Auto merge of #10930 - y21:issue9956, r=blyxyas,xFrednet

[`redundant_closure_call`]: handle nested closures

Fixes #9956.

This ended up being a much larger change than I'd thought, and I ended up having to pretty much rewrite it as a late lint pass, because it needs access to certain things that I don't think are available in early lint passes (e.g. getting the parent expr). I think this'll be required to fi-x #10922 anyway, so this is probably fine.
(edit: had to write "fi-x" because "fix" makes github think that this PR fixes it, which it doesn't 😅 )

Previously, it would suggest changing `(|| || 42)()()` to `|| 42()`, which is a type error (it needs parens: `(|| 42)()`). In my opinion, though, the suggested fix should have really been `42`, so that's what this PR changes.

changelog: [`redundant_closure_call`]: handle nested closures and rewrite as a late lint pass

tests/ui/redundant_closure_call_fixable.fixed

@@ -3,6 +3,7 @@
 #![feature(async_closure)]
 #![warn(clippy::redundant_closure_call)]
 #![allow(clippy::redundant_async_block)]
+#![allow(clippy::type_complexity)]
 #![allow(unused)]
 
 async fn something() -> u32 {
@@ -38,4 +39,50 @@ fn main() {
         };
     }
     m2!();
+    issue9956();
+}
+
+fn issue9956() {
+    assert_eq!(43, 42);
+
+    // ... and some more interesting cases I've found while implementing the fix
+
+    // not actually immediately calling the closure:
+    let a = (|| 42);
+    dbg!(a());
+
+    // immediately calling it inside of a macro
+    dbg!(42);
+
+    // immediately calling only one closure, so we can't remove the other ones
+    let a = (|| || 123);
+    dbg!(a()());
+
+    // nested async closures
+    let a = async { 1 };
+    let h = async { a.await };
+
+    // macro expansion tests
+    macro_rules! echo {
+        ($e:expr) => {
+            $e
+        };
+    }
+    let a = 1;
+    assert_eq!(a, 1);
+    let a = 123;
+    assert_eq!(a, 123);
+
+    // chaining calls, but not closures
+    fn x() -> fn() -> fn() -> fn() -> i32 {
+        || || || 42
+    }
+    let _ = x()()()();
+
+    fn bar() -> fn(i32, i32) {
+        foo
+    }
+    fn foo(_: i32, _: i32) {}
+    bar()(42, 5);
+    foo(42, 5);
 }

tests/ui/redundant_closure_call_fixable.rs

@@ -3,6 +3,7 @@
 #![feature(async_closure)]
 #![warn(clippy::redundant_closure_call)]
 #![allow(clippy::redundant_async_block)]
+#![allow(clippy::type_complexity)]
 #![allow(unused)]
 
 async fn something() -> u32 {
@@ -38,4 +39,50 @@ fn main() {
         };
     }
     m2!();
+    issue9956();
+}
+
+fn issue9956() {
+    assert_eq!((|| || 43)()(), 42);
+
+    // ... and some more interesting cases I've found while implementing the fix
+
+    // not actually immediately calling the closure:
+    let a = (|| 42);
+    dbg!(a());
+
+    // immediately calling it inside of a macro
+    dbg!((|| 42)());
+
+    // immediately calling only one closure, so we can't remove the other ones
+    let a = (|| || || 123)();
+    dbg!(a()());
+
+    // nested async closures
+    let a = (|| || || || async || 1)()()()()();
+    let h = async { a.await };
+
+    // macro expansion tests
+    macro_rules! echo {
+        ($e:expr) => {
+            $e
+        };
+    }
+    let a = (|| echo!(|| echo!(|| 1)))()()();
+    assert_eq!(a, 1);
+    let a = (|| echo!((|| 123)))()();
+    assert_eq!(a, 123);
+
+    // chaining calls, but not closures
+    fn x() -> fn() -> fn() -> fn() -> i32 {
+        || || || 42
+    }
+    let _ = x()()()();
+
+    fn bar() -> fn(i32, i32) {
+        foo
+    }
+    fn foo(_: i32, _: i32) {}
+    bar()((|| || 42)()(), 5);
+    foo((|| || 42)()(), 5);
 }

tests/ui/redundant_closure_call_fixable.stderr

@@ -1,5 +1,5 @@
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:17:13
+  --> $DIR/redundant_closure_call_fixable.rs:18:13
    |
 LL |     let a = (|| 42)();
    |             ^^^^^^^^^ help: try doing something like: `42`
@@ -7,7 +7,7 @@ LL |     let a = (|| 42)();
    = note: `-D clippy::redundant-closure-call` implied by `-D warnings`
 
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:18:13
+  --> $DIR/redundant_closure_call_fixable.rs:19:13
    |
 LL |       let b = (async || {
    |  _____________^
@@ -27,7 +27,7 @@ LL ~     };
    |
 
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:23:13
+  --> $DIR/redundant_closure_call_fixable.rs:24:13
    |
 LL |       let c = (|| {
    |  _____________^
@@ -47,13 +47,13 @@ LL ~     };
    |
 
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:28:13
+  --> $DIR/redundant_closure_call_fixable.rs:29:13
    |
 LL |     let d = (async || something().await)();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `async { something().await }`
 
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:37:13
+  --> $DIR/redundant_closure_call_fixable.rs:38:13
    |
 LL |             (|| m!())()
    |             ^^^^^^^^^^^ help: try doing something like: `m!()`
@@ -64,7 +64,7 @@ LL |     m2!();
    = note: this error originates in the macro `m2` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: try not to call a closure in the expression where it is declared
-  --> $DIR/redundant_closure_call_fixable.rs:32:13
+  --> $DIR/redundant_closure_call_fixable.rs:33:13
    |
 LL |             (|| 0)()
    |             ^^^^^^^^ help: try doing something like: `0`
@@ -74,5 +74,53 @@ LL |     m2!();
    |
    = note: this error originates in the macro `m` which comes from the expansion of the macro `m2` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 6 previous errors
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:46:16
+   |
+LL |     assert_eq!((|| || 43)()(), 42);
+   |                ^^^^^^^^^^^^^^ help: try doing something like: `43`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:55:10
+   |
+LL |     dbg!((|| 42)());
+   |          ^^^^^^^^^ help: try doing something like: `42`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:58:13
+   |
+LL |     let a = (|| || || 123)();
+   |             ^^^^^^^^^^^^^^^^ help: try doing something like: `(|| || 123)`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:62:13
+   |
+LL |     let a = (|| || || || async || 1)()()()()();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `async { 1 }`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:71:13
+   |
+LL |     let a = (|| echo!(|| echo!(|| 1)))()()();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `1`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:73:13
+   |
+LL |     let a = (|| echo!((|| 123)))()();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `123`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:86:11
+   |
+LL |     bar()((|| || 42)()(), 5);
+   |           ^^^^^^^^^^^^^^ help: try doing something like: `42`
+
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:87:9
+   |
+LL |     foo((|| || 42)()(), 5);
+   |         ^^^^^^^^^^^^^^ help: try doing something like: `42`
+
+error: aborting due to 14 previous errors