Treat MSVC "performing full link" message as informational

When the MSVC incremental linker finds a .ilk file for incremental
linking but its associated .exe file is missing, this message is
printed to stdout in 1 line:

LINK : ...\foo.exe not found or not build by the last incremental link;
performing full link

However, if both the .ilk and .exe files are missing (for a clean
build), the message isn't printed and it still does a full link. So, the
presence of the message doesn't affect the result of the build.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -759,10 +759,15 @@ fn for_each(bytes: &[u8], mut f: impl FnMut(&str, &mut String)) -> String {
         escaped_stdout = for_each(&stdout, |line, output| {
             // Hide some progress messages from link.exe that we don't care about.
             // See https://github.com/chromium/chromium/blob/bfa41e41145ffc85f041384280caf2949bb7bd72/build/toolchain/win/tool_wrapper.py#L144-L146
+            // When incremental linking is enabled and an .ilk exists, but its associated .exe is
+            // missing, link.exe prints the path of the missing .exe followed by:
+            let ilk_but_no_exe =
+                "not found or not built by the last incremental link; performing full link";
             let trimmed = line.trim_start();
             if trimmed.starts_with("Creating library")
                 || trimmed.starts_with("Generating code")
                 || trimmed.starts_with("Finished generating code")
+                || trimmed.ends_with(ilk_but_no_exe)
             {
                 linker_info += line;
                 linker_info += "\r\n";

tests/run-make/msvc-incremental-full-link-info/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/msvc-incremental-full-link-info/rmake.rs

@@ -0,0 +1,47 @@
+//@ only-msvc
+//! Tests that the MSVC incremental linker's "performing full link" message is classified as
+//! `linker_info`, not `linker_messages`.
+//!
+//! The MSVC incremental linker prints this message to stdout when it finds a `.ilk` file for
+//! incremental linking  but its associated `.exe` file is missing:
+//!
+//! ```
+//! LINK : ...\main.exe not found or not built by the last incremental link; performing full link
+//! ```
+
+use std::fs;
+
+use run_make_support::bare_rustc;
+
+fn incremental_rustc() -> run_make_support::Rustc {
+    let mut r = bare_rustc();
+    r.input("main.rs")
+        // Overrides `rust.lld=true` on CI.
+        .arg("-Clinker=link.exe")
+        .arg("-Clinker-flavor=msvc")
+        // Without this, Rust passes /OPT:REF to link.exe, which
+        // disables incremental linking entirely and suppresses the message.
+        .arg("-Clink-dead-code")
+        // /DEBUG is required: it implies /INCREMENTAL, which is what makes
+        // link.exe create the .ilk file and later emit the message.
+        .arg("-Cdebuginfo=2");
+    r
+}
+
+fn main() {
+    // First link: produces main.exe and main.ilk.
+    incremental_rustc().run();
+
+    // Delete the .exe but leave the .ilk.
+    fs::remove_file("main.exe").unwrap();
+
+    // Second link: link.exe finds the .ilk but not the .exe and emits the
+    // "performing full link" message on stdout.
+    let output = incremental_rustc()
+        .arg("-Dlinker_messages") // Fail if the message is misclassified.
+        .arg("-Wlinker_info")
+        .run();
+
+    // Verify the message was actually emitted and routed to linker_info.
+    output.assert_stderr_contains("performing full link");
+}