Add suggestion to the "missing native library" error

If we fail to locate a native library that we are linking with, it could
be the case the user entered a complete file name like `foo.lib` or
`libfoo.a` when we expect them to simply provide `foo`.

In this situation, we now detect that case and suggest the user only
provide the library name itself.

compiler/rustc_error_messages/locales/en-US/metadata.ftl

@@ -165,6 +165,8 @@ metadata_failed_write_error =
 metadata_missing_native_library =
     could not find native static library `{$libname}`, perhaps an -L flag is missing?
 
+metadata_only_provide_library_name = only provide the library name `{$suggested_name}`, not the full filename
+
 metadata_failed_create_tempdir =
     couldn't create a temp dir: {$err}
 

compiler/rustc_metadata/src/errors.rs

@@ -372,7 +372,41 @@ pub struct FailedWriteError {
 #[derive(Diagnostic)]
 #[diag(metadata::missing_native_library)]
 pub struct MissingNativeLibrary<'a> {
-    pub libname: &'a str,
+    libname: &'a str,
+    #[subdiagnostic]
+    suggest_name: Option<SuggestLibraryName<'a>>,
+}
+
+impl<'a> MissingNativeLibrary<'a> {
+    pub fn new(libname: &'a str, verbatim: bool) -> Self {
+        // if it looks like the user has provided a complete filename rather just the bare lib name,
+        // then provide a note that they might want to try trimming the name
+        let suggested_name = if !verbatim {
+            if let Some(libname) = libname.strip_prefix("lib") && let Some(libname) = libname.strip_suffix(".a") {
+                // this is a unix style filename so trim prefix & suffix
+                Some(libname)
+            } else if let Some(libname) = libname.strip_suffix(".lib") {
+                // this is a Windows style filename so just trim the suffix
+                Some(libname)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+
+        Self {
+            libname,
+            suggest_name: suggested_name
+                .map(|suggested_name| SuggestLibraryName { suggested_name }),
+        }
+    }
+}
+
+#[derive(Subdiagnostic)]
+#[help(metadata::only_provide_library_name)]
+pub struct SuggestLibraryName<'a> {
+    suggested_name: &'a str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_metadata/src/native_libs.rs

@@ -52,7 +52,7 @@ pub fn find_native_static_library(
         }
     }
 
-    sess.emit_fatal(MissingNativeLibrary { libname: name });
+    sess.emit_fatal(MissingNativeLibrary::new(name, verbatim.unwrap_or(false)));
 }
 
 fn find_bundled_library(

src/test/ui/native-library-link-flags/suggest-libname-only-1.rs

@@ -0,0 +1,9 @@
+// build-fail
+// compile-flags: --crate-type rlib
+// error-pattern: could not find native static library `libfoo.a`
+// error-pattern: only provide the library name `foo`, not the full filename
+
+#[link(name = "libfoo.a", kind = "static")]
+extern { }
+
+pub fn main() { }

src/test/ui/native-library-link-flags/suggest-libname-only-1.stderr

@@ -0,0 +1,6 @@
+error: could not find native static library `libfoo.a`, perhaps an -L flag is missing?
+   |
+   = help: only provide the library name `foo`, not the full filename
+
+error: aborting due to previous error
+

src/test/ui/native-library-link-flags/suggest-libname-only-2.rs

@@ -0,0 +1,9 @@
+// build-fail
+// compile-flags: --crate-type rlib
+// error-pattern: could not find native static library `bar.lib`
+// error-pattern: only provide the library name `bar`, not the full filename
+
+#[link(name = "bar.lib", kind = "static")]
+extern { }
+
+pub fn main() { }

src/test/ui/native-library-link-flags/suggest-libname-only-2.stderr

@@ -0,0 +1,6 @@
+error: could not find native static library `bar.lib`, perhaps an -L flag is missing?
+   |
+   = help: only provide the library name `bar`, not the full filename
+
+error: aborting due to previous error
+