Auto merge of #9770 - sgued:missnamed-getters, r=llogiq

Add new lint  [`misnamed-getters`]

```
changelog: Add new lint  [`misnamed-getters`]
```

Closes #9769

The current lint matches all methods with a body of just one expression under the form `(&mut?)? <expr>.field` where field doesn't match the name of the method but there is a field of the same type in `<expr>` that matches the name. This allows matching nested structs, for example for newtype wrappers. This may cast the net a bit too wide and cause false positives. I'll run [clippy_lint_tester](https://github.com/mikerite/clippy_lint_tester) on the top crates to see how frequently false positives happen.

There also may be room for improvement by checking that the replacement field would work taking into account implementations of `Deref` and `DerefMut` even if the types don't exactly match but I don't know yet how this could be done.

6 files changed, 462 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6f1f73c1fd2..5b6b12c623a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4188,6 +4188,7 @@ Released 2018-09-13
 [`misaligned_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#misaligned_transmute
 [`mismatched_target_os`]: https://rust-lang.github.io/rust-clippy/master/index.html#mismatched_target_os
 [`mismatching_type_param_order`]: https://rust-lang.github.io/rust-clippy/master/index.html#mismatching_type_param_order
+[`misnamed_getters`]: https://rust-lang.github.io/rust-clippy/master/index.html#misnamed_getters
 [`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
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index 0d3fc43a644..eb3210946f1 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -177,6 +177,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::from_raw_with_void_ptr::FROM_RAW_WITH_VOID_PTR_INFO,
     crate::from_str_radix_10::FROM_STR_RADIX_10_INFO,
     crate::functions::DOUBLE_MUST_USE_INFO,
+    crate::functions::MISNAMED_GETTERS_INFO,
     crate::functions::MUST_USE_CANDIDATE_INFO,
     crate::functions::MUST_USE_UNIT_INFO,
     crate::functions::NOT_UNSAFE_PTR_ARG_DEREF_INFO,
diff --git a/clippy_lints/src/functions/misnamed_getters.rs b/clippy_lints/src/functions/misnamed_getters.rs
new file mode 100644
index 00000000000..27acad45ccf
--- /dev/null
+++ b/clippy_lints/src/functions/misnamed_getters.rs
@@ -0,0 +1,125 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::{intravisit::FnKind, Body, ExprKind, FnDecl, HirId, ImplicitSelfKind, Unsafety};
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::Span;
+
+use std::iter;
+
+use super::MISNAMED_GETTERS;
+
+pub fn check_fn(
+    cx: &LateContext<'_>,
+    kind: FnKind<'_>,
+    decl: &FnDecl<'_>,
+    body: &Body<'_>,
+    span: Span,
+    _hir_id: HirId,
+) {
+    let FnKind::Method(ref ident, sig) = kind else {
+            return;
+        };
+
+    // Takes only &(mut) self
+    if decl.inputs.len() != 1 {
+        return;
+    }
+
+    let name = ident.name.as_str();
+
+    let name = match decl.implicit_self {
+        ImplicitSelfKind::MutRef => {
+            let Some(name) = name.strip_suffix("_mut") else {
+                    return;
+                };
+            name
+        },
+        ImplicitSelfKind::Imm | ImplicitSelfKind::Mut | ImplicitSelfKind::ImmRef => name,
+        ImplicitSelfKind::None => return,
+    };
+
+    let name = if sig.header.unsafety == Unsafety::Unsafe {
+        name.strip_suffix("_unchecked").unwrap_or(name)
+    } else {
+        name
+    };
+
+    // Body must be &(mut) <self_data>.name
+    // self_data is not neccessarilly self, to also lint sub-getters, etc…
+
+    let block_expr = if_chain! {
+        if let ExprKind::Block(block,_) = body.value.kind;
+        if block.stmts.is_empty();
+        if let Some(block_expr) = block.expr;
+        then {
+            block_expr
+        } else {
+            return;
+        }
+    };
+    let expr_span = block_expr.span;
+
+    // Accept &<expr>, &mut <expr> and <expr>
+    let expr = if let ExprKind::AddrOf(_, _, tmp) = block_expr.kind {
+        tmp
+    } else {
+        block_expr
+    };
+    let (self_data, used_ident) = if_chain! {
+        if let ExprKind::Field(self_data, ident) = expr.kind;
+        if ident.name.as_str() != name;
+        then {
+            (self_data, ident)
+        } else {
+            return;
+        }
+    };
+
+    let mut used_field = None;
+    let mut correct_field = None;
+    let typeck_results = cx.typeck_results();
+    for adjusted_type in iter::once(typeck_results.expr_ty(self_data))
+        .chain(typeck_results.expr_adjustments(self_data).iter().map(|adj| adj.target))
+    {
+        let ty::Adt(def,_) = adjusted_type.kind() else {
+            continue;
+        };
+
+        for f in def.all_fields() {
+            if f.name.as_str() == name {
+                correct_field = Some(f);
+            }
+            if f.name == used_ident.name {
+                used_field = Some(f);
+            }
+        }
+    }
+
+    let Some(used_field) = used_field else {
+        // Can happen if the field access is a tuple. We don't lint those because the getter name could not start with a number.
+        return;
+    };
+
+    let Some(correct_field) = correct_field else {
+        // There is no field corresponding to the getter name.
+        // FIXME: This can be a false positive if the correct field is reachable trought deeper autodereferences than used_field is
+        return;
+    };
+
+    if cx.tcx.type_of(used_field.did) == cx.tcx.type_of(correct_field.did) {
+        let left_span = block_expr.span.until(used_ident.span);
+        let snippet = snippet(cx, left_span, "..");
+        let sugg = format!("{snippet}{name}");
+        span_lint_and_then(
+            cx,
+            MISNAMED_GETTERS,
+            span,
+            "getter function appears to return the wrong field",
+            |diag| {
+                diag.span_suggestion(expr_span, "consider using", sugg, Applicability::MaybeIncorrect);
+            },
+        );
+    }
+}
diff --git a/clippy_lints/src/functions/mod.rs b/clippy_lints/src/functions/mod.rs
index ae0e0833446..91e6ffe6447 100644
--- a/clippy_lints/src/functions/mod.rs
+++ b/clippy_lints/src/functions/mod.rs
@@ -1,3 +1,4 @@
+mod misnamed_getters;
 mod must_use;
 mod not_unsafe_ptr_arg_deref;
 mod result;
@@ -260,6 +261,48 @@ declare_clippy_lint! {
     "function returning `Result` with large `Err` type"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for getter methods that return a field that doesn't correspond
+    /// to the name of the method, when there is a field's whose name matches that of the method.
+    ///
+    /// ### Why is this bad?
+    /// It is most likely that such a  method is a bug caused by a typo or by copy-pasting.
+    ///
+    /// ### Example
+
+    /// ```rust
+    /// struct A {
+    ///     a: String,
+    ///     b: String,
+    /// }
+    ///
+    /// impl A {
+    ///     fn a(&self) -> &str{
+    ///         &self.b
+    ///     }
+    /// }
+
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct A {
+    ///     a: String,
+    ///     b: String,
+    /// }
+    ///
+    /// impl A {
+    ///     fn a(&self) -> &str{
+    ///         &self.a
+    ///     }
+    /// }
+    /// ```
+    #[clippy::version = "1.67.0"]
+    pub MISNAMED_GETTERS,
+    suspicious,
+    "getter method returning the wrong field"
+}
+
 #[derive(Copy, Clone)]
 pub struct Functions {
     too_many_arguments_threshold: u64,
@@ -286,6 +329,7 @@ impl_lint_pass!(Functions => [
     MUST_USE_CANDIDATE,
     RESULT_UNIT_ERR,
     RESULT_LARGE_ERR,
+    MISNAMED_GETTERS,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Functions {
@@ -301,6 +345,7 @@ impl<'tcx> LateLintPass<'tcx> for Functions {
         too_many_arguments::check_fn(cx, kind, decl, span, hir_id, self.too_many_arguments_threshold);
         too_many_lines::check_fn(cx, kind, span, body, self.too_many_lines_threshold);
         not_unsafe_ptr_arg_deref::check_fn(cx, kind, decl, body, hir_id);
+        misnamed_getters::check_fn(cx, kind, decl, body, span, hir_id);
     }
 
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
diff --git a/tests/ui/misnamed_getters.rs b/tests/ui/misnamed_getters.rs
new file mode 100644
index 00000000000..03e7dac7df9
--- /dev/null
+++ b/tests/ui/misnamed_getters.rs
@@ -0,0 +1,124 @@
+#![allow(unused)]
+#![warn(clippy::misnamed_getters)]
+
+struct A {
+    a: u8,
+    b: u8,
+    c: u8,
+}
+
+impl A {
+    fn a(&self) -> &u8 {
+        &self.b
+    }
+    fn a_mut(&mut self) -> &mut u8 {
+        &mut self.b
+    }
+
+    fn b(self) -> u8 {
+        self.a
+    }
+
+    fn b_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+
+    fn c(&self) -> &u8 {
+        &self.b
+    }
+
+    fn c_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+}
+
+union B {
+    a: u8,
+    b: u8,
+}
+
+impl B {
+    unsafe fn a(&self) -> &u8 {
+        &self.b
+    }
+    unsafe fn a_mut(&mut self) -> &mut u8 {
+        &mut self.b
+    }
+
+    unsafe fn b(self) -> u8 {
+        self.a
+    }
+
+    unsafe fn b_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+
+    unsafe fn c(&self) -> &u8 {
+        &self.b
+    }
+
+    unsafe fn c_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+
+    unsafe fn a_unchecked(&self) -> &u8 {
+        &self.b
+    }
+    unsafe fn a_unchecked_mut(&mut self) -> &mut u8 {
+        &mut self.b
+    }
+
+    unsafe fn b_unchecked(self) -> u8 {
+        self.a
+    }
+
+    unsafe fn b_unchecked_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+
+    unsafe fn c_unchecked(&self) -> &u8 {
+        &self.b
+    }
+
+    unsafe fn c_unchecked_mut(&mut self) -> &mut u8 {
+        &mut self.a
+    }
+}
+
+struct D {
+    d: u8,
+    inner: A,
+}
+
+impl core::ops::Deref for D {
+    type Target = A;
+    fn deref(&self) -> &A {
+        &self.inner
+    }
+}
+
+impl core::ops::DerefMut for D {
+    fn deref_mut(&mut self) -> &mut A {
+        &mut self.inner
+    }
+}
+
+impl D {
+    fn a(&self) -> &u8 {
+        &self.b
+    }
+    fn a_mut(&mut self) -> &mut u8 {
+        &mut self.b
+    }
+
+    fn d(&self) -> &u8 {
+        &self.b
+    }
+    fn d_mut(&mut self) -> &mut u8 {
+        &mut self.b
+    }
+}
+
+fn main() {
+    // test code goes here
+}
diff --git a/tests/ui/misnamed_getters.stderr b/tests/ui/misnamed_getters.stderr
new file mode 100644
index 00000000000..1e38a83d019
--- /dev/null
+++ b/tests/ui/misnamed_getters.stderr
@@ -0,0 +1,166 @@
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:11:5
+   |
+LL | /     fn a(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.a`
+LL | |     }
+   | |_____^
+   |
+   = note: `-D clippy::misnamed-getters` implied by `-D warnings`
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:14:5
+   |
+LL | /     fn a_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.b
+   | |         ----------- help: consider using: `&mut self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:18:5
+   |
+LL | /     fn b(self) -> u8 {
+LL | |         self.a
+   | |         ------ help: consider using: `self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:22:5
+   |
+LL | /     fn b_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.a
+   | |         ----------- help: consider using: `&mut self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:26:5
+   |
+LL | /     fn c(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.c`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:30:5
+   |
+LL | /     fn c_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.a
+   | |         ----------- help: consider using: `&mut self.c`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:41:5
+   |
+LL | /     unsafe fn a(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:44:5
+   |
+LL | /     unsafe fn a_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.b
+   | |         ----------- help: consider using: `&mut self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:48:5
+   |
+LL | /     unsafe fn b(self) -> u8 {
+LL | |         self.a
+   | |         ------ help: consider using: `self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:52:5
+   |
+LL | /     unsafe fn b_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.a
+   | |         ----------- help: consider using: `&mut self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:64:5
+   |
+LL | /     unsafe fn a_unchecked(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:67:5
+   |
+LL | /     unsafe fn a_unchecked_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.b
+   | |         ----------- help: consider using: `&mut self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:71:5
+   |
+LL | /     unsafe fn b_unchecked(self) -> u8 {
+LL | |         self.a
+   | |         ------ help: consider using: `self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:75:5
+   |
+LL | /     unsafe fn b_unchecked_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.a
+   | |         ----------- help: consider using: `&mut self.b`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:107:5
+   |
+LL | /     fn a(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:110:5
+   |
+LL | /     fn a_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.b
+   | |         ----------- help: consider using: `&mut self.a`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:114:5
+   |
+LL | /     fn d(&self) -> &u8 {
+LL | |         &self.b
+   | |         ------- help: consider using: `&self.d`
+LL | |     }
+   | |_____^
+
+error: getter function appears to return the wrong field
+  --> $DIR/misnamed_getters.rs:117:5
+   |
+LL | /     fn d_mut(&mut self) -> &mut u8 {
+LL | |         &mut self.b
+   | |         ----------- help: consider using: `&mut self.d`
+LL | |     }
+   | |_____^
+
+error: aborting due to 18 previous errors
+