Auto merge of #12772 - phi-gamma:redundant-struct-recreation, r=y21

add lint for recreation of an entire struct

This lint makes Clippy warn about situations where an owned struct is
essentially recreated by moving all its fields into a new instance of
the struct. The lint is not machine-applicable because the source
struct may have been partially moved.

This lint originated in something I spotted during peer review. While
working on their branch a colleague ended up with a commit where a
function returned a struct that 1:1 replicated one of its owned inputs
from its members. Initially I suspected they hadn’t run their code
through Clippy but AFAICS there is no lint for this situation yet.

changelog: new lint: [`redundant_owned_struct_recreation`]

### New lint checklist

- \[+] Followed [lint naming conventions][lint_naming]
- \[+] Added passing UI tests (including committed `.stderr` file)
- \[+] `cargo test` passes locally
- \[+] Executed `cargo dev update_lints`
- \[+] Added lint documentation
- \[+] Run `cargo dev fmt`

4 files changed, 328 insertions, 43 deletions

diff --git a/clippy_lints/src/unnecessary_struct_initialization.rs b/clippy_lints/src/unnecessary_struct_initialization.rs
index 2da75334344..afdd3505cdd 100644
--- a/clippy_lints/src/unnecessary_struct_initialization.rs
+++ b/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,
@@ -41,42 +47,111 @@ 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;
+        }
+
+        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(base)) if base_is_suitable(cx, expr, base) && path_matches_base(path, 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 !check_references(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
+}
+
+/// Check whether all fields of a struct assignment match.
+/// Returns a [Path] item that one can obtain a span from for the lint suggestion.
+///
+/// Conditions that must be satisfied to trigger this variant of the lint:
+///
+/// - source struct of the assignment must be of same type as the destination
+/// - names of destination struct fields must match the field names of the source
+///
+/// We don’t check here if all struct fields are assigned as the remainder may
+/// be filled in from a base struct.
+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 found = 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 {
+        // fields are assigned from expression
+        if let ExprKind::Field(src_expr, ident) = f.expr.kind
+            // expression type matches
+            && ty == cx.typeck_results().expr_ty(src_expr)
+            // field name matches
+            && f.ident == ident
+            // assigned from a path expression
+            && let ExprKind::Path(QPath::Resolved(None, src_path)) = src_expr.kind
+        {
+            let Some((_, p)) = found else {
+                // this is the first field assignment in the list
+                found = Some((src_expr, src_path));
+                continue;
+            };
+
+            if p.res == src_path.res {
+                // subsequent field assignment with same origin struct as before
+                continue;
+            }
         }
+        // source of field assignment doesn’t qualify
+        return None;
+    }
+
+    if let Some((src_expr, src_path)) = found
+        && check_references(cx, expr, src_expr)
+    {
+        Some(src_path)
+    } else {
+        None
     }
 }
 
@@ -89,3 +164,43 @@ fn is_mutable(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
         true
     }
 }
+
+fn check_references(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 lint 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
+}
diff --git a/tests/ui/unnecessary_struct_initialization.fixed b/tests/ui/unnecessary_struct_initialization.fixed
index f3cf65da2d6..b0e8f454635 100644
--- a/tests/ui/unnecessary_struct_initialization.fixed
+++ b/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 +73,69 @@ 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 };
+
+    // Should lint: all fields of `v` would be consumed anyway
+    let v = W { f1: 42, f2: 1337 };
+    let w = v;
+
+    // Should not lint: source differs between fields and base
+    let x = W { f1: 42, f2: 1337 };
+    let y = W { f1: w.f1, ..x };
+
+    // Should lint: range desugars to struct
+    let r1 = 0..5;
+    let r2 = r1;
+
+    references();
+    shorthand();
+}
+
+fn references() {
+    // Should not lint as `a` is not mutable
+    let a = W { f1: 42, f2: 1337 };
+    let b = &mut W { f1: a.f1, f2: a.f2 };
+
+    // Should lint as `d` is a shared reference
+    let c = W { f1: 42, f2: 1337 };
+    let d = &c;
+
+    // Should not lint as `e` is not mutable
+    let e = W { f1: 42, f2: 1337 };
+    let f = &mut W { f1: e.f1, ..e };
+
+    // Should lint as `h` is a shared reference
+    let g = W { f1: 42, f2: 1337 };
+    let h = &g;
+
+    // Should not lint as `j` is copy
+    let i = V { f: 0x1701d };
+    let j = &V { ..i };
+
+    // Should not lint as `k` is copy
+    let k = V { f: 0x1701d };
+    let l = &V { f: k.f };
+}
+
+fn shorthand() {
+    struct S1 {
+        a: i32,
+        b: i32,
+    }
+
+    let a = 42;
+    let s = S1 { a: 3, b: 4 };
+
+    // Should not lint: `a` is not from `s`
+    let s = S1 { a, b: s.b };
 }
diff --git a/tests/ui/unnecessary_struct_initialization.rs b/tests/ui/unnecessary_struct_initialization.rs
index bd5302f9d85..b0db71af4d4 100644
--- a/tests/ui/unnecessary_struct_initialization.rs
+++ b/tests/ui/unnecessary_struct_initialization.rs
@@ -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`
@@ -72,4 +77,69 @@ fn main() {
     let p = &mut T {
         ..*Box::new(T { f: 5 })
     };
+
+    // Should lint: all fields of `q` would be consumed anyway
+    let q = W { f1: 42, f2: 1337 };
+    let r = W { f1: q.f1, f2: q.f2 };
+
+    // 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 };
+
+    // Should lint: all fields of `v` would be consumed anyway
+    let v = W { f1: 42, f2: 1337 };
+    let w = W { f1: v.f1, ..v };
+
+    // Should not lint: source differs between fields and base
+    let x = W { f1: 42, f2: 1337 };
+    let y = W { f1: w.f1, ..x };
+
+    // Should lint: range desugars to struct
+    let r1 = 0..5;
+    let r2 = r1.start..r1.end;
+
+    references();
+    shorthand();
+}
+
+fn references() {
+    // Should not lint as `a` is not mutable
+    let a = W { f1: 42, f2: 1337 };
+    let b = &mut W { f1: a.f1, f2: a.f2 };
+
+    // Should lint as `d` is a shared reference
+    let c = W { f1: 42, f2: 1337 };
+    let d = &W { f1: c.f1, f2: c.f2 };
+
+    // Should not lint as `e` is not mutable
+    let e = W { f1: 42, f2: 1337 };
+    let f = &mut W { f1: e.f1, ..e };
+
+    // Should lint as `h` is a shared reference
+    let g = W { f1: 42, f2: 1337 };
+    let h = &W { f1: g.f1, ..g };
+
+    // Should not lint as `j` is copy
+    let i = V { f: 0x1701d };
+    let j = &V { ..i };
+
+    // Should not lint as `k` is copy
+    let k = V { f: 0x1701d };
+    let l = &V { f: k.f };
+}
+
+fn shorthand() {
+    struct S1 {
+        a: i32,
+        b: i32,
+    }
+
+    let a = 42;
+    let s = S1 { a: 3, b: 4 };
+
+    // Should not lint: `a` is not from `s`
+    let s = S1 { a, b: s.b };
 }
diff --git a/tests/ui/unnecessary_struct_initialization.stderr b/tests/ui/unnecessary_struct_initialization.stderr
index 8bc308c7567..56982cc0a39 100644
--- a/tests/ui/unnecessary_struct_initialization.stderr
+++ b/tests/ui/unnecessary_struct_initialization.stderr
@@ -1,5 +1,5 @@
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:32:9
+  --> tests/ui/unnecessary_struct_initialization.rs:37:9
    |
 LL |         Self { ..*self }
    |         ^^^^^^^^^^^^^^^^ help: replace with: `*self`
@@ -8,25 +8,25 @@ LL |         Self { ..*self }
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_struct_initialization)]`
 
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:39:17
+  --> tests/ui/unnecessary_struct_initialization.rs:44:17
    |
 LL |     let mut b = S { ..a };
    |                 ^^^^^^^^^ help: replace with: `a`
 
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:42:18
+  --> tests/ui/unnecessary_struct_initialization.rs:47:18
    |
 LL |     let c = &mut S { ..b };
    |                  ^^^^^^^^^ help: replace with: `b`
 
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:50:14
+  --> tests/ui/unnecessary_struct_initialization.rs:55:14
    |
 LL |     let g = &S { ..f };
    |              ^^^^^^^^^ help: replace with: `f`
 
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:53:18
+  --> tests/ui/unnecessary_struct_initialization.rs:58:18
    |
 LL |       let h = &mut S {
    |  __________________^
@@ -35,7 +35,7 @@ LL | |     };
    | |_____^ help: replace with: `*Box::new(S { f: String::from("foo") })`
 
 error: unnecessary struct building
-  --> tests/ui/unnecessary_struct_initialization.rs:72:18
+  --> tests/ui/unnecessary_struct_initialization.rs:77:18
    |
 LL |       let p = &mut T {
    |  __________________^
@@ -43,5 +43,35 @@ LL | |         ..*Box::new(T { f: 5 })
 LL | |     };
    | |_____^ help: replace with: `*Box::new(T { f: 5 })`
 
-error: aborting due to 6 previous errors
+error: unnecessary struct building
+  --> tests/ui/unnecessary_struct_initialization.rs:83:13
+   |
+LL |     let r = W { f1: q.f1, f2: q.f2 };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `q`
+
+error: unnecessary struct building
+  --> tests/ui/unnecessary_struct_initialization.rs:94:13
+   |
+LL |     let w = W { f1: v.f1, ..v };
+   |             ^^^^^^^^^^^^^^^^^^^ help: replace with: `v`
+
+error: unnecessary struct building
+  --> tests/ui/unnecessary_struct_initialization.rs:102:14
+   |
+LL |     let r2 = r1.start..r1.end;
+   |              ^^^^^^^^^^^^^^^^ help: replace with: `r1`
+
+error: unnecessary struct building
+  --> tests/ui/unnecessary_struct_initialization.rs:115:14
+   |
+LL |     let d = &W { f1: c.f1, f2: c.f2 };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `c`
+
+error: unnecessary struct building
+  --> tests/ui/unnecessary_struct_initialization.rs:123:14
+   |
+LL |     let h = &W { f1: g.f1, ..g };
+   |              ^^^^^^^^^^^^^^^^^^^ help: replace with: `g`
+
+error: aborting due to 11 previous errors