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

This commit adds support for rustc's `-Z terminal-width` flag, which is
used to trim diagnostic output to fit within the current terminal.

Co-authored-by: David Wood <david@davidtw.co>
Signed-off-by: David Wood <david@davidtw.co>

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};
@@ -709,6 +710,7 @@ 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");
@@ -716,6 +718,16 @@ fn add_error_format_and_color(
         _ => {}
     }
     cmd.arg(json);
+
+    if nightly_features_allowed() {
+        if let (Some(width), _) | (_, Some(width)) = (
+            cx.bcx.config.cli_unstable().terminal_width,
+            cx.bcx.config.shell().accurate_err_width(),
+        ) {
+            cmd.arg(format!("-Zterminal-width={}", width));
+        }
+    }
+
     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<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 = parse_usize_opt(v)?,
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }
 

src/cargo/core/shell.rs

@@ -134,6 +134,15 @@ impl Shell {
         }
     }
 
+    /// Returns the width of the terminal in spaces, if any. Always `None` in Windows.
+    pub fn accurate_err_width(&self) -> Option<usize> {
+        if self.is_err_tty() {
+            imp::accurate_stderr_width()
+        } else {
+            None
+        }
+    }
+
     /// Returns `true` if stderr is a tty.
     pub fn is_err_tty(&self) -> bool {
         match self.output {
@@ -411,6 +420,10 @@ mod imp {
     use super::Shell;
     use std::mem;
 
+    pub fn accurate_stderr_width() -> Option<usize> {
+        stderr_width()
+    }
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let mut winsize: libc::winsize = mem::zeroed();
@@ -447,6 +460,10 @@ mod imp {
 
     pub(super) use super::default_err_erase_line as err_erase_line;
 
+    pub fn accurate_stderr_width() -> Option<usize> {
+        None
+    }
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let stdout = GetStdHandle(STD_ERROR_HANDLE);

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();
+}