Remove serialization of diagnostics to files

This is no longer used by the index generator and was always an unstable
compiler detail, so strip it out.

This also leaves in RUSTC_ERROR_METADATA_DST since the stage0 compiler
still needs it to be set.

src/bootstrap/doc.rs

@@ -825,8 +825,7 @@ impl Step for ErrorIndex {
         index.arg(crate::channel::CFG_RELEASE_NUM);
 
         // FIXME: shouldn't have to pass this env var
-        index.env("CFG_BUILD", &builder.config.build)
-             .env("RUSTC_ERROR_METADATA_DST", builder.extended_error_dir());
+        index.env("CFG_BUILD", &builder.config.build);
 
         builder.run(&mut index);
     }

src/bootstrap/test.rs

@@ -1535,8 +1535,7 @@ impl Step for ErrorIndex {
         );
         tool.arg("markdown")
             .arg(&output)
-            .env("CFG_BUILD", &builder.config.build)
-            .env("RUSTC_ERROR_METADATA_DST", builder.extended_error_dir());
+            .env("CFG_BUILD", &builder.config.build);
 
         builder.info(&format!("Testing error-index stage{}", compiler.stage));
         let _time = util::timeit(&builder);

src/libsyntax/diagnostics/plugin.rs

@@ -1,5 +1,4 @@
 use std::collections::BTreeMap;
-use std::env;
 
 use crate::ast::{self, Ident, Name};
 use crate::source_map;
@@ -12,8 +11,6 @@ use crate::tokenstream::{TokenTree};
 use smallvec::smallvec;
 use syntax_pos::Span;
 
-use crate::diagnostics::metadata::output_metadata;
-
 pub use errors::*;
 
 // Maximum width of any line in an extended error description (inclusive).
@@ -127,36 +124,13 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt<'_>,
                                           token_tree: &[TokenTree])
                                           -> Box<dyn MacResult+'cx> {
     assert_eq!(token_tree.len(), 3);
-    let (crate_name, ident) = match (&token_tree[0], &token_tree[2]) {
-        (
-            // Crate name.
-            &TokenTree::Token(Token { kind: token::Ident(crate_name, _), .. }),
-            // DIAGNOSTICS ident.
-            &TokenTree::Token(Token { kind: token::Ident(name, _), span })
-        ) => (crate_name, Ident::new(name, span)),
+    let ident = match &token_tree[2] {
+        // DIAGNOSTICS ident.
+        &TokenTree::Token(Token { kind: token::Ident(name, _), span })
+        => Ident::new(name, span),
         _ => unreachable!()
     };
 
-    // Output error metadata to `tmp/extended-errors/<target arch>/<crate name>.json`
-    if let Ok(target_triple) = env::var("CFG_COMPILER_HOST_TRIPLE") {
-        ecx.parse_sess.registered_diagnostics.with_lock(|diagnostics| {
-            if let Err(e) = output_metadata(ecx,
-                                            &target_triple,
-                                            &crate_name.as_str(),
-                                            diagnostics) {
-                ecx.span_bug(span, &format!(
-                    "error writing metadata for triple `{}` and crate `{}`, error: {}, \
-                     cause: {:?}",
-                    target_triple, crate_name, e.description(), e.source()
-                ));
-            }
-        });
-    } else {
-        ecx.span_err(span, &format!(
-            "failed to write metadata for crate `{}` because $CFG_COMPILER_HOST_TRIPLE is not set",
-            crate_name));
-    }
-
     // Construct the output expression.
     let (count, expr) =
         ecx.parse_sess.registered_diagnostics.with_lock(|diagnostics| {

src/libsyntax/lib.rs

@@ -124,7 +124,6 @@ pub mod diagnostics {
     #[macro_use]
     pub mod macros;
     pub mod plugin;
-    pub mod metadata;
 }
 
 // N.B., this module needs to be declared first so diagnostics are

src/tools/error_index_generator/main.rs

@@ -13,10 +13,16 @@ use std::path::PathBuf;
 use std::cell::RefCell;
 
 use syntax::edition::DEFAULT_EDITION;
-use syntax::diagnostics::metadata::{ErrorMetadataMap, ErrorMetadata};
 
 use rustdoc::html::markdown::{Markdown, IdMap, ErrorCodes, Playground};
 
+pub struct ErrorMetadata {
+    pub description: Option<String>,
+}
+
+/// Mapping from error codes to metadata that can be (de)serialized.
+pub type ErrorMetadataMap = BTreeMap<String, ErrorMetadata>;
+
 enum OutputFormat {
     HTML(HTMLFormatter),
     Markdown(MarkdownFormatter),
@@ -214,9 +220,6 @@ fn main_with_result(format: OutputFormat, dst: &Path) -> Result<(), Box<dyn Erro
     for (code, desc) in long_codes {
         err_map.insert(code.to_string(), ErrorMetadata {
             description: desc.map(String::from),
-            // FIXME: this indicates that the error code is not used, which may not be true.
-            // We currently do not use this information.
-            use_site: None,
         });
     }
     match format {