Warn when SWIFT_EXEC or SWIFT_EXEC_MANIFEST point to invalid paths.

Previously when SWIFT_EXEC or SWIFT_EXEC_MANIFEST were set to non-existent
or non-executable paths, swiftpm would silently ignore them and fall back to
the default Swift compiler. This caused confusion during development when
typos or incorrect paths were used.

Author: rjmansfield

Sources/PackageModel/UserToolchain.swift

@@ -351,10 +351,34 @@ public final class UserToolchain: Toolchain {
         }
 
         let lookup = { UserToolchain.lookup(variable: $0, searchPaths: searchPaths, environment: environment) }
+
+        // Warn if SWIFT_EXEC or SWIFT_EXEC_MANIFEST is set but points to a non-existent or non-executable path
+        func warnIfInvalid(envVar: String, value: String, resolved: AbsolutePath?) {
+            guard resolved == nil else { return }
+
+            if let absolutePath = try? AbsolutePath(validating: value) {
+                if fileSystem.exists(absolutePath) {
+                    fputs("warning: \(envVar) is set to '\(value)' which exists but is not executable; ignoring\n", stderr)
+                } else {
+                    fputs("warning: \(envVar) is set to '\(value)' but the file does not exist; ignoring\n", stderr)
+                }
+            } else {
+                fputs("warning: \(envVar) is set to '\(value)' but no executable was found in search paths; ignoring\n", stderr)
+            }
+        }
+
         // Get overrides.
         let SWIFT_EXEC_MANIFEST = lookup("SWIFT_EXEC_MANIFEST")
         let SWIFT_EXEC = lookup("SWIFT_EXEC")
 
+        // Emit warnings if environment variables are set but lookup failed
+        if let swiftExecValue = environment["SWIFT_EXEC"], !swiftExecValue.isEmpty {
+            warnIfInvalid(envVar: "SWIFT_EXEC", value: swiftExecValue, resolved: SWIFT_EXEC)
+        }
+        if let swiftExecManifestValue = environment["SWIFT_EXEC_MANIFEST"], !swiftExecManifestValue.isEmpty {
+            warnIfInvalid(envVar: "SWIFT_EXEC_MANIFEST", value: swiftExecManifestValue, resolved: SWIFT_EXEC_MANIFEST)
+        }
+
         // Validate the overrides.
         try validateCompiler(at: SWIFT_EXEC)
         try validateCompiler(at: SWIFT_EXEC_MANIFEST)