Rollup merge of #48765 - Phlosioneer:10234-wall-help-message, r=estebank

Add info message for -Wall command

Users coming from other languages (namely C and C++) often expect
to use a -Wall flag. Rustc doesn't support that, and previously it
simply printed that it didn't recognize the "all" lint.

This change makes rustc print out a help message, explaining:
- Why there is no -Wall flag
- How to view all the available warnings
- Point out that the most commonly used warning is -Wunused
- Instead of using a command-line flag, the user should consider
  a !#[warn(unused)] directive in the root of their crate.

I tried to keep the language consistent with the other usage help. Comment if I should change anything.

closes #10234, if accepted.

src/librustc_driver/lib.rs

@@ -1147,6 +1147,15 @@ fn usage(verbose: bool, include_unstable_options: bool) {
              verbose_help);
 }
 
+fn print_wall_help() {
+    println!("
+The flag `-Wall` does not exist in `rustc`. Most useful lints are enabled by
+default. Use `rustc -W help` to see all available lints. It's more common to put
+warning settings in the crate root using `#![warn(LINT_NAME)]` instead of using
+the command line flag directly.
+");
+}
+
 fn describe_lints(sess: &Session, lint_store: &lint::LintStore, loaded_plugins: bool) {
     println!("
 Available lint options:
@@ -1391,6 +1400,13 @@ pub fn handle_options(args: &[String]) -> Option<getopts::Matches> {
         return None;
     }
 
+    // Handle the special case of -Wall.
+    let wall = matches.opt_strs("W");
+    if wall.iter().any(|x| *x == "all") {
+        print_wall_help();
+        return None;
+    }
+
     // Don't handle -W help here, because we might first load plugins.
     let r = matches.opt_strs("Z");
     if r.iter().any(|x| *x == "help") {