Add support for rustc's -Z terminal-width.

src/bin/cargo/cli.rs

@@ -37,6 +37,7 @@ Available unstable (nightly-only) flags:
     -Z timings          -- Display concurrency information
     -Z doctest-xcompile -- Compile and run doctests for non-host target using runner config
     -Z crate-versions   -- Add crate versions to generated docs
+    -Z terminal-width   -- Provide a terminal width to rustc for error truncation
 
 Run with 'cargo -Z [FLAG] [SUBCOMMAND]'"
         );

src/cargo/core/compiler/mod.rs

@@ -47,6 +47,7 @@ pub use self::lto::Lto;
 use self::output_depinfo::output_depinfo;
 use self::unit_graph::UnitDep;
 pub use crate::core::compiler::unit::{Unit, UnitInterner};
+use crate::core::features::nightly_features_allowed;
 use crate::core::manifest::TargetSourcePath;
 use crate::core::profiles::{PanicStrategy, Profile, Strip};
 use crate::core::{Edition, Feature, PackageId, Target};
@@ -709,13 +710,34 @@ fn add_error_format_and_color(
         // to emit a message that cargo will intercept.
         json.push_str(",artifacts");
     }
+
     match cx.bcx.build_config.message_format {
         MessageFormat::Short | MessageFormat::Json { short: true, .. } => {
             json.push_str(",diagnostic-short");
         }
         _ => {}
     }
     cmd.arg(json);
+
+    if nightly_features_allowed() {
+        let config = cx.bcx.config;
+        match (
+            config.cli_unstable().terminal_width,
+            config.shell().err_width().diagnostic_terminal_width(),
+        ) {
+            // Terminal width explicitly provided - only useful for testing.
+            (Some(Some(width)), _) => {
+                cmd.arg(format!("-Zterminal-width={}", width));
+            }
+            // Terminal width was not explicitly provided but flag was provided - common case.
+            (Some(None), Some(width)) => {
+                cmd.arg(format!("-Zterminal-width={}", width));
+            }
+            // User didn't opt-in.
+            _ => (),
+        }
+    }
+
     Ok(())
 }
 

src/cargo/core/features.rs

@@ -357,6 +357,7 @@ pub struct CliUnstable {
     pub separate_nightlies: bool,
     pub multitarget: bool,
     pub rustdoc_map: bool,
+    pub terminal_width: Option<Option<usize>>,
 }
 
 impl CliUnstable {
@@ -411,6 +412,16 @@ impl CliUnstable {
             Ok(true)
         };
 
+        fn parse_usize_opt(value: Option<&str>) -> CargoResult<Option<usize>> {
+            Ok(match value {
+                Some(value) => match value.parse::<usize>() {
+                    Ok(value) => Some(value),
+                    Err(e) => bail!("expected a number, found: {}", e),
+                },
+                None => None,
+            })
+        }
+
         match k {
             "print-im-a-teapot" => self.print_im_a_teapot = parse_bool(k, v)?,
             "unstable-options" => self.unstable_options = parse_empty(k, v)?,
@@ -437,6 +448,7 @@ impl CliUnstable {
             "separate-nightlies" => self.separate_nightlies = parse_empty(k, v)?,
             "multitarget" => self.multitarget = parse_empty(k, v)?,
             "rustdoc-map" => self.rustdoc_map = parse_empty(k, v)?,
+            "terminal-width" => self.terminal_width = Some(parse_usize_opt(v)?),
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }
 

src/cargo/core/shell.rs

@@ -6,6 +6,31 @@ use termcolor::{self, Color, ColorSpec, StandardStream, WriteColor};
 
 use crate::util::errors::CargoResult;
 
+pub enum TtyWidth {
+    NoTty,
+    Known(usize),
+    Guess(usize),
+}
+
+impl TtyWidth {
+    /// Returns the width provided with `-Z terminal-width` to rustc to truncate diagnostics with
+    /// long lines.
+    pub fn diagnostic_terminal_width(&self) -> Option<usize> {
+        match *self {
+            TtyWidth::NoTty | TtyWidth::Guess(_) => None,
+            TtyWidth::Known(width) => Some(width),
+        }
+    }
+
+    /// Returns the width used by progress bars for the tty.
+    pub fn progress_max_width(&self) -> Option<usize> {
+        match *self {
+            TtyWidth::NoTty => None,
+            TtyWidth::Known(width) | TtyWidth::Guess(width) => Some(width),
+        }
+    }
+}
+
 /// The requested verbosity of output.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Verbosity {
@@ -125,12 +150,12 @@ impl Shell {
     }
 
     /// Returns the width of the terminal in spaces, if any.
-    pub fn err_width(&self) -> Option<usize> {
+    pub fn err_width(&self) -> TtyWidth {
         match self.output {
             ShellOut::Stream {
                 stderr_tty: true, ..
             } => imp::stderr_width(),
-            _ => None,
+            _ => TtyWidth::NoTty,
         }
     }
 
@@ -408,21 +433,21 @@ impl ColorChoice {
 
 #[cfg(unix)]
 mod imp {
-    use super::Shell;
+    use super::{Shell, TtyWidth};
     use std::mem;
 
-    pub fn stderr_width() -> Option<usize> {
+    pub fn stderr_width() -> TtyWidth {
         unsafe {
             let mut winsize: libc::winsize = mem::zeroed();
             // The .into() here is needed for FreeBSD which defines TIOCGWINSZ
             // as c_uint but ioctl wants c_ulong.
             if libc::ioctl(libc::STDERR_FILENO, libc::TIOCGWINSZ.into(), &mut winsize) < 0 {
-                return None;
+                return TtyWidth::NoTty;
             }
             if winsize.ws_col > 0 {
-                Some(winsize.ws_col as usize)
+                TtyWidth::Known(winsize.ws_col as usize)
             } else {
-                None
+                TtyWidth::NoTty
             }
         }
     }
@@ -445,14 +470,14 @@ mod imp {
     use winapi::um::wincon::*;
     use winapi::um::winnt::*;
 
-    pub(super) use super::default_err_erase_line as err_erase_line;
+    pub(super) use super::{default_err_erase_line as err_erase_line, TtyWidth};
 
-    pub fn stderr_width() -> Option<usize> {
+    pub fn stderr_width() -> TtyWidth {
         unsafe {
             let stdout = GetStdHandle(STD_ERROR_HANDLE);
             let mut csbi: CONSOLE_SCREEN_BUFFER_INFO = mem::zeroed();
             if GetConsoleScreenBufferInfo(stdout, &mut csbi) != 0 {
-                return Some((csbi.srWindow.Right - csbi.srWindow.Left) as usize);
+                return TtyWidth::Known((csbi.srWindow.Right - csbi.srWindow.Left) as usize);
             }
 
             // On mintty/msys/cygwin based terminals, the above fails with
@@ -468,7 +493,7 @@ mod imp {
                 ptr::null_mut(),
             );
             if h == INVALID_HANDLE_VALUE {
-                return None;
+                return TtyWidth::NoTty;
             }
 
             let mut csbi: CONSOLE_SCREEN_BUFFER_INFO = mem::zeroed();
@@ -484,17 +509,21 @@ mod imp {
                 // resize the console correctly, but there's no reasonable way
                 // to detect which kind of terminal we are running in, or if
                 // GetConsoleScreenBufferInfo returns accurate information.
-                return Some(cmp::min(60, width));
+                return TtyWidth::Guess(cmp::min(60, width));
             }
-            None
+
+            TtyWidth::NoTty
         }
     }
 }
 
 #[cfg(windows)]
 fn default_err_erase_line(shell: &mut Shell) {
-    if let Some(max_width) = imp::stderr_width() {
-        let blank = " ".repeat(max_width);
-        drop(write!(shell.output.stderr(), "{}\r", blank));
+    match imp::stderr_width() {
+        TtyWidth::Known(max_width) | TtyWidth::Guess(max_width) => {
+            let blank = " ".repeat(max_width);
+            drop(write!(shell.output.stderr(), "{}\r", blank));
+        }
+        _ => (),
     }
 }

src/cargo/util/progress.rs

@@ -50,7 +50,7 @@ impl<'cfg> Progress<'cfg> {
         }
 
         Progress {
-            state: cfg.shell().err_width().map(|n| State {
+            state: cfg.shell().err_width().progress_max_width().map(|n| State {
                 config: cfg,
                 format: Format {
                     style,
@@ -216,7 +216,7 @@ impl<'cfg> State<'cfg> {
     }
 
     fn try_update_max_width(&mut self) {
-        if let Some(n) = self.config.shell().err_width() {
+        if let Some(n) = self.config.shell().err_width().progress_max_width() {
             self.format.max_width = n;
         }
     }

src/doc/src/reference/unstable.md

@@ -826,3 +826,37 @@ sysroot. If you are using rustup, this documentation can be installed with
 The default value is `"remote"`.
 
 The value may also take a URL for a custom location.
+
+### terminal-width
+This feature provides a new flag, `-Z terminal-width`, which is used to pass
+a terminal width to `rustc` so that error messages containing long lines
+can be intelligently truncated.
+
+For example, passing `-Z terminal-width=20` (an arbitrarily low value) might
+produce the following error:
+
+```text
+error[E0308]: mismatched types
+  --> src/main.rs:2:17
+  |
+2 | ..._: () = 42;
+  |       --   ^^ expected `()`, found integer
+  |       |
+  |       expected due to this
+
+error: aborting due to previous error
+```
+
+In contrast, without `-Z terminal-width`, the error would look as shown below:
+
+```text
+error[E0308]: mismatched types
+ --> src/main.rs:2:17
+  |
+2 |     let _: () = 42;
+  |            --   ^^ expected `()`, found integer
+  |            |
+  |            expected due to this
+
+error: aborting due to previous error
+```

tests/testsuite/build.rs

@@ -7,8 +7,8 @@ use cargo::{
 use cargo_test_support::paths::{root, CargoPathExt};
 use cargo_test_support::registry::Package;
 use cargo_test_support::{
-    basic_bin_manifest, basic_lib_manifest, basic_manifest, lines_match, main_file, project,
-    rustc_host, sleep_ms, symlink_supported, t, Execs, ProjectBuilder,
+    basic_bin_manifest, basic_lib_manifest, basic_manifest, is_nightly, lines_match, main_file,
+    project, rustc_host, sleep_ms, symlink_supported, t, Execs, ProjectBuilder,
 };
 use std::env;
 use std::fs;
@@ -5103,3 +5103,27 @@ fn target_directory_backup_exclusion() {
     p.cargo("build").run();
     assert!(!&cachedir_tag.is_file());
 }
+
+#[cargo_test]
+fn simple_terminal_width() {
+    if !is_nightly() {
+        // --terminal-width is unstable
+        return;
+    }
+    let p = project()
+        .file(
+            "src/lib.rs",
+            r#"
+                fn main() {
+                    let _: () = 42;
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build -Zterminal-width=20")
+        .masquerade_as_nightly_cargo()
+        .with_status(101)
+        .with_stderr_contains("3 | ..._: () = 42;")
+        .run();
+}