Auto merge of #129464 - GuillaumeGomez:rollup-ckfqd7h, r=GuillaumeGomez

Rollup of 9 pull requests

Successful merges:

 - #128511 (Document WebAssembly target feature expectations)
 - #129243 (do not build `cargo-miri` by default on stable channel)
 - #129263 (Add a missing compatibility note in the 1.80.0 release notes)
 - #129276 (Stabilize feature `char_indices_offset`)
 - #129350 (adapt integer comparison tests for LLVM 20 IR changes)
 - #129408 (Fix handling of macro arguments within the `dropping_copy_types` lint)
 - #129426 (rustdoc-search: use tighter json for names and parents)
 - #129437 (Fix typo in a help diagnostic)
 - #129457 (kobzol vacation)

r? `@ghost`
`@rustbot` modify labels: rollup

9 files changed, 267 insertions, 15 deletions

diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index ff8ca2ad74a..3a1eb43b801 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -1096,7 +1096,7 @@ tool_extended!((self, builder),
     CargoClippy, "src/tools/clippy", "cargo-clippy", stable=true;
     Clippy, "src/tools/clippy", "clippy-driver", stable=true, add_bins_to_sysroot = ["clippy-driver", "cargo-clippy"];
     Miri, "src/tools/miri", "miri", stable=false, add_bins_to_sysroot = ["miri"];
-    CargoMiri, "src/tools/miri/cargo-miri", "cargo-miri", stable=true, add_bins_to_sysroot = ["cargo-miri"];
+    CargoMiri, "src/tools/miri/cargo-miri", "cargo-miri", stable=false, add_bins_to_sysroot = ["cargo-miri"];
     Rls, "src/tools/rls", "rls", stable=true;
     Rustfmt, "src/tools/rustfmt", "rustfmt", stable=true, add_bins_to_sysroot = ["rustfmt", "cargo-fmt"];
 );
diff --git a/src/doc/rustc/src/SUMMARY.md b/src/doc/rustc/src/SUMMARY.md
index cb0b2e63452..4e34e2667cf 100644
--- a/src/doc/rustc/src/SUMMARY.md
+++ b/src/doc/rustc/src/SUMMARY.md
@@ -81,6 +81,7 @@
     - [wasm32-wasip1](platform-support/wasm32-wasip1.md)
     - [wasm32-wasip1-threads](platform-support/wasm32-wasip1-threads.md)
     - [wasm32-wasip2](platform-support/wasm32-wasip2.md)
+    - [wasm32-unknown-unknown](platform-support/wasm32-unknown-unknown.md)
     - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md)
     - [\*-win7-windows-msvc](platform-support/win7-windows-msvc.md)
     - [x86_64-fortanix-unknown-sgx](platform-support/x86_64-fortanix-unknown-sgx.md)
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 667758a1203..dd478e9df37 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -192,7 +192,7 @@ target | std | notes
 [`thumbv8m.main-none-eabi`](platform-support/thumbv8m.main-none-eabi.md) | * | Bare Armv8-M Mainline
 [`thumbv8m.main-none-eabihf`](platform-support/thumbv8m.main-none-eabi.md) | * | Bare Armv8-M Mainline, hardfloat
 `wasm32-unknown-emscripten` | ✓ | WebAssembly via Emscripten
-`wasm32-unknown-unknown` | ✓ | WebAssembly
+[`wasm32-unknown-unknown`](platform-support/wasm32-unknown-unknown.md) | ✓ | WebAssembly
 `wasm32-wasi` | ✓ | WebAssembly with WASI (undergoing a [rename to `wasm32-wasip1`][wasi-rename])
 [`wasm32-wasip1`](platform-support/wasm32-wasip1.md) | ✓ | WebAssembly with WASI
 [`wasm32-wasip1-threads`](platform-support/wasm32-wasip1-threads.md) | ✓ | WebAssembly with WASI Preview 1 and threads
diff --git a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
new file mode 100644
index 00000000000..ee37e5e90cf
--- /dev/null
+++ b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
@@ -0,0 +1,198 @@
+# `wasm32-unknown-unknown`
+
+**Tier: 2**
+
+The `wasm32-unknown-unknown` target is a WebAssembly compilation target which
+does not import any functions from the host for the standard library. This is
+the "minimal" WebAssembly in the sense of making the fewest assumptions about
+the host environment. This target is often used when compiling to the web or
+JavaScript environments as there is no standard for what functions can be
+imported on the web. This target can also be useful for creating minimal or
+bare-bones WebAssembly binaries.
+
+The `wasm32-unknown-unknown` target has support for the Rust standard library
+but many parts of the standard library do not work and return errors. For
+example `println!` does nothing, `std::fs` always return errors, and
+`std::thread::spawn` will panic. There is no means by which this can be
+overridden. For a WebAssembly target that more fully supports the standard
+library see the [`wasm32-wasip1`](./wasm32-wasip1.md) or
+[`wasm32-wasip2`](./wasm32-wasip2.md) targets.
+
+The `wasm32-unknown-unknown` target has full support for the `core` and `alloc`
+crates. It additionally supports the `HashMap` type in the `std` crate, although
+hash maps are not randomized like they are on other platforms.
+
+One existing user of this target (please feel free to edit and expand this list
+too) is the [`wasm-bindgen` project](https://github.com/rustwasm/wasm-bindgen)
+which facilitates Rust code interoperating with JavaScript code. Note, though,
+that not all uses of `wasm32-unknown-unknown` are using JavaScript and the web.
+
+## Target maintainers
+
+When this target was added to the compiler platform-specific documentation here
+was not maintained at that time. This means that the list below is not
+exhaustive and there are more interested parties in this target. That being
+said since when this document was last updated those interested in maintaining
+this target are:
+
+- Alex Crichton, https://github.com/alexcrichton
+
+## Requirements
+
+This target is cross-compiled. The target includes support for `std` itself,
+but as mentioned above many pieces of functionality that require an operating
+system do not work and will return errors.
+
+This target currently has no equivalent in C/C++. There is no C/C++ toolchain
+for this target. While interop is theoretically possible it's recommended to
+instead use one of:
+
+* `wasm32-unknown-emscripten` - for web-based use cases the Emscripten
+  toolchain is typically chosen for running C/C++.
+* [`wasm32-wasip1`](./wasm32-wasip1.md) - the wasi-sdk toolchain is used to
+  compile C/C++ on this target and can interop with Rust code. WASI works on
+  the web so far as there's no blocker, but an implementation of WASI APIs
+  must be either chosen or reimplemented.
+
+This target has no build requirements beyond what's in-tree in the Rust
+repository. Linking binaries requires LLD to be enabled for the `wasm-ld`
+driver. This target uses the `dlmalloc` crate as the default global allocator.
+
+## Building the target
+
+Building this target can be done by:
+
+* Configure the `wasm32-unknown-unknown` target to get built.
+* Configure LLD to be built.
+* Ensure the `WebAssembly` target backend is not disabled in LLVM.
+
+These are all controlled through `config.toml` options. It should be possible
+to build this target on any platform.
+
+## Building Rust programs
+
+Rust programs can be compiled by adding this target via rustup:
+
+```sh
+$ rustup target add wasm32-unknown-unknown
+```
+
+and then compiling with the target:
+
+```sh
+$ rustc foo.rs --target wasm32-unknown-unknown
+$ file foo.wasm
+```
+
+## Cross-compilation
+
+This target can be cross-compiled from any host.
+
+## Testing
+
+This target is not tested in CI for the rust-lang/rust repository. Many tests
+must be disabled to run on this target and failures are non-obvious because
+`println!` doesn't work in the standard library. It's recommended to test the
+`wasm32-wasip1` target instead for WebAssembly compatibility.
+
+## Conditionally compiling code
+
+It's recommended to conditionally compile code for this target with:
+
+```text
+#[cfg(all(target_family = "wasm", target_os = "unknown"))]
+```
+
+Note that there is no way to tell via `#[cfg]` whether code will be running on
+the web or not.
+
+## Enabled WebAssembly features
+
+WebAssembly is an evolving standard which adds new features such as new
+instructions over time. This target's default set of supported WebAssembly
+features will additionally change over time. The `wasm32-unknown-unknown` target
+inherits the default settings of LLVM which typically matches the default
+settings of Emscripten as well.
+
+Changes to WebAssembly go through a [proposals process][proposals] but reaching
+the final stage (stage 5) does not automatically mean that the feature will be
+enabled in LLVM and Rust by default. At this time the general guidance is that
+features must be present in most engines for a "good chunk of time" before
+they're enabled in LLVM by default. There is currently no exact number of
+months or engines that are required to enable features by default.
+
+[proposals]: https://github.com/WebAssembly/proposals
+
+As of the time of this writing the proposals that are enabled by default (the
+`generic` CPU in LLVM terminology) are:
+
+* `multivalue`
+* `mutable-globals`
+* `reference-types`
+* `sign-ext`
+
+If you're compiling WebAssembly code for an engine that does not support a
+feature in LLVM's default feature set then the feature must be disabled at
+compile time. Note, though, that enabled features may be used in the standard
+library or precompiled libraries shipped via rustup. This means that not only
+does your own code need to be compiled with the correct set of flags but the
+Rust standard library additionally must be recompiled.
+
+Compiling all code for the initial release of WebAssembly looks like:
+
+```sh
+$ export RUSTFLAGS=-Ctarget-cpu=mvp
+$ cargo +nightly build -Zbuild-std=panic_abort,std --target wasm32-unknown-unknown
+```
+
+Here the `mvp` "cpu" is a placeholder in LLVM for disabling all supported
+features by default. Cargo's `-Zbuild-std` feature, a Nightly Rust feature, is
+then used to recompile the standard library in addition to your own code. This
+will produce a binary that uses only the original WebAssembly features by
+default and no proposals since its inception.
+
+To enable individual features it can be done with `-Ctarget-feature=+foo`.
+Available features for Rust code itself are documented in the [reference] and
+can also be found through:
+
+```sh
+$ rustc -Ctarget-feature=help --target wasm32-unknown-unknown
+```
+
+You'll need to consult your WebAssembly engine's documentation to learn more
+about the supported WebAssembly features the engine has.
+
+[reference]: https://doc.rust-lang.org/reference/attributes/codegen.html#wasm32-or-wasm64
+
+Note that it is still possible for Rust crates and libraries to enable
+WebAssembly features on a per-function level. This means that the build
+command above may not be sufficient to disable all WebAssembly features. If the
+final binary still has SIMD instructions, for example, the function in question
+will need to be found and the crate in question will likely contain something
+like:
+
+```rust,ignore (not-always-compiled-to-wasm)
+#[target_feature(enable = "simd128")]
+fn foo() {
+    // ...
+}
+```
+
+In this situation there is no compiler flag to disable emission of SIMD
+instructions and the crate must instead be modified to not include this function
+at compile time either by default or through a Cargo feature. For crate authors
+it's recommended to avoid `#[target_feature(enable = "...")]` except where
+necessary and instead use:
+
+```rust,ignore (not-always-compiled-to-wasm)
+#[cfg(target_feature = "simd128")]
+fn foo() {
+    // ...
+}
+```
+
+That is to say instead of enabling target features it's recommended to
+conditionally compile code instead. This is notably different to the way native
+platforms such as x86\_64 work, and this is due to the fact that WebAssembly
+binaries must only contain code the engine understands. Native binaries work so
+long as the CPU doesn't execute unknown code dynamically at runtime.
diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md b/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md
index c3eda26ca8e..994c0f4bbb3 100644
--- a/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md
+++ b/src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md
@@ -162,3 +162,17 @@ It's recommended to conditionally compile code for this target with:
 Prior to Rust 1.80 the `target_env = "p1"` key was not set. Currently the
 `target_feature = "atomics"` is Nightly-only. Note that the precise `#[cfg]`
 necessary to detect this target may change as the target becomes more stable.
+
+## Enabled WebAssembly features
+
+The default set of WebAssembly features enabled for compilation includes two
+more features in addition to that which
+[`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md) enables:
+
+* `bulk-memory`
+* `atomics`
+
+For more information about features see the documentation for
+[`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md), but note that the
+`mvp` CPU in LLVM does not support this target as it's required that
+`bulk-memory`, `atomics`, and `mutable-globals` are all enabled.
diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip1.md b/src/doc/rustc/src/platform-support/wasm32-wasip1.md
index fb70bbdc2b4..0e4def6768d 100644
--- a/src/doc/rustc/src/platform-support/wasm32-wasip1.md
+++ b/src/doc/rustc/src/platform-support/wasm32-wasip1.md
@@ -132,3 +132,9 @@ It's recommended to conditionally compile code for this target with:
 
 Note that the `target_env = "p1"` condition first appeared in Rust 1.80. Prior
 to Rust 1.80 the `target_env` condition was not set.
+
+## Enabled WebAssembly features
+
+The default set of WebAssembly features enabled for compilation is currently the
+same as [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md). See the
+documentation there for more information.
diff --git a/src/doc/rustc/src/platform-support/wasm32-wasip2.md b/src/doc/rustc/src/platform-support/wasm32-wasip2.md
index 1e53fbc178e..bb2348b201e 100644
--- a/src/doc/rustc/src/platform-support/wasm32-wasip2.md
+++ b/src/doc/rustc/src/platform-support/wasm32-wasip2.md
@@ -61,3 +61,9 @@ It's recommended to conditionally compile code for this target with:
 ```text
 #[cfg(all(target_os = "wasi", target_env = "p2"))]
 ```
+
+## Enabled WebAssembly features
+
+The default set of WebAssembly features enabled for compilation is currently the
+same as [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md). See the
+documentation there for more information.
diff --git a/src/librustdoc/html/render/search_index.rs b/src/librustdoc/html/render/search_index.rs
index 8a12bdef69b..d7682bd1c19 100644
--- a/src/librustdoc/html/render/search_index.rs
+++ b/src/librustdoc/html/render/search_index.rs
@@ -579,12 +579,14 @@ pub(crate) fn build_index<'tcx>(
             let mut names = Vec::with_capacity(self.items.len());
             let mut types = String::with_capacity(self.items.len());
             let mut full_paths = Vec::with_capacity(self.items.len());
-            let mut parents = Vec::with_capacity(self.items.len());
+            let mut parents = String::with_capacity(self.items.len());
+            let mut parents_backref_queue = VecDeque::new();
             let mut functions = String::with_capacity(self.items.len());
             let mut deprecated = Vec::with_capacity(self.items.len());
 
-            let mut backref_queue = VecDeque::new();
+            let mut type_backref_queue = VecDeque::new();
 
+            let mut last_name = None;
             for (index, item) in self.items.iter().enumerate() {
                 let n = item.ty as u8;
                 let c = char::try_from(n + b'A').expect("item types must fit in ASCII");
@@ -597,17 +599,39 @@ pub(crate) fn build_index<'tcx>(
                     "`{}` is missing idx",
                     item.name
                 );
-                // 0 is a sentinel, everything else is one-indexed
-                parents.push(item.parent_idx.map(|x| x + 1).unwrap_or(0));
+                assert!(
+                    parents_backref_queue.len() <= 16,
+                    "the string encoding only supports 16 slots of lookback"
+                );
+                let parent: i32 = item.parent_idx.map(|x| x + 1).unwrap_or(0).try_into().unwrap();
+                if let Some(idx) = parents_backref_queue.iter().position(|p: &i32| *p == parent) {
+                    parents.push(
+                        char::try_from('0' as u32 + u32::try_from(idx).unwrap())
+                            .expect("last possible value is '?'"),
+                    );
+                } else if parent == 0 {
+                    write_vlqhex_to_string(parent, &mut parents);
+                } else {
+                    parents_backref_queue.push_front(parent);
+                    write_vlqhex_to_string(parent, &mut parents);
+                    if parents_backref_queue.len() > 16 {
+                        parents_backref_queue.pop_back();
+                    }
+                }
 
-                names.push(item.name.as_str());
+                if Some(item.name.as_str()) == last_name {
+                    names.push("");
+                } else {
+                    names.push(item.name.as_str());
+                    last_name = Some(item.name.as_str());
+                }
 
                 if !item.path.is_empty() {
                     full_paths.push((index, &item.path));
                 }
 
                 match &item.search_type {
-                    Some(ty) => ty.write_to_string(&mut functions, &mut backref_queue),
+                    Some(ty) => ty.write_to_string(&mut functions, &mut type_backref_queue),
                     None => functions.push('`'),
                 }
 
diff --git a/src/librustdoc/html/static/js/search.js b/src/librustdoc/html/static/js/search.js
index 3eb27ea087c..4e3b532ae08 100644
--- a/src/librustdoc/html/static/js/search.js
+++ b/src/librustdoc/html/static/js/search.js
@@ -3546,7 +3546,7 @@ ${item.displayPath}<span class="${type}">${name}</span>\
             // Used to de-duplicate inlined and re-exported stuff
             const itemReexports = new Map(crateCorpus.r);
             // an array of (Number) the parent path index + 1 to `paths`, or 0 if none
-            const itemParentIdxs = crateCorpus.i;
+            const itemParentIdxDecoder = new VlqHexDecoder(crateCorpus.i, noop => noop);
             // a map Number, string for impl disambiguators
             const implDisambiguator = new Map(crateCorpus.b);
             // an array of [(Number) item type,
@@ -3593,6 +3593,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
             // faster analysis operations
             lastPath = "";
             len = itemTypes.length;
+            let lastName = "";
+            let lastWord = "";
             for (let i = 0; i < len; ++i) {
                 const bitIndex = i + 1;
                 if (descIndex >= descShard.len &&
@@ -3608,10 +3610,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                     descIndex = 0;
                     descShardList.push(descShard);
                 }
-                let word = "";
-                if (typeof itemNames[i] === "string") {
-                    word = itemNames[i].toLowerCase();
-                }
+                const name = itemNames[i] === "" ? lastName : itemNames[i];
+                const word = itemNames[i] === "" ? lastWord : itemNames[i].toLowerCase();
                 const path = itemPaths.has(i) ? itemPaths.get(i) : lastPath;
                 const type = itemFunctionDecoder.next();
                 if (type !== null) {
@@ -3633,15 +3633,16 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 }
                 // This object should have exactly the same set of fields as the "crateRow"
                 // object defined above.
+                const itemParentIdx = itemParentIdxDecoder.next();
                 const row = {
                     crate,
                     ty: itemTypes.charCodeAt(i) - 65, // 65 = "A"
-                    name: itemNames[i],
+                    name,
                     path,
                     descShard,
                     descIndex,
                     exactPath: itemReexports.has(i) ? itemPaths.get(itemReexports.get(i)) : path,
-                    parent: itemParentIdxs[i] > 0 ? paths[itemParentIdxs[i] - 1] : undefined,
+                    parent: itemParentIdx > 0 ? paths[itemParentIdx - 1] : undefined,
                     type,
                     id,
                     word,
@@ -3655,6 +3656,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 if (!searchIndexEmptyDesc.get(crate).contains(bitIndex)) {
                     descIndex += 1;
                 }
+                lastName = name;
+                lastWord = word;
             }
 
             if (aliases) {