add lint for recreation of an entire struct

clippy_lints/src/redundant_owned_struct_recreation.rs

@@ -0,0 +1,107 @@
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use std::fmt::{self, Write as _};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for struct literals that recreate a struct we already have an owned
+    /// version of.
+    ///
+    /// ### Why is this bad?
+    /// This is essentially a no-op as all members are being moved into the new
+    /// struct and the old one becomes inaccessible.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// fn redundant () {
+    ///     struct Foo { a: i32, b: String }
+    ///
+    ///     let foo = Foo { a: 42, b: "Hello, Foo!".into() };
+    ///
+    ///     let bar = Foo { a: foo.a, b: foo.b };
+    ///
+    ///     println!("redundant: bar = Foo {{ a: {}, b: \"{}\" }}", bar.a, bar.b);
+    /// }
+    /// ```
+    ///
+    /// The struct literal in the assignment to ``bar`` could be replaced with
+    /// ``let bar = foo``.
+    ///
+    /// Empty structs are ignored by the lint.
+    #[clippy::version = "pre 1.29.0"]
+    pub REDUNDANT_OWNED_STRUCT_RECREATION,
+    //pedantic,
+    correctness,
+    "recreation of owned structs from identical parts"
+}
+
+declare_lint_pass!(RedundantOwnedStructRecreation => [REDUNDANT_OWNED_STRUCT_RECREATION]);
+
+impl<'tcx> LateLintPass<'tcx> for RedundantOwnedStructRecreation {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
+        if !expr.span.from_expansion()
+            && let ExprKind::Struct(qpath, fields, base) = expr.kind
+        {
+            let ty = cx.typeck_results().expr_ty(expr);
+
+            // Conditions that must be satisfied to trigger the lint:
+            // - source of the assignment must be a struct
+            // - type of struct in expr must match
+            // - names of assignee struct fields (lhs) must match the names of the assigned on (rhs)
+            // - all fields musts be assigned (no Default)
+
+            if base.is_some() {
+                // one or more fields assigned from `..Default`
+                return;
+            }
+
+            let matching = fields.iter().filter(|f| !f.is_shorthand).try_fold(
+                None,
+                |seen_path, f| -> Result<Option<&'tcx Path<'tcx>>, ()> {
+                    // fields are assigned from expression
+                    if let ExprKind::Field(expr, ident) = f.expr.kind
+                        // expression type matches
+                        && ty == cx.typeck_results().expr_ty(expr)
+                        // field name matches
+                        && f.ident == ident
+                        && let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind
+                    {
+                        match seen_path {
+                            None => {
+                                // first field assignment
+                                Ok(Some(path))
+                            },
+                            Some(p) if p.res == path.res => {
+                                // subsequent field assignment, same origin struct as before
+                                Ok(seen_path)
+                            },
+                            Some(_) => {
+                                // subsequent field assignment, different origin struct as before
+                                Err(())
+                            },
+                        }
+                    } else {
+                        // source of field assignment not an expression
+                        Err(())
+                    }
+                },
+            );
+
+            let Ok(Some(path)) = matching else { return };
+
+            let sugg = format!("{}", snippet(cx, path.span, ".."),);
+
+            span_lint_and_sugg(
+                cx,
+                REDUNDANT_OWNED_STRUCT_RECREATION,
+                expr.span,
+                "recreation of owned struct from identical parts",
+                "try replacing it with",
+                sugg,
+                // the lint may be incomplete due to partial moves
+                // of struct fields
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+}

clippy_lints/src/unnecessary_struct_initialization.rs

@@ -2,14 +2,15 @@ use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
 use clippy_utils::ty::is_copy;
 use clippy_utils::{get_parent_expr, path_to_local};
-use rustc_hir::{BindingMode, Expr, ExprKind, Node, PatKind, UnOp};
+use rustc_hir::{BindingMode, Expr, ExprField, ExprKind, Node, PatKind, Path, QPath, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for initialization of a `struct` by copying a base without setting
-    /// any field.
+    /// Checks for initialization of an identical `struct` from another instance
+    /// of the type, either by copying a base without setting any field or by
+    /// moving all fields individually.
     ///
     /// ### Why is this bad?
     /// Readability suffers from unnecessary struct building.
@@ -29,9 +30,14 @@ declare_clippy_lint! {
     /// let b = a;
     /// ```
     ///
+    /// The struct literal ``S { ..a }`` in the assignment to ``b`` could be replaced
+    /// with just ``a``.
+    ///
     /// ### Known Problems
     /// Has false positives when the base is a place expression that cannot be
     /// moved out of, see [#10547](https://github.com/rust-lang/rust-clippy/issues/10547).
+    ///
+    /// Empty structs are ignored by the lint.
     #[clippy::version = "1.70.0"]
     pub UNNECESSARY_STRUCT_INITIALIZATION,
     nursery,
@@ -41,43 +47,113 @@ declare_lint_pass!(UnnecessaryStruct => [UNNECESSARY_STRUCT_INITIALIZATION]);
 
 impl LateLintPass<'_> for UnnecessaryStruct {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
-        if let ExprKind::Struct(_, &[], Some(base)) = expr.kind {
-            if let Some(parent) = get_parent_expr(cx, expr)
-                && let parent_ty = cx.typeck_results().expr_ty_adjusted(parent)
-                && parent_ty.is_any_ptr()
-            {
-                if is_copy(cx, cx.typeck_results().expr_ty(expr)) && path_to_local(base).is_some() {
-                    // When the type implements `Copy`, a reference to the new struct works on the
-                    // copy. Using the original would borrow it.
-                    return;
-                }
-
-                if parent_ty.is_mutable_ptr() && !is_mutable(cx, base) {
-                    // The original can be used in a mutable reference context only if it is mutable.
-                    return;
-                }
-            }
+        let ExprKind::Struct(_, fields, base) = expr.kind else {
+            return;
+        };
 
-            // TODO: do not propose to replace *XX if XX is not Copy
-            if let ExprKind::Unary(UnOp::Deref, target) = base.kind
-                && matches!(target.kind, ExprKind::Path(..))
-                && !is_copy(cx, cx.typeck_results().expr_ty(expr))
-            {
-                // `*base` cannot be used instead of the struct in the general case if it is not Copy.
-                return;
-            }
+        if expr.span.from_expansion() {
+            // Prevent lint from hitting inside macro code
+            return;
+        }
+
+        // Conditions that must be satisfied to trigger this variant of the lint:
+        // - source of the assignment must be a struct
+        // - type of struct in expr must match
+        // - of destination struct fields must match the names of the source
+        // - all fields musts be assigned (no Default)
+
+        let field_path = same_path_in_all_fields(cx, expr, fields);
+
+        let sugg = match (field_path, base) {
+            (Some(&path), None) => {
+                // all fields match, no base given
+                path.span
+            },
+            /*
+            (Some(&path), Some(&Expr{ kind: ExprKind::Struct(&base_path, _, _), .. }))
+                //if ExprKind::Path(base_path) == path => {
+                if base_path == path => {
+                // all fields match, has base: ensure that the path of the base matches
+                path.span
+            },
+            */
+            (Some(path), Some(base)) if base_is_suitable(cx, expr, base) && path_matches_base(path, base) => {
+                eprintln!("[phg] »»» path: {:?}", path);
+                eprintln!("[phg] »»» base: {:?}", base);
+
+                // all fields match, has base: ensure that the path of the base matches
+                base.span
+            },
+            (None, Some(base)) if fields.is_empty() && base_is_suitable(cx, expr, base) => {
+                // just the base, no explicit fields
+
+                base.span
+            },
+            _ => return,
+        };
+
+        span_lint_and_sugg(
+            cx,
+            UNNECESSARY_STRUCT_INITIALIZATION,
+            expr.span,
+            "unnecessary struct building",
+            "replace with",
+            snippet(cx, sugg, "..").into_owned(),
+            rustc_errors::Applicability::MachineApplicable,
+        );
+    }
+}
+
+fn base_is_suitable(cx: &LateContext<'_>, expr: &Expr<'_>, base: &Expr<'_>) -> bool {
+    if !mutability_matches(cx, expr, base) {
+        return false;
+    }
+    // TODO: do not propose to replace *XX if XX is not Copy
+    if let ExprKind::Unary(UnOp::Deref, target) = base.kind
+        && matches!(target.kind, ExprKind::Path(..))
+        && !is_copy(cx, cx.typeck_results().expr_ty(expr))
+    {
+        // `*base` cannot be used instead of the struct in the general case if it is not Copy.
+        return false;
+    }
+    true
+}
+
+fn same_path_in_all_fields<'tcx>(
+    cx: &LateContext<'_>,
+    expr: &Expr<'_>,
+    fields: &[ExprField<'tcx>],
+) -> Option<&'tcx Path<'tcx>> {
+    let ty = cx.typeck_results().expr_ty(expr);
+
+    let mut seen_path = None;
 
-            span_lint_and_sugg(
-                cx,
-                UNNECESSARY_STRUCT_INITIALIZATION,
-                expr.span,
-                "unnecessary struct building",
-                "replace with",
-                snippet(cx, base.span, "..").into_owned(),
-                rustc_errors::Applicability::MachineApplicable,
-            );
+    for f in fields.iter().filter(|f| !f.is_shorthand) {
+        // fields are assigned from expression
+        if let ExprKind::Field(expr, ident) = f.expr.kind
+            // expression type matches
+            && ty == cx.typeck_results().expr_ty(expr)
+            // field name matches
+            && f.ident == ident
+            // assigned from a path expression
+            && let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind
+        {
+            let Some(p) = seen_path else {
+                // this is the first field assignment in the list
+                seen_path = Some(path);
+                continue;
+            };
+
+            if p.res == path.res {
+                // subsequent field assignment with same origin struct as before
+                continue;
+            }
         }
+        // source of field assignment doesn’t qualify
+        return None;
     }
+
+    seen_path
 }
 
 fn is_mutable(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
@@ -89,3 +165,43 @@ fn is_mutable(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
         true
     }
 }
+
+fn mutability_matches(cx: &LateContext<'_>, expr_a: &Expr<'_>, expr_b: &Expr<'_>) -> bool {
+    if let Some(parent) = get_parent_expr(cx, expr_a)
+        && let parent_ty = cx.typeck_results().expr_ty_adjusted(parent)
+        && parent_ty.is_any_ptr()
+    {
+        if is_copy(cx, cx.typeck_results().expr_ty(expr_a)) && path_to_local(expr_b).is_some() {
+            // When the type implements `Copy`, a reference to the new struct works on the
+            // copy. Using the original would borrow it.
+            return false;
+        }
+
+        if parent_ty.is_mutable_ptr() && !is_mutable(cx, expr_b) {
+            // The original can be used in a mutable reference context only if it is mutable.
+            return false;
+        }
+    }
+
+    true
+}
+
+/// When some fields are assigned from a base struct and others individually
+/// the lint applies only if the source of the field is the same as the base.
+/// This is enforced here by comparing the path of the base expression;
+/// needless to say the link only applies if it (or whatever expression it is
+/// a reference of) actually has a path.
+fn path_matches_base(path: &Path<'_>, base: &Expr<'_>) -> bool {
+    let base_path = match base.kind {
+        ExprKind::Unary(UnOp::Deref, base_expr) => {
+            if let ExprKind::Path(QPath::Resolved(_, base_path)) = base_expr.kind {
+                base_path
+            } else {
+                return false;
+            }
+        },
+        ExprKind::Path(QPath::Resolved(_, base_path)) => base_path,
+        _ => return false,
+    };
+    path.res == base_path.res
+}

tests/ui/unnecessary_struct_initialization.fixed

@@ -26,6 +26,11 @@ struct V {
     f: u32,
 }
 
+struct W {
+    f1: u32,
+    f2: u32,
+}
+
 impl Clone for V {
     fn clone(&self) -> Self {
         // Lint: `Self` implements `Copy`
@@ -68,4 +73,15 @@ fn main() {
 
     // Should lint: the result of an expression is mutable and temporary
     let p = &mut *Box::new(T { f: 5 });
+
+    // Should lint: all fields of `q` would be consumed anyway
+    let q = W { f1: 42, f2: 1337 };
+    let r = q;
+
+    // Should not lint: not all fields of `t` from same source
+    let s = W { f1: 1337, f2: 42 };
+    let t = W { f1: s.f1, f2: r.f2 };
+
+    // Should not lint: different fields of `s` assigned
+    let u = W { f1: s.f2, f2: s.f1 };
 }