Write to stdout if `-` is given as output file

If `-o -` or `--emit KIND=-` is provided, output will be written
to stdout instead. Binary output (`obj`, `llvm-bc`, `link` and
`metadata`) being written this way will result in an error unless
stdout is not a tty. Multiple output types going to stdout will
trigger an error too, as they will all be mixded together.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/doc/rustc/src/command-line-arguments.md b/src/doc/rustc/src/command-line-arguments.md
index 3be4382b0a3..2c15d1b1110 100644
--- a/src/doc/rustc/src/command-line-arguments.md
+++ b/src/doc/rustc/src/command-line-arguments.md
@@ -202,6 +202,12 @@ flag](codegen-options/index.md#extra-filename). The files are written to the
 current directory unless the [`--out-dir` flag](#option-out-dir) is used. Each
 emission type may also specify the output filename with the form `KIND=PATH`,
 which takes precedence over the `-o` flag.
+Specifying `-o -` or `--emit KIND=-` asks rustc to emit to stdout.
+Text output types (`asm`, `dep-info`, `llvm-ir` and `mir`) can be written to
+stdout despite it being a tty or not. This will result in an error if any
+binary output type is written to stdout that is a tty.
+This will also result in an error if multiple output types
+would be written to stdout, because they would be all mixed together.
 
 [LLVM bitcode]: https://llvm.org/docs/BitCodeFormat.html
 [LLVM IR]: https://llvm.org/docs/LangRef.html