Strip code to the left and right in diagnostics for long lines

Cargo.lock

@@ -3219,6 +3219,7 @@ dependencies = [
  "rustc_data_structures",
  "serialize",
  "syntax_pos",
+ "term_size",
  "termcolor",
  "unicode-width",
 ]
@@ -4090,6 +4091,17 @@ dependencies = [
  "winapi 0.3.6",
 ]
 
+[[package]]
+name = "term_size"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9e5b9a66db815dcfd2da92db471106457082577c3c278d4138ab3e3b4e189327"
+dependencies = [
+ "kernel32-sys",
+ "libc",
+ "winapi 0.2.8",
+]
+
 [[package]]
 name = "termcolor"
 version = "1.0.4"

src/librustc/session/config.rs

@@ -1292,6 +1292,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "show macro backtraces even for non-local macros"),
     teach: bool = (false, parse_bool, [TRACKED],
         "show extended diagnostic help"),
+    terminal_width: Option<usize> = (None, parse_opt_uint, [UNTRACKED],
+        "set the current terminal width"),
     continue_parse_after_error: bool = (false, parse_bool, [TRACKED],
         "attempt to recover from parse errors (experimental)"),
     dep_tasks: bool = (false, parse_bool, [UNTRACKED],

src/librustc/session/mod.rs

@@ -1055,13 +1055,15 @@ fn default_emitter(
                         Some(source_map.clone()),
                         short,
                         sopts.debugging_opts.teach,
+                        sopts.debugging_opts.terminal_width,
                     ),
                     Some(dst) => EmitterWriter::new(
                         dst,
                         Some(source_map.clone()),
                         short,
                         false, // no teach messages when writing to a buffer
                         false, // no colors when writing to a buffer
+                        None,  // no terminal width
                     ),
                 };
                 Box::new(emitter.ui_testing(sopts.debugging_opts.ui_testing))
@@ -1375,7 +1377,7 @@ pub fn early_error(output: config::ErrorOutputType, msg: &str) -> ! {
     let emitter: Box<dyn Emitter + sync::Send> = match output {
         config::ErrorOutputType::HumanReadable(kind) => {
             let (short, color_config) = kind.unzip();
-            Box::new(EmitterWriter::stderr(color_config, None, short, false))
+            Box::new(EmitterWriter::stderr(color_config, None, short, false, None))
         }
         config::ErrorOutputType::Json { pretty, json_rendered } =>
             Box::new(JsonEmitter::basic(pretty, json_rendered)),
@@ -1389,7 +1391,7 @@ pub fn early_warn(output: config::ErrorOutputType, msg: &str) {
     let emitter: Box<dyn Emitter + sync::Send> = match output {
         config::ErrorOutputType::HumanReadable(kind) => {
             let (short, color_config) = kind.unzip();
-            Box::new(EmitterWriter::stderr(color_config, None, short, false))
+            Box::new(EmitterWriter::stderr(color_config, None, short, false, None))
         }
         config::ErrorOutputType::Json { pretty, json_rendered } =>
             Box::new(JsonEmitter::basic(pretty, json_rendered)),

src/librustc_driver/lib.rs

@@ -1135,11 +1135,13 @@ pub fn report_ices_to_stderr_if_any<F: FnOnce() -> R, R>(f: F) -> Result<R, Erro
             // Thread panicked without emitting a fatal diagnostic
             eprintln!("");
 
-            let emitter =
-                Box::new(errors::emitter::EmitterWriter::stderr(errors::ColorConfig::Auto,
-                                                                None,
-                                                                false,
-                                                                false));
+            let emitter = Box::new(errors::emitter::EmitterWriter::stderr(
+                errors::ColorConfig::Auto,
+                None,
+                false,
+                false,
+                None,
+            ));
             let handler = errors::Handler::with_emitter(true, None, emitter);
 
             // a .span_bug or .bug call has already printed what

src/librustc_errors/Cargo.toml

@@ -18,3 +18,4 @@ unicode-width = "0.1.4"
 atty = "0.2"
 termcolor = "1.0"
 annotate-snippets = "0.6.1"
+term_size = "0.3.1"