unnecessary_struct_initialization: extend to assignments moving all fields

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. Until now this lint only triggered
for structs recreated from a base struct.

NB: The new functionality too will cause false positives for the
situation where a non-copy struct consisting of all copy members
is touched again in subsequent code.

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 };
 }

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 };
 }

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