Lint on invalid values passed to x.py --warnings

This also introduces support for `--warnings allow` and fixes --warnings
being overridden by the configuration file, config.toml.

src/bootstrap/config.rs

@@ -405,7 +405,7 @@ impl Config {
         config.incremental = flags.incremental;
         config.dry_run = flags.dry_run;
         config.keep_stage = flags.keep_stage;
-        if let Some(value) = flags.warnings {
+        if let Some(value) = flags.deny_warnings {
             config.deny_warnings = value;
         }
 
@@ -571,7 +571,7 @@ impl Config {
             config.rustc_default_linker = rust.default_linker.clone();
             config.musl_root = rust.musl_root.clone().map(PathBuf::from);
             config.save_toolstates = rust.save_toolstates.clone().map(PathBuf::from);
-            set(&mut config.deny_warnings, rust.deny_warnings.or(flags.warnings));
+            set(&mut config.deny_warnings, flags.deny_warnings.or(rust.deny_warnings));
             set(&mut config.backtrace_on_ice, rust.backtrace_on_ice);
             set(&mut config.rust_verify_llvm_ir, rust.verify_llvm_ir);
             set(&mut config.rust_remap_debuginfo, rust.remap_debuginfo);

src/bootstrap/flags.rs

@@ -33,8 +33,11 @@ pub struct Flags {
     pub rustc_error_format: Option<String>,
     pub dry_run: bool,
 
-    // true => deny
-    pub warnings: Option<bool>,
+    // This overrides the deny-warnings configuation option,
+    // which passes -Dwarnings to the compiler invocations.
+    //
+    // true => deny, false => allow
+    pub deny_warnings: Option<bool>,
 }
 
 pub enum Subcommand {
@@ -468,7 +471,7 @@ Arguments:
                 .into_iter()
                 .map(|p| p.into())
                 .collect::<Vec<_>>(),
-            warnings: matches.opt_str("warnings").map(|v| v == "deny"),
+            deny_warnings: parse_deny_warnings(&matches),
         }
     }
 }
@@ -549,3 +552,18 @@ fn split(s: &[String]) -> Vec<String> {
         .map(|s| s.to_string())
         .collect()
 }
+
+fn parse_deny_warnings(matches: &getopts::Matches) -> Option<bool> {
+    match matches.opt_str("warnings").as_ref().map(|v| v.as_str()) {
+        Some("deny") => Some(true),
+        Some("allow") => Some(false),
+        Some(value) => {
+            eprintln!(
+                r#"invalid value for --warnings: {:?}, expected "allow" or "deny""#,
+                value,
+                );
+            process::exit(1);
+        },
+        None => None,
+    }
+}