Rollup merge of #89468 - FabianWolff:issue-89358, r=jackh726

Report fatal lexer errors in `--cfg` command line arguments

Fixes #89358. The erroneous behavior was apparently introduced by `@Mark-Simulacrum` in a678e31; the idea is to silence individual parser errors and instead emit one catch-all error message after parsing. However, for the example in #89358, a fatal lexer error is created here:
https://github.com/rust-lang/rust/blob/edebf77e0090195bf80c0d8cda821e1bf9d03053/compiler/rustc_parse/src/lexer/mod.rs#L340-L349

This fatal error aborts the compilation, and so the call to `new_parser_from_source_str()` never returns and the catch-all error message is never emitted. I have therefore changed the `SilentEmitter` to silence only non-fatal errors; with my changes, for the rustc invocation described in #89358:
```sh
rustc --cfg "abc\""
```
I get the following output:
```
error[E0765]: unterminated double quote string
  |
  = note: this error occurred on the command line: `--cfg=abc"`
```

compiler/rustc_errors/src/emitter.rs

@@ -15,7 +15,7 @@ use rustc_span::{MultiSpan, SourceFile, Span};
 use crate::snippet::{Annotation, AnnotationType, Line, MultilineAnnotation, Style, StyledString};
 use crate::styled_buffer::StyledBuffer;
 use crate::{
-    CodeSuggestion, Diagnostic, DiagnosticId, Level, SubDiagnostic, SubstitutionHighlight,
+    CodeSuggestion, Diagnostic, DiagnosticId, Handler, Level, SubDiagnostic, SubstitutionHighlight,
     SuggestionStyle,
 };
 
@@ -523,14 +523,27 @@ impl Emitter for EmitterWriter {
     }
 }
 
-/// An emitter that does nothing when emitting a diagnostic.
-pub struct SilentEmitter;
+/// An emitter that does nothing when emitting a non-fatal diagnostic.
+/// Fatal diagnostics are forwarded to `fatal_handler` to avoid silent
+/// failures of rustc, as witnessed e.g. in issue #89358.
+pub struct SilentEmitter {
+    pub fatal_handler: Handler,
+    pub fatal_note: Option<String>,
+}
 
 impl Emitter for SilentEmitter {
     fn source_map(&self) -> Option<&Lrc<SourceMap>> {
         None
     }
-    fn emit_diagnostic(&mut self, _: &Diagnostic) {}
+    fn emit_diagnostic(&mut self, d: &Diagnostic) {
+        if d.level == Level::Fatal {
+            let mut d = d.clone();
+            if let Some(ref note) = self.fatal_note {
+                d.note(note);
+            }
+            self.fatal_handler.emit_diagnostic(&d);
+        }
+    }
 }
 
 /// Maximum number of lines we will print for a multiline suggestion; arbitrary.

compiler/rustc_interface/src/interface.rs

@@ -75,7 +75,10 @@ pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String
         let cfg = cfgspecs
             .into_iter()
             .map(|s| {
-                let sess = ParseSess::with_silent_emitter();
+                let sess = ParseSess::with_silent_emitter(Some(format!(
+                    "this error occurred on the command line: `--cfg={}`",
+                    s
+                )));
                 let filename = FileName::cfg_spec_source_code(&s);
                 let mut parser = new_parser_from_source_str(&sess, filename, s.to_string());
 

compiler/rustc_session/src/parse.rs

@@ -174,9 +174,14 @@ impl ParseSess {
         }
     }
 
-    pub fn with_silent_emitter() -> Self {
+    pub fn with_silent_emitter(fatal_note: Option<String>) -> Self {
         let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
-        let handler = Handler::with_emitter(false, None, Box::new(SilentEmitter));
+        let fatal_handler = Handler::with_tty_emitter(ColorConfig::Auto, false, None, None);
+        let handler = Handler::with_emitter(
+            false,
+            None,
+            Box::new(SilentEmitter { fatal_handler, fatal_note }),
+        );
         ParseSess::with_span_handler(handler, sm)
     }
 

src/test/ui/conditional-compilation/cfg-arg-invalid-7.rs

@@ -0,0 +1,5 @@
+// Regression test for issue #89358.
+
+// compile-flags: --cfg a"
+// error-pattern: unterminated double quote string
+// error-pattern: this error occurred on the command line

src/test/ui/conditional-compilation/cfg-arg-invalid-7.stderr

@@ -0,0 +1,4 @@
+error[E0765]: unterminated double quote string
+  |
+  = note: this error occurred on the command line: `--cfg=a"`
+