Add hover for closure

crates/ide/src/hover.rs

@@ -119,8 +119,8 @@ fn hover_simple(
         | T![crate]
         | T![Self]
         | T![_] => 4,
-        // index and prefix ops
-        T!['['] | T![']'] | T![?] | T![*] | T![-] | T![!] => 3,
+        // index and prefix ops and closure pipe
+        T!['['] | T![']'] | T![?] | T![*] | T![-] | T![!] | T![|] => 3,
         kind if kind.is_keyword() => 2,
         T!['('] | T![')'] => 2,
         kind if kind.is_trivia() => 0,
@@ -219,6 +219,16 @@ fn hover_simple(
                 };
                 render::type_info_of(sema, config, &Either::Left(call_expr))
             })
+        })
+        // try closure
+        .or_else(|| {
+            descended().find_map(|token| {
+                if token.kind() != T![|] {
+                    return None;
+                }
+                let c = token.parent().and_then(|x| x.parent()).and_then(ast::ClosureExpr::cast)?;
+                render::closure_expr(sema, c)
+            })
         });
 
     result.map(|mut res: HoverResult| {

crates/ide/src/hover/render.rs

@@ -42,6 +42,38 @@ pub(super) fn type_info_of(
     type_info(sema, _config, original, adjusted)
 }
 
+pub(super) fn closure_expr(
+    sema: &Semantics<'_, RootDatabase>,
+    c: ast::ClosureExpr,
+) -> Option<HoverResult> {
+    let ty = &sema.type_of_expr(&c.into())?.original;
+    let layout = ty
+        .layout(sema.db)
+        .map(|x| format!(" // size = {}, align = {}", x.size.bytes(), x.align.abi.bytes()))
+        .unwrap_or_default();
+    let c = ty.as_closure()?;
+    let mut captures = c
+        .captured_items(sema.db)
+        .into_iter()
+        .map(|x| {
+            format!("* `{}` by {}", x.display_place(c.clone().into(), sema.db), x.display_kind())
+        })
+        .join("\n");
+    if captures.trim().is_empty() {
+        captures = "This closure captures nothing".to_string();
+    }
+    let mut res = HoverResult::default();
+    res.markup = format!(
+        "```rust\n{}{}\n{}\n```\n\n## Captures\n{}",
+        c.display_with_id(sema.db),
+        layout,
+        c.display_with_impl(sema.db),
+        captures,
+    )
+    .into();
+    Some(res)
+}
+
 pub(super) fn try_expr(
     sema: &Semantics<'_, RootDatabase>,
     _config: &HoverConfig,

crates/ide/src/hover/tests.rs

@@ -198,6 +198,85 @@ fn main() {
     );
 }
 
+#[test]
+fn hover_closure() {
+    check(
+        r#"
+//- minicore: copy
+fn main() {
+    let x = 2;
+    let y = $0|z| x + z;
+}
+"#,
+        expect![[r#"
+            *|*
+            ```rust
+            {closure#0} // size = 8, align = 8
+            impl Fn(i32) -> i32
+            ```
+
+            ## Captures
+            * `x` by immutable borrow
+        "#]],
+    );
+
+    check(
+        r#"
+//- minicore: copy
+fn foo(x: impl Fn(i32) -> i32) {
+
+}
+fn main() {
+    foo($0|x: i32| x)
+}
+"#,
+        expect![[r#"
+            *|*
+            ```rust
+            {closure#0} // size = 0, align = 1
+            impl Fn(i32) -> i32
+            ```
+
+            ## Captures
+            This closure captures nothing
+        "#]],
+    );
+
+    check(
+        r#"
+//- minicore: copy
+
+struct Z { f: i32 }
+
+struct Y(&'static mut Z)
+
+struct X {
+    f1: Y,
+    f2: (Y, Y),
+}
+
+fn main() {
+    let x: X;
+    let y = $0|| {
+        x.f1;
+        &mut x.f2.0 .0.f;
+    };
+}
+"#,
+        expect![[r#"
+            *|*
+            ```rust
+            {closure#0} // size = 16, align = 8
+            impl FnOnce()
+            ```
+
+            ## Captures
+            * `x.f1` by move
+            * `(*x.f2.0.0).f` by mutable borrow
+        "#]],
+    );
+}
+
 #[test]
 fn hover_shows_long_type_of_an_expression() {
     check(