #6958
+pub struct AsyncLen;
+impl AsyncLen {
+    async fn async_task(&self) -> bool {
+        true
+    }
+
+    pub async fn len(&self) -> usize {
+        if self.async_task().await { 0 } else { 1 }
+    }
+
+    pub async fn is_empty(&self) -> bool {
+        self.len().await == 0
+    }
+}
+
 fn main() {}

tests/ui/len_without_is_empty.stderr

@@ -1,64 +1,123 @@
 error: struct `PubOne` has a public `len` method, but no `is_empty` method
-  --> $DIR/len_without_is_empty.rs:7:5
+  --> $DIR/len_without_is_empty.rs:9:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::len-without-is-empty` implied by `-D warnings`
 
 error: trait `PubTraitsToo` has a `len` method but no (possibly inherited) `is_empty` method
-  --> $DIR/len_without_is_empty.rs:55:1
+  --> $DIR/len_without_is_empty.rs:57:1
    |
 LL | / pub trait PubTraitsToo {
 LL | |     fn len(&self) -> isize;
 LL | | }
    | |_^
 
 error: struct `HasIsEmpty` has a public `len` method, but a private `is_empty` method
-  --> $DIR/len_without_is_empty.rs:68:5
+  --> $DIR/len_without_is_empty.rs:70:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:72:5
+  --> $DIR/len_without_is_empty.rs:74:5
    |
 LL |     fn is_empty(&self) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: struct `HasWrongIsEmpty` has a public `len` method, but the `is_empty` method has an unexpected signature
-  --> $DIR/len_without_is_empty.rs:80:5
+  --> $DIR/len_without_is_empty.rs:82:5
    |
 LL |     pub fn len(&self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:84:5
+  --> $DIR/len_without_is_empty.rs:86:5
    |
 LL |     pub fn is_empty(&self, x: u32) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(&self) -> bool`
 
 error: struct `MismatchedSelf` has a public `len` method, but the `is_empty` method has an unexpected signature
-  --> $DIR/len_without_is_empty.rs:92:5
+  --> $DIR/len_without_is_empty.rs:94:5
    |
 LL |     pub fn len(self) -> isize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: `is_empty` defined here
-  --> $DIR/len_without_is_empty.rs:96:5
+  --> $DIR/len_without_is_empty.rs:98:5
    |
 LL |     pub fn is_empty(&self) -> bool {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(self) -> bool`
 
 error: trait `DependsOnFoo` has a `len` method but no (possibly inherited) `is_empty` method
-  --> $DIR/len_without_is_empty.rs:171:1
+  --> $DIR/len_without_is_empty.rs:173:1
    |
 LL | / pub trait DependsOnFoo: Foo {
 LL | |     fn len(&mut self) -> usize;
 LL | | }
    | |_^
 
-error: aborting due to 6 previous errors
+error: struct `OptionalLen3` has a public `len` method, but the `is_empty` method has an unexpected signature
+  --> $DIR/len_without_is_empty.rs:218:5
+   |
+LL |     pub fn len(&self) -> usize {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `is_empty` defined here
+  --> $DIR/len_without_is_empty.rs:223:5
+   |
+LL |     pub fn is_empty(&self) -> Option<bool> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected signature: `(&self) -> bool`
+
+error: struct `ResultLen` has a public `len` method, but the `is_empty` method has an unexpected signature
+  --> $DIR/len_without_is_empty.rs:230:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `is_empty` defined here
+  --> $DIR/len_without_is_empty.rs:235:5
+   |
+LL |     pub fn is_empty(&self) -> Option<bool> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected signature: `(&self) -> bool` or `(&self) -> Result<bool>
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:230:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::result-unit-err` implied by `-D warnings`
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:242:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:246:5
+   |
+LL |     pub fn is_empty(&self) -> Result<bool, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: this returns a `Result<_, ()>
+  --> $DIR/len_without_is_empty.rs:253:5
+   |
+LL |     pub fn len(&self) -> Result<usize, ()> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom Error type instead
+
+error: aborting due to 12 previous errors