fix(R): detect and warn about x64 R on Windows ARM

When x64 R fails on Windows ARM, parse YAML output to detect the architecture
mismatch and provide a specific error message with ARM64 R download links
instead of the generic "check your R installation" message.

Also adds platform information to quarto check output with warning when
x64 R is detected on ARM Windows.

Author: cderv

src/core/knitr.ts

@@ -11,13 +11,15 @@ import { rBinaryPath, resourcePath } from "./resources.ts";
 import { readYamlFromString } from "./yaml.ts";
 import { coerce, satisfies } from "semver/mod.ts";
 import { debug } from "../deno_ral/log.ts";
+import { isWindows } from "../deno_ral/platform.ts";
 
 export interface KnitrCapabilities {
   versionMajor: number;
   versionMinor: number;
   versionPatch: number;
   home: string;
   libPaths: string[];
+  platform?: string;
   packages: KnitrRequiredPackages;
 }
 
@@ -115,6 +117,42 @@ export async function knitrCapabilities(rBin: string | undefined) {
       if (result.stderr) {
         debug(`    with stderr from R:\n${result.stderr}`);
       }
+
+      // Check if this is x64 R on Windows ARM
+      if (result.stdout) {
+        try {
+          const yamlMatch = result.stdout.match(/--- YAML_START ---(.*)--- YAML_END ---/s);
+          if (yamlMatch) {
+            const yamlLines = yamlMatch[1];
+            const caps = readYamlFromString(yamlLines) as KnitrCapabilities;
+
+            if (caps.platform) {
+              const isWindowsArm = isWindows && Deno.build.arch === "aarch64";
+              const isX64R = caps.platform.includes("x86_64") || caps.platform.includes("i386");
+
+              if (isWindowsArm && isX64R) {
+                throw new Error(
+                  "x64 R detected on Windows ARM.\n\n" +
+                  "x64 R runs under emulation and is not reliable for Quarto.\n" +
+                  "Please install native ARM64 R:\n" +
+                  "  Download: https://www.r-project.org/nosvn/winutf8/aarch64/R-4-signed/\n" +
+                  "  RTools: https://cran.r-project.org/bin/windows/Rtools/rtools45/files/\n\n" +
+                  "After installation, set QUARTO_R environment variable:\n" +
+                  "  set QUARTO_R=C:\\Program Files\\R-aarch64\\R-4.5.0\\bin\\Rscript.exe"
+                );
+              }
+            }
+          }
+        } catch (e) {
+          // If it's our specific x64-on-ARM error, rethrow it
+          if (e instanceof Error && e.message.includes("x64 R detected")) {
+            throw e;
+          }
+          // Otherwise YAML parse failed, continue to return undefined
+          debug("    Failed to parse YAML for architecture detection");
+        }
+      }
+
       return undefined;
     }
   } catch {
@@ -136,6 +174,20 @@ export function knitrCapabilitiesMessage(caps: KnitrCapabilities, indent = "") {
   for (const path of caps.libPaths) {
     lines.push(`  - ${path}`);
   }
+
+  // Show platform information if available
+  if (caps.platform) {
+    lines.push(`Platform: ${caps.platform}`);
+
+    // Check for x64 R on Windows ARM and show warning
+    const isWindowsArm = isWindows && Deno.build.arch === "aarch64";
+    const isX64R = caps.platform.includes("x86_64") || caps.platform.includes("i386");
+    if (isWindowsArm && isX64R) {
+      lines.push(`⚠️  WARNING: x64 R on Windows ARM is not supported`);
+      lines.push(`    Install ARM64 R: https://www.r-project.org/nosvn/winutf8/aarch64/R-4-signed/`);
+    }
+  }
+
   lines.push(`knitr: ${caps.packages.knitr || "(None)"}`);
   if (caps.packages.knitr && !caps.packages.knitrVersOk) {
     lines.push(

src/resources/capabilities/knitr.R

@@ -8,6 +8,7 @@ cat("libPaths:\n")
 for (lib in .libPaths()) {
   cat(paste0('  - ', shQuote(lib)), "\n")
 }
+cat("platform:", R.version[['platform']], "\n")
 cat("packages:\n")
 cat("  knitr: ")
 if (requireNamespace("knitr", quietly = TRUE)) {