feat: Render padding information when hovering on structs

7 files changed, 272 insertions, 23 deletions

diff --git a/src/tools/rust-analyzer/crates/hir/src/lib.rs b/src/tools/rust-analyzer/crates/hir/src/lib.rs
index 3a91050d15f..e8218cf8611 100644
--- a/src/tools/rust-analyzer/crates/hir/src/lib.rs
+++ b/src/tools/rust-analyzer/crates/hir/src/lib.rs
@@ -5972,6 +5972,59 @@ impl Layout {
         }
     }
 
+    pub fn tail_padding(&self, field_size: &mut impl FnMut(usize) -> Option<u64>) -> Option<u64> {
+        match self.0.fields {
+            layout::FieldsShape::Primitive => None,
+            layout::FieldsShape::Union(_) => None,
+            layout::FieldsShape::Array { stride, count } => count.checked_sub(1).and_then(|tail| {
+                let tail_field_size = field_size(tail as usize)?;
+                let offset = stride.bytes() * tail;
+                self.0.size.bytes().checked_sub(offset)?.checked_sub(tail_field_size)
+            }),
+            layout::FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                let tail = memory_index.last_index()?;
+                let tail_field_size = field_size(tail.0.into_raw().into_u32() as usize)?;
+                let offset = offsets.get(tail)?.bytes();
+                self.0.size.bytes().checked_sub(offset)?.checked_sub(tail_field_size)
+            }
+        }
+    }
+
+    pub fn largest_padding(
+        &self,
+        field_size: &mut impl FnMut(usize) -> Option<u64>,
+    ) -> Option<u64> {
+        match self.0.fields {
+            layout::FieldsShape::Primitive => None,
+            layout::FieldsShape::Union(_) => None,
+            layout::FieldsShape::Array { stride: _, count: 0 } => None,
+            layout::FieldsShape::Array { stride, .. } => {
+                let size = field_size(0)?;
+                stride.bytes().checked_sub(size)
+            }
+            layout::FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                let mut reverse_index = vec![None; memory_index.len()];
+                for (src, (mem, offset)) in memory_index.iter().zip(offsets.iter()).enumerate() {
+                    reverse_index[*mem as usize] = Some((src, offset.bytes()));
+                }
+                if reverse_index.iter().any(|it| it.is_none()) {
+                    stdx::never!();
+                    return None;
+                }
+                reverse_index
+                    .into_iter()
+                    .flatten()
+                    .chain(std::iter::once((0, self.0.size.bytes())))
+                    .tuple_windows()
+                    .filter_map(|((i, start), (_, end))| {
+                        let size = field_size(i)?;
+                        end.checked_sub(start)?.checked_sub(size)
+                    })
+                    .max()
+            }
+        }
+    }
+
     pub fn enum_tag_size(&self) -> Option<usize> {
         let tag_size =
             if let layout::Variants::Multiple { tag, tag_encoding, .. } = &self.0.variants {
diff --git a/src/tools/rust-analyzer/crates/ide/src/hover.rs b/src/tools/rust-analyzer/crates/ide/src/hover.rs
index 873e31b4a33..8bb1c708e25 100644
--- a/src/tools/rust-analyzer/crates/ide/src/hover.rs
+++ b/src/tools/rust-analyzer/crates/ide/src/hover.rs
@@ -58,6 +58,7 @@ pub struct MemoryLayoutHoverConfig {
     pub size: Option<MemoryLayoutHoverRenderKind>,
     pub offset: Option<MemoryLayoutHoverRenderKind>,
     pub alignment: Option<MemoryLayoutHoverRenderKind>,
+    pub padding: Option<MemoryLayoutHoverRenderKind>,
     pub niches: bool,
 }
 
diff --git a/src/tools/rust-analyzer/crates/ide/src/hover/render.rs b/src/tools/rust-analyzer/crates/ide/src/hover/render.rs
index ad720c8a627..c24864a18bd 100644
--- a/src/tools/rust-analyzer/crates/ide/src/hover/render.rs
+++ b/src/tools/rust-analyzer/crates/ide/src/hover/render.rs
@@ -630,27 +630,57 @@ pub(super) fn definition(
                 }
             },
             |_| None,
+            |_| None,
+        ),
+        Definition::Adt(it @ Adt::Struct(strukt)) => render_memory_layout(
+            config.memory_layout,
+            || it.layout(db),
+            |_| None,
+            |layout| {
+                let mut field_size =
+                    |i: usize| Some(strukt.fields(db).get(i)?.layout(db).ok()?.size());
+                if strukt.repr(db).is_some_and(|it| it.inhibit_struct_field_reordering()) {
+                    Some(("tail padding", layout.tail_padding(&mut field_size)?))
+                } else {
+                    Some(("largest padding", layout.largest_padding(&mut field_size)?))
+                }
+            },
+            |_| None,
+        ),
+        Definition::Adt(it) => render_memory_layout(
+            config.memory_layout,
+            || it.layout(db),
+            |_| None,
+            |_| None,
+            |_| None,
         ),
-        Definition::Adt(it) => {
-            render_memory_layout(config.memory_layout, || it.layout(db), |_| None, |_| None)
-        }
         Definition::Variant(it) => render_memory_layout(
             config.memory_layout,
             || it.layout(db),
             |_| None,
+            |_| None,
             |layout| layout.enum_tag_size(),
         ),
-        Definition::TypeAlias(it) => {
-            render_memory_layout(config.memory_layout, || it.ty(db).layout(db), |_| None, |_| None)
-        }
-        Definition::Local(it) => {
-            render_memory_layout(config.memory_layout, || it.ty(db).layout(db), |_| None, |_| None)
-        }
+        Definition::TypeAlias(it) => render_memory_layout(
+            config.memory_layout,
+            || it.ty(db).layout(db),
+            |_| None,
+            |_| None,
+            |_| None,
+        ),
+        Definition::Local(it) => render_memory_layout(
+            config.memory_layout,
+            || it.ty(db).layout(db),
+            |_| None,
+            |_| None,
+            |_| None,
+        ),
         Definition::SelfType(it) => render_memory_layout(
             config.memory_layout,
             || it.self_ty(db).layout(db),
             |_| None,
             |_| None,
+            |_| None,
         ),
         _ => None,
     };
@@ -1055,9 +1085,13 @@ fn closure_ty(
     if let Some(trait_) = c.fn_trait(sema.db).get_id(sema.db, original.krate(sema.db).into()) {
         push_new_def(hir::Trait::from(trait_).into())
     }
-    if let Some(layout) =
-        render_memory_layout(config.memory_layout, || original.layout(sema.db), |_| None, |_| None)
-    {
+    if let Some(layout) = render_memory_layout(
+        config.memory_layout,
+        || original.layout(sema.db),
+        |_| None,
+        |_| None,
+        |_| None,
+    ) {
         format_to!(markup, "\n___\n{layout}");
     }
     format_to!(markup, "{adjusted}\n\n## Captures\n{}", captures_rendered,);
@@ -1142,6 +1176,7 @@ fn render_memory_layout(
     config: Option<MemoryLayoutHoverConfig>,
     layout: impl FnOnce() -> Result<Layout, LayoutError>,
     offset: impl FnOnce(&Layout) -> Option<u64>,
+    padding: impl FnOnce(&Layout) -> Option<(&str, u64)>,
     tag: impl FnOnce(&Layout) -> Option<usize>,
 ) -> Option<String> {
     let config = config?;
@@ -1199,6 +1234,23 @@ fn render_memory_layout(
         }
     }
 
+    if let Some(render) = config.padding {
+        if let Some((padding_name, padding)) = padding(&layout) {
+            format_to!(label, "{padding_name} = ");
+            match render {
+                MemoryLayoutHoverRenderKind::Decimal => format_to!(label, "{padding}"),
+                MemoryLayoutHoverRenderKind::Hexadecimal => format_to!(label, "{padding:#X}"),
+                MemoryLayoutHoverRenderKind::Both if padding >= 10 => {
+                    format_to!(label, "{padding} ({padding:#X})")
+                }
+                MemoryLayoutHoverRenderKind::Both => {
+                    format_to!(label, "{padding}")
+                }
+            }
+            format_to!(label, ", ");
+        }
+    }
+
     if config.niches {
         if let Some(niches) = layout.niches() {
             if niches > 1024 {
diff --git a/src/tools/rust-analyzer/crates/ide/src/hover/tests.rs b/src/tools/rust-analyzer/crates/ide/src/hover/tests.rs
index 06ca24c3ec3..a281a491525 100644
--- a/src/tools/rust-analyzer/crates/ide/src/hover/tests.rs
+++ b/src/tools/rust-analyzer/crates/ide/src/hover/tests.rs
@@ -12,6 +12,7 @@ const HOVER_BASE_CONFIG: HoverConfig = HoverConfig {
         size: Some(MemoryLayoutHoverRenderKind::Both),
         offset: Some(MemoryLayoutHoverRenderKind::Both),
         alignment: Some(MemoryLayoutHoverRenderKind::Both),
+        padding: Some(MemoryLayoutHoverRenderKind::Both),
         niches: true,
     }),
     documentation: true,
@@ -933,7 +934,7 @@ struct Foo$0(pub u32) where u32: Copy;
 
             ---
 
-            size = 4, align = 4, no Drop
+            size = 4, align = 4, largest padding = 0, no Drop
         "#]],
     );
 }
@@ -959,7 +960,7 @@ struct Foo$0 { field: u32 }
 
             ---
 
-            size = 4, align = 4, no Drop
+            size = 4, align = 4, largest padding = 0, no Drop
         "#]],
     );
     check(
@@ -984,7 +985,7 @@ struct Foo$0 where u32: Copy { field: u32 }
 
             ---
 
-            size = 4, align = 4, no Drop
+            size = 4, align = 4, largest padding = 0, no Drop
         "#]],
     );
 }
@@ -1013,7 +1014,7 @@ fn hover_record_struct_limit() {
 
             ---
 
-            size = 12 (0xC), align = 4, no Drop
+            size = 12 (0xC), align = 4, largest padding = 0, no Drop
         "#]],
     );
     check_hover_fields_limit(
@@ -1036,7 +1037,7 @@ fn hover_record_struct_limit() {
 
             ---
 
-            size = 4, align = 4, no Drop
+            size = 4, align = 4, largest padding = 0, no Drop
         "#]],
     );
     check_hover_fields_limit(
@@ -1062,7 +1063,7 @@ fn hover_record_struct_limit() {
 
             ---
 
-            size = 16 (0x10), align = 4, no Drop
+            size = 16 (0x10), align = 4, largest padding = 0, no Drop
         "#]],
     );
     check_hover_fields_limit(
@@ -1083,7 +1084,7 @@ fn hover_record_struct_limit() {
 
             ---
 
-            size = 12 (0xC), align = 4, no Drop
+            size = 12 (0xC), align = 4, largest padding = 0, no Drop
         "#]],
     );
     check_hover_fields_limit(
@@ -1104,7 +1105,7 @@ fn hover_record_struct_limit() {
 
             ---
 
-            size = 12 (0xC), align = 4, no Drop
+            size = 12 (0xC), align = 4, largest padding = 0, no Drop
         "#]],
     );
 
@@ -3114,7 +3115,7 @@ struct S$0<T>(core::marker::PhantomData<T>);
 
             ---
 
-            size = 0, align = 1, no Drop
+            size = 0, align = 1, largest padding = 0, no Drop
         "#]],
     );
 }
@@ -3148,6 +3149,111 @@ fn test_hover_layout_of_enum() {
 }
 
 #[test]
+fn test_hover_layout_padding_info() {
+    check(
+        r#"struct $0Foo {
+            x: bool,
+            y: i64,
+            z: u32,
+        }"#,
+        expect![[r#"
+            *Foo*
+
+            ```rust
+            ra_test_fixture
+            ```
+
+            ```rust
+            struct Foo {
+                x: bool,
+                y: i64,
+                z: u32,
+            }
+            ```
+
+            ---
+
+            size = 16 (0x10), align = 8, largest padding = 3, niches = 254, no Drop
+        "#]],
+    );
+
+    check(
+        r#"#[repr(align(32))]
+        struct $0Foo {
+            x: bool,
+            y: i64,
+            z: u32,
+        }"#,
+        expect![[r#"
+            *Foo*
+
+            ```rust
+            ra_test_fixture
+            ```
+
+            ```rust
+            struct Foo {
+                x: bool,
+                y: i64,
+                z: u32,
+            }
+            ```
+
+            ---
+
+            size = 32 (0x20), align = 32 (0x20), largest padding = 19 (0x13), niches = 254, no Drop
+        "#]],
+    );
+
+    check(
+        r#"#[repr(C)]
+        struct $0Foo {
+            x: bool,
+            y: i64,
+            z: u32,
+        }"#,
+        expect![[r#"
+            *Foo*
+
+            ```rust
+            ra_test_fixture
+            ```
+
+            ```rust
+            struct Foo {
+                x: bool,
+                y: i64,
+                z: u32,
+            }
+            ```
+
+            ---
+
+            size = 24 (0x18), align = 8, tail padding = 4, niches = 254, no Drop
+        "#]],
+    );
+
+    check(
+        r#"struct $0Foo(i16, u128, u64)"#,
+        expect![[r#"
+            *Foo*
+
+            ```rust
+            ra_test_fixture
+            ```
+
+            ```rust
+            struct Foo(i16, u128, u64)
+            ```
+
+            ---
+
+            size = 32 (0x20), align = 8, largest padding = 6, no Drop
+        "#]],
+    );
+}
+
+#[test]
 fn test_hover_no_memory_layout() {
     check_hover_no_memory_layout(
         r#"struct Foo { fiel$0d_a: u8, field_b: i32, field_c: i16 }"#,
@@ -9198,7 +9304,7 @@ struct Pedro$0<'a> {
 
             ---
 
-            size = 16 (0x10), align = 8, niches = 1, no Drop
+            size = 16 (0x10), align = 8, largest padding = 0, niches = 1, no Drop
         "#]],
     )
 }
@@ -10559,7 +10665,7 @@ struct DropField$0 {
 
             ---
 
-            size = 4, align = 4, needs Drop
+            size = 4, align = 4, largest padding = 0, needs Drop
         "#]],
     );
     check(
diff --git a/src/tools/rust-analyzer/crates/rust-analyzer/src/config.rs b/src/tools/rust-analyzer/crates/rust-analyzer/src/config.rs
index f7158235ca4..d1ca8c1a91a 100644
--- a/src/tools/rust-analyzer/crates/rust-analyzer/src/config.rs
+++ b/src/tools/rust-analyzer/crates/rust-analyzer/src/config.rs
@@ -149,6 +149,8 @@ config_data! {
         hover_memoryLayout_niches: Option<bool> = Some(false),
         /// How to render the offset information in a memory layout hover.
         hover_memoryLayout_offset: Option<MemoryLayoutHoverRenderKindDef> = Some(MemoryLayoutHoverRenderKindDef::Hexadecimal),
+        /// How to render the padding information in a memory layout hover.
+        hover_memoryLayout_padding: Option<MemoryLayoutHoverRenderKindDef> = None,
         /// How to render the size information in a memory layout hover.
         hover_memoryLayout_size: Option<MemoryLayoutHoverRenderKindDef> = Some(MemoryLayoutHoverRenderKindDef::Both),
 
@@ -1635,6 +1637,7 @@ impl Config {
                 size: self.hover_memoryLayout_size().map(mem_kind),
                 offset: self.hover_memoryLayout_offset().map(mem_kind),
                 alignment: self.hover_memoryLayout_alignment().map(mem_kind),
+                padding: self.hover_memoryLayout_padding().map(mem_kind),
                 niches: self.hover_memoryLayout_niches().unwrap_or_default(),
             }),
             documentation: self.hover_documentation_enable().to_owned(),
diff --git a/src/tools/rust-analyzer/docs/book/src/configuration_generated.md b/src/tools/rust-analyzer/docs/book/src/configuration_generated.md
index de1d0ea4a35..0e07dadfb7c 100644
--- a/src/tools/rust-analyzer/docs/book/src/configuration_generated.md
+++ b/src/tools/rust-analyzer/docs/book/src/configuration_generated.md
@@ -763,6 +763,13 @@ Default: `"hexadecimal"`
 How to render the offset information in a memory layout hover.
 
 
+## rust-analyzer.hover.memoryLayout.padding {#hover.memoryLayout.padding}
+
+Default: `null`
+
+How to render the padding information in a memory layout hover.
+
+
 ## rust-analyzer.hover.memoryLayout.size {#hover.memoryLayout.size}
 
 Default: `"both"`
diff --git a/src/tools/rust-analyzer/editors/code/package.json b/src/tools/rust-analyzer/editors/code/package.json
index 88a90aad8c9..c8c36cd85c8 100644
--- a/src/tools/rust-analyzer/editors/code/package.json
+++ b/src/tools/rust-analyzer/editors/code/package.json
@@ -1782,6 +1782,33 @@
             {
                 "title": "hover",
                 "properties": {
+                    "rust-analyzer.hover.memoryLayout.padding": {
+                        "markdownDescription": "How to render the padding information in a memory layout hover.",
+                        "default": null,
+                        "anyOf": [
+                            {
+                                "type": "null"
+                            },
+                            {
+                                "type": "string",
+                                "enum": [
+                                    "both",
+                                    "decimal",
+                                    "hexadecimal"
+                                ],
+                                "enumDescriptions": [
+                                    "Render as 12 (0xC)",
+                                    "Render as 12",
+                                    "Render as 0xC"
+                                ]
+                            }
+                        ]
+                    }
+                }
+            },
+            {
+                "title": "hover",
+                "properties": {
                     "rust-analyzer.hover.memoryLayout.size": {
                         "markdownDescription": "How to render the size information in a memory layout hover.",
                         "default": "both",