Use flag in Shell to track progress clearing.

src/cargo/core/compiler/job_queue.rs

@@ -12,7 +12,7 @@ use jobserver::{Acquired, HelperThread};
 use log::{debug, info, trace};
 
 use crate::core::profiles::Profile;
-use crate::core::{PackageId, Target, TargetKind, Verbosity};
+use crate::core::{PackageId, Target, TargetKind};
 use crate::handle_error;
 use crate::util;
 use crate::util::diagnostic_server::{self, DiagnosticPrinter};
@@ -289,14 +289,8 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
             };
 
             for event in events {
-                // CAUTION: Care must be taken to clear the progress bar if a
-                // message is to be actually displayed. Try to avoid
-                // unnecessarily clearing it to avoid flickering.
                 match event {
                     Message::Run(cmd) => {
-                        if cx.bcx.config.shell().verbosity() == Verbosity::Verbose {
-                            self.progress.clear();
-                        }
                         cx.bcx
                             .config
                             .shell()
@@ -306,17 +300,14 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
                         plan.update(&module_name, &cmd, &filenames)?;
                     }
                     Message::Stdout(out) => {
-                        self.progress.clear();
                         println!("{}", out);
                     }
                     Message::Stderr(err) => {
-                        self.progress.clear();
                         let mut shell = cx.bcx.config.shell();
                         shell.print_ansi(err.as_bytes())?;
                         shell.err().write_all(b"\n")?;
                     }
                     Message::FixDiagnostic(msg) => {
-                        self.progress.clear();
                         print.print(&msg)?;
                     }
                     Message::Finish(key, result) => {
@@ -336,7 +327,6 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
                         match result {
                             Ok(()) => self.finish(key, cx)?,
                             Err(e) => {
-                                self.progress.clear();
                                 let msg = "The following warnings were emitted during compilation:";
                                 self.emit_warnings(Some(msg), &key, cx)?;
 
@@ -460,7 +450,6 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
         let bcx = &mut cx.bcx;
         if let Some(output) = output.get(&(key.pkg, key.kind)) {
             if !output.warnings.is_empty() {
-                self.progress.clear();
                 if let Some(msg) = msg {
                     writeln!(bcx.config.shell().err(), "{}\n", msg)?;
                 }
@@ -521,12 +510,10 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
                     // Skip Doctest
                     if !key.mode.is_any_test() {
                         self.documented.insert(key.pkg);
-                        self.progress.clear();
                         config.shell().status("Documenting", key.pkg)?;
                     }
                 } else {
                     self.compiled.insert(key.pkg);
-                    self.progress.clear();
                     if key.mode.is_check() {
                         config.shell().status("Checking", key.pkg)?;
                     } else {
@@ -540,9 +527,6 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
                     && !(key.mode == CompileMode::Doctest && self.compiled.contains(&key.pkg))
                 {
                     self.compiled.insert(key.pkg);
-                    if config.shell().verbosity() == Verbosity::Verbose {
-                        self.progress.clear();
-                    }
                     config.shell().verbose(|c| c.status("Fresh", key.pkg))?;
                 }
             }

src/cargo/core/shell.rs

@@ -23,6 +23,9 @@ pub struct Shell {
     err: ShellOut,
     /// How verbose messages should be
     verbosity: Verbosity,
+    /// Flag that indicates the current line needs to be cleared before
+    /// printing. Used when a progress bar is currently displayed.
+    needs_clear: bool,
 }
 
 impl fmt::Debug for Shell {
@@ -75,6 +78,7 @@ impl Shell {
                 tty: atty::is(atty::Stream::Stderr),
             },
             verbosity: Verbosity::Verbose,
+            needs_clear: false,
         }
     }
 
@@ -83,6 +87,7 @@ impl Shell {
         Shell {
             err: ShellOut::Write(out),
             verbosity: Verbosity::Verbose,
+            needs_clear: false,
         }
     }
 
@@ -97,10 +102,25 @@ impl Shell {
     ) -> CargoResult<()> {
         match self.verbosity {
             Verbosity::Quiet => Ok(()),
-            _ => self.err.print(status, message, color, justified),
+            _ => {
+                if self.needs_clear {
+                    self.err_erase_line();
+                }
+                self.err.print(status, message, color, justified)
+            }
         }
     }
 
+    /// Set whether or not the next print should clear the current line.
+    pub fn set_needs_clear(&mut self, needs_clear: bool) {
+        self.needs_clear = needs_clear;
+    }
+
+    /// True if the `needs_clear` flag is not set.
+    pub fn is_cleared(&self) -> bool {
+        !self.needs_clear
+    }
+
     /// Returns the width of the terminal in spaces, if any
     pub fn err_width(&self) -> Option<usize> {
         match self.err {
@@ -119,13 +139,17 @@ impl Shell {
 
     /// Get a reference to the underlying writer
     pub fn err(&mut self) -> &mut dyn Write {
+        if self.needs_clear {
+            self.err_erase_line();
+        }
         self.err.as_write()
     }
 
     /// Erase from cursor to end of line.
     pub fn err_erase_line(&mut self) {
         if let ShellOut::Stream { tty: true, .. } = self.err {
             imp::err_erase_line(self);
+            self.needs_clear = false;
         }
     }
 
@@ -251,6 +275,9 @@ impl Shell {
 
     /// Prints a message and translates ANSI escape code into console colors.
     pub fn print_ansi(&mut self, message: &[u8]) -> CargoResult<()> {
+        if self.needs_clear {
+            self.err_erase_line();
+        }
         #[cfg(windows)]
         {
             if let ShellOut::Stream { stream, .. } = &mut self.err {
@@ -361,7 +388,7 @@ mod imp {
         // This is the "EL - Erase in Line" sequence. It clears from the cursor
         // to the end of line.
         // https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_sequences
-        let _ = shell.err().write_all(b"\x1B[K");
+        let _ = shell.err.as_write().write_all(b"\x1B[K");
     }
 }
 
@@ -434,6 +461,6 @@ mod imp {
 fn default_err_erase_line(shell: &mut Shell) {
     if let Some(max_width) = imp::stderr_width() {
         let blank = " ".repeat(max_width);
-        drop(write!(shell.err(), "{}\r", blank));
+        drop(write!(shell.err.as_write(), "{}\r", blank));
     }
 }

src/cargo/util/progress.rs

@@ -195,18 +195,21 @@ impl<'cfg> State<'cfg> {
         }
 
         // Only update if the line has changed.
-        if self.last_line.as_ref() != Some(&line) {
-            self.config.shell().status_header(&self.name)?;
-            write!(self.config.shell().err(), "{}\r", line)?;
+        if self.config.shell().is_cleared() || self.last_line.as_ref() != Some(&line) {
+            let mut shell = self.config.shell();
+            shell.set_needs_clear(false);
+            shell.status_header(&self.name)?;
+            write!(shell.err(), "{}\r", line)?;
             self.last_line = Some(line);
+            shell.set_needs_clear(true);
         }
 
         Ok(())
     }
 
     fn clear(&mut self) {
         // No need to clear if the progress is not currently being displayed.
-        if self.last_line.is_some() {
+        if self.last_line.is_some() && !self.config.shell().is_cleared() {
             self.config.shell().err_erase_line();
             self.last_line = None;
         }