Rollup merge of #131487 - graydon:wasm32v1-none, r=alexcrichton

Add wasm32v1-none target (compiler-team/#791)

This is a preliminary implementation of the MCP discussed in [compiler-team#791](https://github.com/rust-lang/compiler-team/issues/791). It's not especially "major" but you know, process! Anyway it adds a new wasm32v1-none target which just pins down a set of wasm features. I think this is close to the consensus that emerged when discussing it on Zulip so I figured I'd sketch to see how hard it is. Turns out not very.

3 files changed, 59 insertions, 1 deletions

diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 5ca2a339f72..f4cbe47e0f3 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -1803,6 +1803,7 @@ supported_targets! {
 
     ("wasm32-unknown-emscripten", wasm32_unknown_emscripten),
     ("wasm32-unknown-unknown", wasm32_unknown_unknown),
+    ("wasm32v1-none", wasm32v1_none),
     ("wasm32-wasi", wasm32_wasi),
     ("wasm32-wasip1", wasm32_wasip1),
     ("wasm32-wasip2", wasm32_wasip2),
diff --git a/compiler/rustc_target/src/spec/targets/wasm32v1_none.rs b/compiler/rustc_target/src/spec/targets/wasm32v1_none.rs
new file mode 100644
index 00000000000..bf35ae009c6
--- /dev/null
+++ b/compiler/rustc_target/src/spec/targets/wasm32v1_none.rs
@@ -0,0 +1,51 @@
+//! A "bare wasm" target representing a WebAssembly output that does not import
+//! anything from its environment and also specifies an _upper_ bound on the set
+//! of WebAssembly proposals that are supported.
+//!
+//! It's equivalent to the `wasm32-unknown-unknown` target with the additional
+//! flags `-Ctarget-cpu=mvp` and `-Ctarget-feature=+mutable-globals`. This
+//! enables just the features specified in <https://www.w3.org/TR/wasm-core-1/>
+//!
+//! This is a _separate target_ because using `wasm32-unknown-unknown` with
+//! those target flags doesn't automatically rebuild libcore / liballoc with
+//! them, and in order to get those libraries rebuilt you need to use the
+//! nightly Rust feature `-Zbuild-std`. This target is for people who want to
+//! use stable Rust, and target a stable set pf WebAssembly features.
+
+use crate::spec::{Cc, LinkerFlavor, Target, base};
+
+pub(crate) fn target() -> Target {
+    let mut options = base::wasm::options();
+    options.os = "none".into();
+
+    // WebAssembly 1.0 shipped in 2019 and included exactly one proposal
+    // after the initial "MVP" feature set: "mutable-globals".
+    options.cpu = "mvp".into();
+    options.features = "+mutable-globals".into();
+
+    options.add_pre_link_args(LinkerFlavor::WasmLld(Cc::No), &[
+        // For now this target just never has an entry symbol no matter the output
+        // type, so unconditionally pass this.
+        "--no-entry",
+    ]);
+    options.add_pre_link_args(LinkerFlavor::WasmLld(Cc::Yes), &[
+        // Make sure clang uses LLD as its linker and is configured appropriately
+        // otherwise
+        "--target=wasm32-unknown-unknown",
+        "-Wl,--no-entry",
+    ]);
+
+    Target {
+        llvm_target: "wasm32-unknown-unknown".into(),
+        metadata: crate::spec::TargetMetadata {
+            description: Some("WebAssembly".into()),
+            tier: Some(2),
+            host_tools: Some(false),
+            std: Some(false),
+        },
+        pointer_width: 32,
+        data_layout: "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20".into(),
+        arch: "wasm32".into(),
+        options,
+    }
+}
diff --git a/compiler/rustc_target/src/spec/tests/tests_impl.rs b/compiler/rustc_target/src/spec/tests/tests_impl.rs
index 7ed21954fba..cc5931be860 100644
--- a/compiler/rustc_target/src/spec/tests/tests_impl.rs
+++ b/compiler/rustc_target/src/spec/tests/tests_impl.rs
@@ -121,7 +121,13 @@ impl Target {
         // Check dynamic linking stuff
         // BPF: when targeting user space vms (like rbpf), those can load dynamic libraries.
         // hexagon: when targeting QuRT, that OS can load dynamic libraries.
-        if self.os == "none" && (self.arch != "bpf" && self.arch != "hexagon") {
+        // wasm{32,64}: dynamic linking is inherent in the definition of the VM.
+        if self.os == "none"
+            && (self.arch != "bpf"
+                && self.arch != "hexagon"
+                && self.arch != "wasm32"
+                && self.arch != "wasm64")
+        {
             assert!(!self.dynamic_linking);
         }
         if self.only_cdylib