Warn when rustc output conflicts with existing directories

When the compiled executable would conflict with a directory, display a
rustc error instead of a verbose and potentially-confusing linker
error. This is a usability improvement, and doesn’t actually change
behaviour with regards to compilation success. This addresses the
concern in #35887.

src/librustc/session/config.rs

@@ -549,23 +549,43 @@ impl OutputFilenames {
         format!("{}{}", self.out_filestem, self.extra)
     }
 
+    fn check_output<F, T>(&self, f: F) -> Option<T> where F: Fn(PathBuf) -> Option<T> {
+        match self.single_output_file {
+            Some(ref output_path) => {
+                f(output_path.clone())
+            },
+            None => {
+                for k in self.outputs.keys() {
+                    let output_path = self.path(k.to_owned());
+                    if let Some(result) = f(output_path) {
+                        return Some(result);
+                    }
+                }
+                None
+            }
+        }
+    }
+
     pub fn contains_path(&self, input_path: &PathBuf) -> bool {
         let input_path = input_path.canonicalize().ok();
         if input_path.is_none() {
             return false
         }
-        match self.single_output_file {
-            Some(ref output_path) => output_path.canonicalize().ok() == input_path,
-            None => {
-                for k in self.outputs.keys() {
-                    let output_path = self.path(k.to_owned());
+        let check = |output_path: PathBuf| {
             if output_path.canonicalize().ok() == input_path {
-                        return true;
-                    }
-                }
-                false
-            }
+                Some(())
+            } else { None }
+        };
+        self.check_output(check).is_some()
     }
+
+    pub fn conflicts_with_dir(&self) -> Option<PathBuf> {
+        let check = |output_path: PathBuf| {
+            if output_path.is_dir() {
+                Some(output_path)
+            } else { None }
+        };
+        self.check_output(check)
     }
 }
 

src/librustc_driver/driver.rs

@@ -125,12 +125,21 @@ pub fn compile_input(trans: Box<TransCrate>,
         // Ensure the source file isn't accidentally overwritten during compilation.
         match *input_path {
             Some(ref input_path) => {
-                if outputs.contains_path(input_path) && sess.opts.will_create_output_file() {
+                if sess.opts.will_create_output_file() {
+                    if outputs.contains_path(input_path) {
                         sess.err(&format!(
                             "the input file \"{}\" would be overwritten by the generated executable",
                             input_path.display()));
                         return Err(CompileIncomplete::Stopped);
                     }
+                    if let Some(dir_path) = outputs.conflicts_with_dir() {
+                        sess.err(&format!(
+                            "the generated executable for the input file \"{}\" conflicts with the \
+                            existing directory \"{}\'",
+                            input_path.display(), dir_path.display()));
+                        return Err(CompileIncomplete::Stopped);
+                    }
+                }
             },
             None => {}
         }

src/test/run-make/output-filename-conflicts-with-directory/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	cp foo.rs $(TMPDIR)/foo.rs
+	mkdir $(TMPDIR)/foo
+	$(RUSTC) $(TMPDIR)/foo.rs 2>&1 \
+		| $(CGREP) -e "the generated executable for the input file \".*foo\.rs\" conflicts with the existing directory \".*foo\'"

src/test/run-make/output-filename-conflicts-with-directory/foo.rs

@@ -0,0 +1,11 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {}