Use target exe_suffix for doctests

This will use rust_out.exe for doctests on Windows,
rust_out.wasm for doctests in the wasm case, and
also handles cross-compiling or user-provided targets.

src/librustdoc/doctest.rs

@@ -19,7 +19,7 @@ use rustc_span::edition::Edition;
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::sym;
 use rustc_span::{BytePos, FileName, Pos, Span, DUMMY_SP};
-use rustc_target::spec::TargetTriple;
+use rustc_target::spec::{Target, TargetTriple};
 use tempfile::Builder as TempFileBuilder;
 
 use std::env;
@@ -293,6 +293,16 @@ struct UnusedExterns {
     unused_extern_names: Vec<String>,
 }
 
+fn add_exe_suffix(input: String, target: &TargetTriple) -> String {
+    let exe_suffix = match target {
+        TargetTriple::TargetTriple(_) => Target::expect_builtin(target).options.exe_suffix,
+        TargetTriple::TargetJson { contents, .. } => {
+            Target::from_json(contents.parse().unwrap()).unwrap().0.options.exe_suffix
+        }
+    };
+    input + &exe_suffix
+}
+
 fn run_test(
     test: &str,
     crate_name: &str,
@@ -313,7 +323,9 @@ fn run_test(
     let (test, line_offset, supports_color) =
         make_test(test, Some(crate_name), lang_string.test_harness, opts, edition, Some(test_id));
 
-    let output_file = outdir.path().join("rust_out");
+    // Make sure we emit well-formed executable names for our target.
+    let rust_out = add_exe_suffix("rust_out".to_owned(), &target);
+    let output_file = outdir.path().join(rust_out);
 
     let rustc_binary = rustdoc_options
         .test_builder