Cranelift: rework isle-in-source-tree functionality.

Per bytecodealliance#9625 and bytecodealliance#9588, a downstream user of Cranelift was misusing the
`isle-in-source-tree` feature, enabling it indiscriminately even in
published crates (wasmerio/wasmer#5202). This went against the intent of
the feature, to be a way for local developers to observe the generated
source. As such, it ran afoul of some safety checks for that purpose,
and also polluted users' Cargo caches in a way that we cannot now fix
except in future releases.

The best we can do is probably to take away features that are liable to
be misused. Following discussion in today's Cranelift weekly meeting, we
decided to provide this functionality under an environment variable
instead. This allows folks who know what they're doing to get the same
behavior, but does not expose a feature to crate users.

Fixes bytecodealliance#9625.

cranelift/codegen/Cargo.toml

@@ -128,10 +128,6 @@ souper-harvest = ["souper-ir", "souper-ir/stringify"]
 # Report any ISLE errors in pretty-printed style.
 isle-errors = ["cranelift-isle/fancy-errors"]
 
-# Put ISLE generated files in isle_generated_code/, for easier
-# inspection, rather than inside of target/.
-isle-in-source-tree = []
-
 # Enable tracking how long passes take in Cranelift.
 #
 # Enabled by default.

cranelift/codegen/build.rs

@@ -63,37 +63,24 @@ fn main() {
 
     println!("cargo:rerun-if-changed=build.rs");
 
-    let explicit_isle_dir = &crate_dir.join("isle_generated_code");
-    #[cfg(feature = "isle-in-source-tree")]
-    let isle_dir = explicit_isle_dir;
-    #[cfg(not(feature = "isle-in-source-tree"))]
-    let isle_dir = &out_dir;
-
-    #[cfg(feature = "isle-in-source-tree")]
-    {
-        std::fs::create_dir_all(isle_dir).expect("Could not create ISLE source directory");
-    }
-    #[cfg(not(feature = "isle-in-source-tree"))]
-    {
-        if explicit_isle_dir.is_dir() {
-            eprintln!(concat!(
-                "Error: directory isle_generated_code/ exists but is only used when\n",
-                "`--feature isle-in-source-tree` is specified. To prevent confusion,\n",
-                "this build script requires the directory to be removed when reverting\n",
-                "to the usual generated code in target/. Please delete the directory and\n",
-                "re-run this build.\n",
-            ));
-            std::process::exit(1);
-        }
-    }
+    let isle_dir = if let Ok(path) = std::env::var("ISLE_SOURCE_DIR") {
+        // This will canonicalize any relative path in terms of the
+        // crate root, and will take any absolute path as overriding the
+        // `crate_dir`.
+        crate_dir.join(&path)
+    } else {
+        out_dir.into()
+    };
+
+    std::fs::create_dir_all(&isle_dir).expect("Could not create ISLE source directory");
 
-    if let Err(err) = meta::generate(&isas, &out_dir, isle_dir) {
+    if let Err(err) = meta::generate(&isas, &out_dir, &isle_dir) {
         eprintln!("Error: {err}");
         process::exit(1);
     }
 
     if &std::env::var("SKIP_ISLE").unwrap_or("0".to_string()) != "1" {
-        if let Err(err) = build_isle(crate_dir, isle_dir) {
+        if let Err(err) = build_isle(crate_dir, &isle_dir) {
             eprintln!("Error: {err}");
             process::exit(1);
         }