Auto merge of #47052 - fschutt:master, r=estebank

Improved error messages for linking failure

Partial fix for #46998

It's unnecessary to print the linker options if there is no linker installed in the first place. Currently, for libraries, the output is still printed, but that should be cleaned up in the future. If you don't have gcc or g++ installed, so that no linker is installed on the system, the output is now this:

```
$ ./rustc hello.rs
error: linker `cc` not found
  |
  = note: No such file or directory (os error 2)

error: aborting due to previous error
```

For libraries, the linker arguments are still printed, but they should be cleaned up in further commits.

Author: bors

Diff for: src/Cargo.lock

@@ -708,10 +708,25 @@ fn link_natively(sess: &Session,
             info!("linker stdout:\n{}", escape_string(&prog.stdout));
         },
         Err(e) => {
-            sess.struct_err(&format!("could not exec the linker `{}`: {}", pname.display(), e))
-                .note(&format!("{:?}", &cmd))
-                .emit();
-            if sess.target.target.options.is_like_msvc && e.kind() == io::ErrorKind::NotFound {
+            let linker_not_found = e.kind() == io::ErrorKind::NotFound;
+
+            let mut linker_error = {
+                if linker_not_found {
+                    sess.struct_err(&format!("linker `{}` not found", pname.display()))
+                } else {
+                    sess.struct_err(&format!("could not exec the linker `{}`", pname.display()))
+                }
+            };
+
+            linker_error.note(&format!("{}", e));
+
+            if !linker_not_found {
+                linker_error.note(&format!("{:?}", &cmd));
+            }
+
+            linker_error.emit();
+
+            if sess.target.target.options.is_like_msvc && linker_not_found {
                 sess.note_without_error("the msvc targets depend on the msvc linker \
                     but `link.exe` was not found");
                 sess.note_without_error("please ensure that VS 2013 or VS 2015 was installed \

Diff for: src/librustc_trans/back/link.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 // compile-flags: -C linker=llllll -Z linker-flavor=ld
-// error-pattern: the linker `llllll`
+// error-pattern: linker `llllll` not found
 
 fn main() {
 }