Auto merge of #116207 - Ayush1325:uefi_stdio, r=Mark-Simulacrum

Stdio support for UEFI

- Uses Simple Text Output Protocol and Simple Text Input Protocol
- Reading is done one character at a time
- Writing is done with max 4096 characters

# Quirks
## Output Newline
- UEFI uses CRLF for newline. So when running the application in UEFI shell (qemu VGA), the output of `println` looks weird.
- However, since the UEFI shell supports piping output, I am unsure if doing any output post-processing is a good idea. UEFI shell `cat` command seems to work fine with just LF.

## Input Newline
- `Stdin.read_line()` method is broken in UEFI shell. Pressing enter seems to be read as CR, which means LF is never encountered.
- Works fine with input redirection from file.

CC `@dvdhrm`

1 files changed, 7 insertions, 1 deletions

diff --git a/src/doc/rustc/src/platform-support/unknown-uefi.md b/src/doc/rustc/src/platform-support/unknown-uefi.md
index 68cd7fae319..370939520dc 100644
--- a/src/doc/rustc/src/platform-support/unknown-uefi.md
+++ b/src/doc/rustc/src/platform-support/unknown-uefi.md
@@ -265,9 +265,12 @@ cargo build --target x86_64-unknown-uefi -Zbuild-std=std,panic_abort
 #### os_str
 - While the strings in UEFI should be valid UCS-2, in practice, many implementations just do not care and use UTF-16 strings.
 - Thus, the current implementation supports full UTF-16 strings.
+#### stdio
+- Uses `Simple Text Input Protocol` and `Simple Text Output Protocol`.
+- Note: UEFI uses CRLF for new line. This means Enter key is registered as CR instead of LF.
 
 ## Example: Hello World With std
-The following code features a valid UEFI application, including stdio and `alloc` (`OsString` and `Vec`):
+The following code features a valid UEFI application, including `stdio` and `alloc` (`OsString` and `Vec`):
 
 This example can be compiled as binary crate via `cargo` using the toolchain
 compiled from the above source (named custom):
@@ -286,6 +289,9 @@ use std::{
 };
 
 pub fn main() {
+  println!("Starting Rust Application...");
+
+  // Use System Table Directly
   let st = env::system_table().as_ptr() as *mut efi::SystemTable;
   let mut s: Vec<u16> = OsString::from("Hello World!\n").encode_wide().collect();
   s.push(0);