New lint: detect unnecessary struct building

CHANGELOG.md

@@ -4987,6 +4987,7 @@ Released 2018-09-13
 [`unnecessary_safety_doc`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_doc
 [`unnecessary_self_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_self_imports
 [`unnecessary_sort_by`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_sort_by
+[`unnecessary_struct_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_struct_initialization
 [`unnecessary_to_owned`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned
 [`unnecessary_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_unwrap
 [`unnecessary_wraps`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_wraps

clippy_lints/src/declared_lints.rs

@@ -618,6 +618,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::unnamed_address::VTABLE_ADDRESS_COMPARISONS_INFO,
     crate::unnecessary_owned_empty_strings::UNNECESSARY_OWNED_EMPTY_STRINGS_INFO,
     crate::unnecessary_self_imports::UNNECESSARY_SELF_IMPORTS_INFO,
+    crate::unnecessary_struct_initialization::UNNECESSARY_STRUCT_INITIALIZATION_INFO,
     crate::unnecessary_wraps::UNNECESSARY_WRAPS_INFO,
     crate::unnested_or_patterns::UNNESTED_OR_PATTERNS_INFO,
     crate::unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME_INFO,

clippy_lints/src/lib.rs

@@ -302,6 +302,7 @@ mod unit_types;
 mod unnamed_address;
 mod unnecessary_owned_empty_strings;
 mod unnecessary_self_imports;
+mod unnecessary_struct_initialization;
 mod unnecessary_wraps;
 mod unnested_or_patterns;
 mod unsafe_removed_from_name;
@@ -938,6 +939,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(let_with_type_underscore::UnderscoreTyped));
     store.register_late_pass(|_| Box::new(allow_attributes::AllowAttribute));
     store.register_late_pass(move |_| Box::new(manual_main_separator_str::ManualMainSeparatorStr::new(msrv())));
+    store.register_late_pass(|_| Box::new(unnecessary_struct_initialization::UnnecessaryStruct));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/unnecessary_struct_initialization.rs

@@ -0,0 +1,84 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, get_parent_expr, path_to_local, source::snippet, ty::is_copy};
+use rustc_hir::{BindingAnnotation, Expr, ExprKind, Node, PatKind, UnOp};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for initialization of a `struct` by copying a base without setting
+    /// any field.
+    ///
+    /// ### Why is this bad?
+    /// Readibility suffers from unnecessary struct building.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct S { s: String }
+    ///
+    /// let a = S { s: String::from("Hello, world!") };
+    /// let b = S { ..a };
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct S { s: String }
+    ///
+    /// let a = S { s: String::from("Hello, world!") };
+    /// let b = a;
+    /// ```
+    #[clippy::version = "1.70.0"]
+    pub UNNECESSARY_STRUCT_INITIALIZATION,
+    complexity,
+    "struct built from a base that can be written mode concisely"
+}
+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;
+                }
+            }
+
+            // 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;
+            }
+
+            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,
+            );
+        }
+    }
+}
+
+fn is_mutable(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    if let Some(hir_id) = path_to_local(expr) &&
+        let Node::Pat(pat) = cx.tcx.hir().get(hir_id)
+    {
+        matches!(pat.kind, PatKind::Binding(BindingAnnotation::MUT, ..))
+    } else {
+        true
+    }
+}

tests/ui/needless_update.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::needless_update)]
-#![allow(clippy::no_effect)]
+#![allow(clippy::no_effect, clippy::unnecessary_struct_initialization)]
 
 struct S {
     pub a: i32,

tests/ui/no_effect.rs

@@ -1,7 +1,12 @@
 #![feature(box_syntax, fn_traits, unboxed_closures)]
 #![warn(clippy::no_effect_underscore_binding)]
 #![allow(dead_code, path_statements)]
-#![allow(clippy::deref_addrof, clippy::redundant_field_names, clippy::uninlined_format_args)]
+#![allow(
+    clippy::deref_addrof,
+    clippy::redundant_field_names,
+    clippy::uninlined_format_args,
+    clippy::unnecessary_struct_initialization
+)]
 
 struct Unit;
 struct Tuple(i32);

tests/ui/no_effect.stderr

@@ -1,5 +1,5 @@
 error: statement with no effect
-  --> $DIR/no_effect.rs:92:5
+  --> $DIR/no_effect.rs:97:5
    |
 LL |     0;
    |     ^^
@@ -7,157 +7,157 @@ LL |     0;
    = note: `-D clippy::no-effect` implied by `-D warnings`
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:93:5
+  --> $DIR/no_effect.rs:98:5
    |
 LL |     s2;
    |     ^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:94:5
+  --> $DIR/no_effect.rs:99:5
    |
 LL |     Unit;
    |     ^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:95:5
+  --> $DIR/no_effect.rs:100:5
    |
 LL |     Tuple(0);
    |     ^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:96:5
+  --> $DIR/no_effect.rs:101:5
    |
 LL |     Struct { field: 0 };
    |     ^^^^^^^^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:97:5
+  --> $DIR/no_effect.rs:102:5
    |
 LL |     Struct { ..s };
    |     ^^^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:98:5
+  --> $DIR/no_effect.rs:103:5
    |
 LL |     Union { a: 0 };
    |     ^^^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:99:5
+  --> $DIR/no_effect.rs:104:5
    |
 LL |     Enum::Tuple(0);
    |     ^^^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:100:5
+  --> $DIR/no_effect.rs:105:5
    |
 LL |     Enum::Struct { field: 0 };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:101:5
+  --> $DIR/no_effect.rs:106:5
    |
 LL |     5 + 6;
    |     ^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:102:5
+  --> $DIR/no_effect.rs:107:5
    |
 LL |     *&42;
    |     ^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:103:5
+  --> $DIR/no_effect.rs:108:5
    |
 LL |     &6;
    |     ^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:104:5
+  --> $DIR/no_effect.rs:109:5
    |
 LL |     (5, 6, 7);
    |     ^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:105:5
+  --> $DIR/no_effect.rs:110:5
    |
 LL |     box 42;
    |     ^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:106:5
+  --> $DIR/no_effect.rs:111:5
    |
 LL |     ..;
    |     ^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:107:5
+  --> $DIR/no_effect.rs:112:5
    |
 LL |     5..;
    |     ^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:108:5
+  --> $DIR/no_effect.rs:113:5
    |
 LL |     ..5;
    |     ^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:109:5
+  --> $DIR/no_effect.rs:114:5
    |
 LL |     5..6;
    |     ^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:110:5
+  --> $DIR/no_effect.rs:115:5
    |
 LL |     5..=6;
    |     ^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:111:5
+  --> $DIR/no_effect.rs:116:5
    |
 LL |     [42, 55];
    |     ^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:112:5
+  --> $DIR/no_effect.rs:117:5
    |
 LL |     [42, 55][1];
    |     ^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:113:5
+  --> $DIR/no_effect.rs:118:5
    |
 LL |     (42, 55).1;
    |     ^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:114:5
+  --> $DIR/no_effect.rs:119:5
    |
 LL |     [42; 55];
    |     ^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:115:5
+  --> $DIR/no_effect.rs:120:5
    |
 LL |     [42; 55][13];
    |     ^^^^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:117:5
+  --> $DIR/no_effect.rs:122:5
    |
 LL |     || x += 5;
    |     ^^^^^^^^^^
 
 error: statement with no effect
-  --> $DIR/no_effect.rs:119:5
+  --> $DIR/no_effect.rs:124:5
    |
 LL |     FooString { s: s };
    |     ^^^^^^^^^^^^^^^^^^^
 
 error: binding to `_` prefixed variable with no side-effect
-  --> $DIR/no_effect.rs:120:5
+  --> $DIR/no_effect.rs:125:5
    |
 LL |     let _unused = 1;
    |     ^^^^^^^^^^^^^^^^
@@ -165,19 +165,19 @@ LL |     let _unused = 1;
    = note: `-D clippy::no-effect-underscore-binding` implied by `-D warnings`
 
 error: binding to `_` prefixed variable with no side-effect
-  --> $DIR/no_effect.rs:121:5
+  --> $DIR/no_effect.rs:126:5
    |
 LL |     let _penguin = || println!("Some helpful closure");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: binding to `_` prefixed variable with no side-effect
-  --> $DIR/no_effect.rs:122:5
+  --> $DIR/no_effect.rs:127:5
    |
 LL |     let _duck = Struct { field: 0 };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: binding to `_` prefixed variable with no side-effect
-  --> $DIR/no_effect.rs:123:5
+  --> $DIR/no_effect.rs:128:5
    |
 LL |     let _cat = [2, 4, 6, 8][2];
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/unnecessary_operation.fixed

@@ -1,7 +1,13 @@
 // run-rustfix
 
 #![feature(box_syntax)]
-#![allow(clippy::deref_addrof, dead_code, unused, clippy::no_effect)]
+#![allow(
+    clippy::deref_addrof,
+    dead_code,
+    unused,
+    clippy::no_effect,
+    clippy::unnecessary_struct_initialization
+)]
 #![warn(clippy::unnecessary_operation)]
 
 struct Tuple(i32);

tests/ui/unnecessary_operation.rs

@@ -1,7 +1,13 @@
 // run-rustfix
 
 #![feature(box_syntax)]
-#![allow(clippy::deref_addrof, dead_code, unused, clippy::no_effect)]
+#![allow(
+    clippy::deref_addrof,
+    dead_code,
+    unused,
+    clippy::no_effect,
+    clippy::unnecessary_struct_initialization
+)]
 #![warn(clippy::unnecessary_operation)]
 
 struct Tuple(i32);

tests/ui/unnecessary_operation.stderr

@@ -1,5 +1,5 @@
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:51:5
+  --> $DIR/unnecessary_operation.rs:57:5
    |
 LL |     Tuple(get_number());
    |     ^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
@@ -7,109 +7,109 @@ LL |     Tuple(get_number());
    = note: `-D clippy::unnecessary-operation` implied by `-D warnings`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:52:5
+  --> $DIR/unnecessary_operation.rs:58:5
    |
 LL |     Struct { field: get_number() };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:53:5
+  --> $DIR/unnecessary_operation.rs:59:5
    |
 LL |     Struct { ..get_struct() };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_struct();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:54:5
+  --> $DIR/unnecessary_operation.rs:60:5
    |
 LL |     Enum::Tuple(get_number());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:55:5
+  --> $DIR/unnecessary_operation.rs:61:5
    |
 LL |     Enum::Struct { field: get_number() };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:56:5
+  --> $DIR/unnecessary_operation.rs:62:5
    |
 LL |     5 + get_number();
    |     ^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `5;get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:57:5
+  --> $DIR/unnecessary_operation.rs:63:5
    |
 LL |     *&get_number();
    |     ^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:58:5
+  --> $DIR/unnecessary_operation.rs:64:5
    |
 LL |     &get_number();
    |     ^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:59:5
+  --> $DIR/unnecessary_operation.rs:65:5
    |
 LL |     (5, 6, get_number());
    |     ^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `5;6;get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:60:5
+  --> $DIR/unnecessary_operation.rs:66:5
    |
 LL |     box get_number();
    |     ^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:61:5
+  --> $DIR/unnecessary_operation.rs:67:5
    |
 LL |     get_number()..;
    |     ^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:62:5
+  --> $DIR/unnecessary_operation.rs:68:5
    |
 LL |     ..get_number();
    |     ^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:63:5
+  --> $DIR/unnecessary_operation.rs:69:5
    |
 LL |     5..get_number();
    |     ^^^^^^^^^^^^^^^^ help: statement can be reduced to: `5;get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:64:5
+  --> $DIR/unnecessary_operation.rs:70:5
    |
 LL |     [42, get_number()];
    |     ^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `42;get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:65:5
+  --> $DIR/unnecessary_operation.rs:71:5
    |
 LL |     [42, 55][get_usize()];
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42, 55].len() > get_usize());`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:66:5
+  --> $DIR/unnecessary_operation.rs:72:5
    |
 LL |     (42, get_number()).1;
    |     ^^^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `42;get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:67:5
+  --> $DIR/unnecessary_operation.rs:73:5
    |
 LL |     [get_number(); 55];
    |     ^^^^^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:68:5
+  --> $DIR/unnecessary_operation.rs:74:5
    |
 LL |     [42; 55][get_usize()];
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42; 55].len() > get_usize());`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:69:5
+  --> $DIR/unnecessary_operation.rs:75:5
    |
 LL | /     {
 LL | |         get_number()
@@ -117,7 +117,7 @@ LL | |     };
    | |______^ help: statement can be reduced to: `get_number();`
 
 error: unnecessary operation
-  --> $DIR/unnecessary_operation.rs:72:5
+  --> $DIR/unnecessary_operation.rs:78:5
    |
 LL | /     FooString {
 LL | |         s: String::from("blah"),

tests/ui/unnecessary_struct_initialization.fixed

@@ -0,0 +1,73 @@
+// run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::unnecessary_struct_initialization)]
+
+struct S {
+    f: String,
+}
+
+#[derive(Clone, Copy)]
+struct T {
+    f: u32,
+}
+
+struct U {
+    f: u32,
+}
+
+impl Clone for U {
+    fn clone(&self) -> Self {
+        // Do not lint: `Self` does not implement `Copy`
+        Self { ..*self }
+    }
+}
+
+#[derive(Copy)]
+struct V {
+    f: u32,
+}
+
+impl Clone for V {
+    fn clone(&self) -> Self {
+        // Lint: `Self` implements `Copy`
+        *self
+    }
+}
+
+fn main() {
+    // Should lint: `a` would be consumed anyway
+    let a = S { f: String::from("foo") };
+    let mut b = a;
+
+    // Should lint: `b` would be consumed, and is mutable
+    let c = &mut b;
+
+    // Should not lint as `d` is not mutable
+    let d = S { f: String::from("foo") };
+    let e = &mut S { ..d };
+
+    // Should lint as `f` would be consumed anyway
+    let f = S { f: String::from("foo") };
+    let g = &f;
+
+    // Should lint: the result of an expression is mutable
+    let h = &mut *Box::new(S { f: String::from("foo") });
+
+    // Should not lint: `m` would be both alive and borrowed
+    let m = T { f: 17 };
+    let n = &T { ..m };
+
+    // Should not lint: `m` should not be modified
+    let o = &mut T { ..m };
+    o.f = 32;
+    assert_eq!(m.f, 17);
+
+    // Should not lint: `m` should not be modified
+    let o = &mut T { ..m } as *mut T;
+    unsafe { &mut *o }.f = 32;
+    assert_eq!(m.f, 17);
+
+    // Should lint: the result of an expression is mutable and temporary
+    let p = &mut *Box::new(T { f: 5 });
+}

tests/ui/unnecessary_struct_initialization.rs

@@ -0,0 +1,77 @@
+// run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::unnecessary_struct_initialization)]
+
+struct S {
+    f: String,
+}
+
+#[derive(Clone, Copy)]
+struct T {
+    f: u32,
+}
+
+struct U {
+    f: u32,
+}
+
+impl Clone for U {
+    fn clone(&self) -> Self {
+        // Do not lint: `Self` does not implement `Copy`
+        Self { ..*self }
+    }
+}
+
+#[derive(Copy)]
+struct V {
+    f: u32,
+}
+
+impl Clone for V {
+    fn clone(&self) -> Self {
+        // Lint: `Self` implements `Copy`
+        Self { ..*self }
+    }
+}
+
+fn main() {
+    // Should lint: `a` would be consumed anyway
+    let a = S { f: String::from("foo") };
+    let mut b = S { ..a };
+
+    // Should lint: `b` would be consumed, and is mutable
+    let c = &mut S { ..b };
+
+    // Should not lint as `d` is not mutable
+    let d = S { f: String::from("foo") };
+    let e = &mut S { ..d };
+
+    // Should lint as `f` would be consumed anyway
+    let f = S { f: String::from("foo") };
+    let g = &S { ..f };
+
+    // Should lint: the result of an expression is mutable
+    let h = &mut S {
+        ..*Box::new(S { f: String::from("foo") })
+    };
+
+    // Should not lint: `m` would be both alive and borrowed
+    let m = T { f: 17 };
+    let n = &T { ..m };
+
+    // Should not lint: `m` should not be modified
+    let o = &mut T { ..m };
+    o.f = 32;
+    assert_eq!(m.f, 17);
+
+    // Should not lint: `m` should not be modified
+    let o = &mut T { ..m } as *mut T;
+    unsafe { &mut *o }.f = 32;
+    assert_eq!(m.f, 17);
+
+    // Should lint: the result of an expression is mutable and temporary
+    let p = &mut T {
+        ..*Box::new(T { f: 5 })
+    };
+}

tests/ui/unnecessary_struct_initialization.stderr

@@ -0,0 +1,46 @@
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:34:9
+   |
+LL |         Self { ..*self }
+   |         ^^^^^^^^^^^^^^^^ help: replace with: `*self`
+   |
+   = note: `-D clippy::unnecessary-struct-initialization` implied by `-D warnings`
+
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:41:17
+   |
+LL |     let mut b = S { ..a };
+   |                 ^^^^^^^^^ help: replace with: `a`
+
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:44:18
+   |
+LL |     let c = &mut S { ..b };
+   |                  ^^^^^^^^^ help: replace with: `b`
+
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:52:14
+   |
+LL |     let g = &S { ..f };
+   |              ^^^^^^^^^ help: replace with: `f`
+
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:55:18
+   |
+LL |       let h = &mut S {
+   |  __________________^
+LL | |         ..*Box::new(S { f: String::from("foo") })
+LL | |     };
+   | |_____^ help: replace with: `*Box::new(S { f: String::from("foo") })`
+
+error: unnecessary struct building
+  --> $DIR/unnecessary_struct_initialization.rs:74:18
+   |
+LL |       let p = &mut T {
+   |  __________________^
+LL | |         ..*Box::new(T { f: 5 })
+LL | |     };
+   | |_____^ help: replace with: `*Box::new(T { f: 5 })`
+
+error: aborting due to 6 previous errors
+