Assume README.md if readme=true

Author: tverghis

src/cargo/util/toml/mod.rs

@@ -1199,11 +1199,18 @@ impl TomlManifest {
             project.namespaced_features.unwrap_or(false),
         )?;
 
+        let readme = readme_for_project(package_root, project);
+        if let Some(ref r) = readme {
+            if !package_root.join(r).is_file() {
+                bail!("readme file with name '{}' was not found", r);
+            }
+        };
+
         let metadata = ManifestMetadata {
             description: project.description.clone(),
             homepage: project.homepage.clone(),
             documentation: project.documentation.clone(),
-            readme: readme_for_project(package_root, project),
+            readme,
             authors: project.authors.clone().unwrap_or_default(),
             license: project.license.clone(),
             license_file: project.license_file.clone(),
@@ -1520,7 +1527,7 @@ fn readme_for_project(package_root: &Path, project: &TomlProject) -> Option<Stri
         None => default_readme_from_package_root(package_root),
         Some(value) => match value {
             StringOrBool::Bool(false) => None,
-            StringOrBool::Bool(true) => default_readme_from_package_root(package_root),
+            StringOrBool::Bool(true) => Some("README.md".to_string()),
             StringOrBool::String(v) => Some(v.clone()),
         },
     }

src/doc/src/reference/manifest.md

@@ -168,7 +168,8 @@ readme = "README.md"
 If no value is specified for this field, and a file named `README.md`,
 `README.txt` or `README` exists in the package root, then the name of that
 file will be used. You can suppress this behavior by setting this field to
-`false`.
+`false`. If the field is set to `true`, a default value of `README.md` will
+be assumed.
 
 #### The `homepage` field
 

tests/testsuite/read_manifest.rs

@@ -185,16 +185,29 @@ fn cargo_read_manifest_suppress_default_readme() {
         .run();
 }
 
-// If a file named README.txt exists, and `readme = true`, the value `README.txt` should be defaulted in.
+// If a file named README.md exists, and `readme = true`, the value `README.md` should be defaulted in.
 #[cargo_test]
 fn cargo_read_manifest_defaults_readme_if_true() {
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest_with_readme("foo", "true"))
-        .file("README.txt", "Sample project")
+        .file("README.md", "Sample project")
         .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
         .build();
 
     p.cargo("read-manifest")
-        .with_json(&manifest_output(&format!(r#""{}""#, "README.txt")))
+        .with_json(&manifest_output(&format!(r#""{}""#, "README.md")))
         .run();
 }
+
+// If a file named README.md does not exist, and `readme = true`, it should panic.
+#[cargo_test]
+#[should_panic]
+fn cargo_read_manifest_panics_if_default_readme_not_found() {
+    let p = project()
+        .file("Cargo.toml", &basic_bin_manifest_with_readme("foo", "true"))
+        .file("README.txt", "Sample project")
+        .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
+        .build();
+
+    p.cargo("read-manifest").run();
+}