Auto merge of #48449 - petrochenkov:uidiff, r=nikomatsakis

Anonymize some line numbers in UI test output

New unstable flag `-Z ui-testing` is introduced. This flag changes diagnostic output of the compiler *in some way* making it more suitable for UI testing (this is intentionally vague).
At the moment this flag anonymizes line numbers at line starts thus solving the largest issue with UI test diffs. If diffs continue to be too noisy, some other tweaks could be applied (e.g. anonymizing lines/columns in `--> $DIR/file.rs:line:column`), but this needs some time and experience (we shouldn't diverge too much from the actual output in general).

If comment `// disable-ui-testing-normalization` is added to an UI test, then `-Z ui-testing` is not passed.

Closes #46643

Author: bors

src/librustc/session/config.rs

@@ -1322,6 +1322,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
          epoch). Crates compiled with different epochs can be linked together."),
     run_dsymutil: Option<bool> = (None, parse_opt_bool, [TRACKED],
           "run `dsymutil` and delete intermediate object files"),
+    ui_testing: bool = (false, parse_bool, [UNTRACKED],
+          "format compiler diagnostics in a way that's better suitable for UI testing"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc/session/mod.rs

@@ -909,21 +909,24 @@ pub fn build_session_with_codemap(sopts: config::Options,
 
     let emitter: Box<Emitter> = match (sopts.error_format, emitter_dest) {
         (config::ErrorOutputType::HumanReadable(color_config), None) => {
-            Box::new(EmitterWriter::stderr(color_config,
-                                           Some(codemap.clone()),
-                                           false,
-                                           sopts.debugging_opts.teach))
+            Box::new(EmitterWriter::stderr(color_config, Some(codemap.clone()),
+                     false, sopts.debugging_opts.teach)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::HumanReadable(_), Some(dst)) => {
-            Box::new(EmitterWriter::new(dst, Some(codemap.clone()), false, false))
+            Box::new(EmitterWriter::new(dst, Some(codemap.clone()),
+                     false, false)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::Json(pretty), None) => {
             Box::new(JsonEmitter::stderr(Some(registry), codemap.clone(),
-                     pretty, sopts.debugging_opts.approximate_suggestions))
+                     pretty, sopts.debugging_opts.approximate_suggestions)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::Json(pretty), Some(dst)) => {
             Box::new(JsonEmitter::new(dst, Some(registry), codemap.clone(),
-                     pretty, sopts.debugging_opts.approximate_suggestions))
+                     pretty, sopts.debugging_opts.approximate_suggestions)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::Short(color_config), None) => {
             Box::new(EmitterWriter::stderr(color_config, Some(codemap.clone()), true, false))